Mammals () are a group of
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
animals constituting the
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differentl ...
Mammalia (), characterized by the presence of
mammary gland
A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in primat ...
s which in
females produce
milk
Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digestion, digest solid food. Immune factors and immune ...
for feeding (nursing) their young, a
neocortex
The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, sp ...
(a region of the brain),
fur
Fur is a thick growth of hair that covers the skin of mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture from reaching the skin; the underfur acts as an insulating blanket t ...
or
hair
Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals.
The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and f ...
, and three
middle ear bones. These characteristics distinguish them from
reptile
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s (including
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
s) from which they
diverged in the
Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
, over 300 million years ago. Around 6,400
extant
Extant is the opposite of the word extinct. It may refer to:
* Extant hereditary titles
* Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English
* Extant taxon, a taxon which is not extinct, ...
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
of mammals have been described divided into 29
orders
Order, ORDER or Orders may refer to:
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
* Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...
. The largest
orders
Order, ORDER or Orders may refer to:
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
* Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...
, in terms of number of species, are the
rodents
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are nat ...
,
bats
Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...
, and
Eulipotyphla (
hedgehogs,
mole
Mole (or Molé) may refer to:
Animals
* Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America
* Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpida ...
s,
shrew
Shrews (family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to different fa ...
s, and others). The next three are the
Primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
s (including
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s,
ape
Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its siste ...
s,
monkey
Monkey is a common name that may refer to most mammals of the infraorder Simiiformes, also known as the simians. Traditionally, all animals in the group now known as simians are counted as monkeys except the apes, which constitutes an incomple ...
s, and others), the
Artiodactyla (
cetacean
Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel them ...
s and
even-toed ungulates), and the
Carnivora
Carnivora is a Clade, monophyletic order of Placentalia, placental mammals consisting of the most recent common ancestor of all felidae, cat-like and canidae, dog-like animals, and all descendants of that ancestor. Members of this group are f ...
(
cats,
dog
The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...
s,
seals
Seals may refer to:
* Pinniped, a diverse group of semi-aquatic marine mammals, many of which are commonly called seals, particularly:
** Earless seal, or "true seal"
** Fur seal
* Seal (emblem), a device to impress an emblem, used as a means of a ...
, and others).
In terms of
cladistics
Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...
, which reflects evolutionary history, mammals are the only living members of the
Synapsida (synapsids); this
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
, together with
Sauropsida
Sauropsida ("lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia. Sauropsida is the sister taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early syna ...
(reptiles and birds), constitutes the larger
Amniota clade. The early synapsids were
sphenacodonts
Sphenacodontia is a stem-based clade of derived synapsids. It was defined by Amson and Laurin (2011) as "the largest clade that includes ''Haptodus baylei'', ''Haptodus garnettensis'' and ''Sphenacodon ferox'', but not ''Edaphosaurus pogonias'' ...
, a group that included the famous ''
Dimetrodon
''Dimetrodon'' ( or ,) meaning "two measures of teeth,” is an extinct genus of non-mammalian synapsid that lived during the Cisuralian (Early Permian), around 295–272 million years ago (Mya). It is a member of the family Sphenacodontid ...
''. The synapsids split into several diverse groups of non-mammalian synapsids — traditionally and incorrectly referred to as mammal-like reptiles or by the term pelycosaurs, and now known as stem mammals or protomammals — before giving rise to
therapsids during the beginning of the
Middle Permian
The Guadalupian is the second and middle series/epoch of the Permian. The Guadalupian was preceded by the Cisuralian and followed by the Lopingian. It is named after the Guadalupe Mountains of New Mexico and Texas, and dates between 272.95 ± 0. ...
period. Mammals originated from
cynodont
The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...
s, an advanced group of therapsids, during the Late
Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
-Early
Jurassic
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
. The modern mammalian orders arose in the
Paleogene
The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...
and
Neogene
The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...
periods of the
Cenozoic
The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...
era, after the
extinction of non-avian dinosaurs, and have been the dominant terrestrial animal group from 66 million years ago to the present.
The basic body type is
quadruped, and most mammals use their four extremities for
terrestrial locomotion
Terrestrial locomotion has evolved as animals adapted from aquatic to terrestrial environments. Locomotion on land raises different problems than that in water, with reduced friction being replaced by the increased effects of gravity.
As viewed ...
; but in some, the extremities are adapted for
life at sea, in
the air,
in trees,
underground
Underground most commonly refers to:
* Subterranea (geography), the regions beneath the surface of the Earth
Underground may also refer to:
Places
* The Underground (Boston), a music club in the Allston neighborhood of Boston
* The Underground (S ...
, or
on two legs. Mammals range in size from the
bumblebee bat to the
blue whale
The blue whale (''Balaenoptera musculus'') is a marine mammal and a baleen whale. Reaching a maximum confirmed length of and weighing up to , it is the largest animal known to have ever existed. The blue whale's long and slender body can ...
—possibly the largest animal to have ever lived. Maximum lifespan varies from two years for the shrew to 211 years for the
bowhead whale
The bowhead whale (''Balaena mysticetus'') is a species of baleen whale belonging to the family Balaenidae and the only living representative of the genus ''Balaena''. They are the only baleen whale endemic to the Arctic and subarctic waters, ...
. All modern mammals give birth to live young, except the five species of
monotreme
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brain ...
s, which are egg-laying mammals. The most species-rich group of mammals, the
cohort
Cohort or cohortes may refer to:
* Cohort (educational group), a group of students working together through the same academic curriculum
* Cohort (floating point), a set of different encodings of the same numerical value
* Cohort (military unit), ...
called
placentals
Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...
, have a
placenta
The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mater ...
, which enables the feeding of the fetus during
gestation
Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregna ...
.
Most mammals are
intelligent, with some possessing large brains,
self-awareness
In philosophy of self, self-awareness is the experience of one's own personality or individuality. It is not to be confused with consciousness in the sense of qualia. While consciousness is being aware of one's environment and body and lifesty ...
, and
tool use
Tool use by animals is a phenomenon in which an animal uses any kind of tool in order to achieve a goal such as acquiring food and water, grooming, defence, communication, recreation or construction. Originally thought to be a skill possessed o ...
. Mammals can communicate and vocalize in several ways, including the production of
ultrasound
Ultrasound is sound waves with frequency, frequencies higher than the upper audible limit of human hearing range, hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hea ...
,
scent-marking,
alarm signals,
singing
Singing is the act of creating musical sounds with the voice. A person who sings is called a singer, artist or vocalist (in jazz and/or popular music). Singers perform music (arias, recitatives, songs, etc.) that can be sung with or without ...
, and
echolocation. Mammals can organize themselves into
fission-fusion societies,
harems
Harem (Persian: حرمسرا ''haramsarā'', ar, حَرِيمٌ ''ḥarīm'', "a sacred inviolable place; harem; female members of the family") refers to domestic spaces that are reserved for the women of the house in a Muslim family. A hare ...
, and
hierarchies—but can also be solitary and
territorial
A territory is an area of land, sea, or space, particularly belonging or connected to a country, person, or animal.
In international politics, a territory is usually either the total area from which a state may extract power resources or a ...
. Most mammals are
polygynous
Polygyny (; from Neoclassical Greek πολυγυνία (); ) is the most common and accepted form of polygamy around the world, entailing the marriage of a man with several women.
Incidence
Polygyny is more widespread in Africa than in any ...
, but some can be
monogamous
Monogamy ( ) is a form of Dyad (sociology), dyadic Intimate relationship, relationship in which an individual has only one Significant other, partner during their lifetime. Alternately, only one partner at any one time (Monogamy#Serial monogamy, ...
or
polyandrous.
Domestication of many types of mammals by humans played a major role in the
Neolithic Revolution
The Neolithic Revolution, or the (First) Agricultural Revolution, was the wide-scale transition of many human cultures during the Neolithic period from a lifestyle of hunting and gathering to one of agriculture and settlement, making an incre ...
, and resulted in
farming
Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to ...
replacing
hunting and gathering
A traditional hunter-gatherer or forager is a human living an ancestrally derived lifestyle in which most or all food is obtained by foraging, that is, by gathering food from local sources, especially edible wild plants but also insects, fungi, ...
as the primary source of food for humans. This led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, and ultimately the development of the first
civilization
A civilization (or civilisation) is any complex society characterized by the development of a state, social stratification, urbanization, and symbolic systems of communication beyond natural spoken language (namely, a writing system).
Ci ...
s. Domesticated mammals provided, and continue to provide, power for transport and agriculture, as well as food (
meat
Meat is animal flesh that is eaten as food. Humans have hunted, farmed, and scavenged animals for meat since prehistoric times. The establishment of settlements in the Neolithic Revolution allowed the domestication of animals such as chic ...
and
dairy product
Dairy products or milk products, also known as lacticinia, are food products made from (or containing) milk. The most common dairy animals are cow, water buffalo, nanny goat, and ewe. Dairy products include common grocery store food items in th ...
s),
fur
Fur is a thick growth of hair that covers the skin of mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture from reaching the skin; the underfur acts as an insulating blanket t ...
, and
leather
Leather is a strong, flexible and durable material obtained from the tanning, or chemical treatment, of animal skins and hides to prevent decay. The most common leathers come from cattle, sheep, goats, equine animals, buffalo, pigs and hogs, ...
. Mammals are also
hunted and raced for sport, and are used as
model organism
A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...
s in science. Mammals have been depicted in
art
Art is a diverse range of human activity, and resulting product, that involves creative or imaginative talent expressive of technical proficiency, beauty, emotional power, or conceptual ideas.
There is no generally agreed definition of wha ...
since
Paleolithic
The Paleolithic or Palaeolithic (), also called the Old Stone Age (from Greek: παλαιός ''palaios'', "old" and λίθος ''lithos'', "stone"), is a period in human prehistory that is distinguished by the original development of stone too ...
times, and appear in literature, film, mythology, and religion. Decline in numbers and
extinction
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
of many mammals is primarily driven by human
poaching and
habitat destruction
Habitat destruction (also termed habitat loss and habitat reduction) is the process by which a natural habitat becomes incapable of supporting its native species. The organisms that previously inhabited the site are displaced or dead, thereby ...
, primarily
deforestation
Deforestation or forest clearance is the removal of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. The most concentrated d ...
.
Classification
![Mammal species pie chart](https://upload.wikimedia.org/wikipedia/commons/0/00/Mammal_species_pie_chart.svg)
Mammal classification has been through several revisions since
Carl Linnaeus
Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the ...
initially defined the class, and at present, no classification system is universally accepted. McKenna & Bell (1997) and Wilson & Reeder (2005) provide useful recent compendiums.
Simpson
Simpson most often refers to:
* Simpson (name), a British surname
*''The Simpsons'', an animated American sitcom
**The Simpson family, central characters of the series ''The Simpsons''
Simpson may also refer to:
Organizations Schools
*Simpso ...
(1945)
provides
systematics
Biological systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: cladograms, phylogenetic tre ...
of mammal origins and relationships that had been taught universally until the end of the 20th century.
However, since 1945, a large amount of new and more detailed information has gradually been found: The
paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of
cladistics
Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...
. Though fieldwork and lab work progressively outdated Simpson's classification, it remains the closest thing to an official classification of mammals, despite its known issues.
Most mammals, including the six most species-rich
orders
Order, ORDER or Orders may refer to:
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
* Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...
, belong to the placental group. The three largest orders in numbers of species are
Rodent
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are na ...
ia:
mice
A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...
,
rat
Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' ( pack rats), ''Bandicota'' (bandicoot ...
s,
porcupine
Porcupines are large rodents with coats of sharp spines, or quills, that protect them against predation. The term covers two families of animals: the Old World porcupines of family Hystricidae, and the New World porcupines of family, Erethizont ...
s,
beaver
Beavers are large, semiaquatic rodents in the genus ''Castor'' native to the temperate Northern Hemisphere. There are two extant species: the North American beaver (''Castor canadensis'') and the Eurasian beaver (''C. fiber''). Beavers ar ...
s,
capybara
The capybaraAlso called capivara (in Brazil), capiguara (in Bolivia), chigüire, chigüiro, or fercho (in Colombia and Venezuela), carpincho (in Argentina, Paraguay and Uruguay) and ronsoco (in Peru). or greater capybara (''Hydrochoerus hydro ...
s, and other gnawing mammals;
Chiroptera: bats; and
Soricomorpha
Soricomorpha (from Greek "shrew-form") is a formerly used taxon within the class of mammals. In the past it formed a significant group within the former order Insectivora. However, Insectivora was shown to be polyphyletic and various new orders w ...
:
shrew
Shrews (family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to different fa ...
s,
moles Moles can refer to:
*Moles de Xert, a mountain range in the Baix Maestrat comarca, Valencian Community, Spain
*The Moles (Australian band)
*The Moles, alter ego of Scottish band Simon Dupree and the Big Sound
People
*Abraham Moles, French engineer ...
, and
solenodons. The next three biggest orders, depending on the
biological classification
In biology, taxonomy () is the scientific study of naming, defining ( circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon) and these groups are given ...
scheme used, are the
Primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
s:
ape
Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its siste ...
s,
monkey
Monkey is a common name that may refer to most mammals of the infraorder Simiiformes, also known as the simians. Traditionally, all animals in the group now known as simians are counted as monkeys except the apes, which constitutes an incomple ...
s, and
lemurs; the
Cetartiodactyla:
whale
Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...
s and
even-toed ungulates
The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...
; and the
Carnivora
Carnivora is a Clade, monophyletic order of Placentalia, placental mammals consisting of the most recent common ancestor of all felidae, cat-like and canidae, dog-like animals, and all descendants of that ancestor. Members of this group are f ...
which includes
cats,
dog
The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...
s,
weasel
Weasels are mammals of the genus ''Mustela'' of the family Mustelidae. The genus ''Mustela'' includes the least weasels, polecats, stoats, ferrets and European mink. Members of this genus are small, active predators, with long and slender bo ...
s,
bear
Bears are carnivoran mammals of the family Ursidae. They are classified as caniforms, or doglike carnivorans. Although only eight species of bears are extant, they are widespread, appearing in a wide variety of habitats throughout the Nor ...
s,
seals
Seals may refer to:
* Pinniped, a diverse group of semi-aquatic marine mammals, many of which are commonly called seals, particularly:
** Earless seal, or "true seal"
** Fur seal
* Seal (emblem), a device to impress an emblem, used as a means of a ...
, and allies.
According to ''
Mammal Species of the World'', 5,416 species were identified in 2006. These were grouped into 1,229
genera
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...
, 153
families
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideall ...
and 29 orders.
In 2008, the
International Union for Conservation of Nature
The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natu ...
(IUCN) completed a five-year Global Mammal Assessment for its
IUCN Red List
The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is the world's most comprehensive inventory of the global conservation status of biol ...
, which counted 5,488 species. According to research published in the ''
Journal of Mammalogy
The ''Journal of Mammalogy'' is a bimonthly peer-reviewed scientific journal published by Oxford University Press on behalf of the American Society of Mammalogists. Both the society and the journal were established in 1919. The journal covers rese ...
'' in 2018, the number of recognized mammal species is 6,495, including 96 recently extinct.
Definitions
The word "
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
" is modern, from the scientific name ''Mammalia'' coined by Carl Linnaeus in 1758, derived from the
Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
''
mamma
Mama(s) or Mamma or Momma may refer to:
Roles
*Mother, a female parent
*Mama-san, in Japan and East Asia, a woman in a position of authority
*Mamas, a name for female associates of the Hells Angels
Places
*Mama, Russia, an urban-type settlement ...
'' ("teat, pap"). In an influential 1988 paper, Timothy Rowe defined Mammalia
phylogenetically
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
as the
crown group
In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...
of mammals, the
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
consisting of the
most recent common ancestor of living
monotreme
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brain ...
s (
echidnas and
platypus
The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal Endemic (ecology), endemic to Eastern states of Australia, eastern Australia, including Tasmania. The platypu ...
es) and
Theria
Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes.
Ch ...
n mammals (
marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...
s and
placentals) and all descendants of that ancestor. Since this ancestor lived in the
Jurassic
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
period, Rowe's definition excludes all animals from the earlier
Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
, despite the fact that Triassic fossils in the
Haramiyida
Haramiyida ("thief" from Arabic الحرامية (al ḥarāmiyah), "thief, bandit") is a possibly polyphyletic order of mammaliaform cynodonts or mammals of controversial taxonomic affinites. Their teeth, which are by far the most common remains ...
have been referred to the Mammalia since the mid-19th century. If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. ''
Ambondro'' is more closely related to monotremes than to therian mammals while ''
Amphilestes
''Amphilestes'' is a genus of extinct eutriconodont mammal from the Middle Jurassic of the United Kingdom. It was one of the first Mesozoic mammals discovered and described.
Discovery
The first specimen of ''Amphilestes'' was discovered along ...
'' and ''
Amphitherium
''Amphitherium'' is an extinct genus of stem cladotherian mammal that lived during the Middle Jurassic of England. It was one of the first Mesozoic mammals ever described. A recent phylogenetic study found it to be the sister taxon of ''Palaeoxo ...
'' are more closely related to the therians; as fossils of all three genera are dated about in the
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations co ...
, this is a reasonable estimate for the appearance of the crown group.
T.S. Kemp has provided a more traditional definition: "
Synapsids that possess a
dentary–
squamosal jaw articulation and
occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of ''
Sinoconodon
''Sinoconodon'' is an extinct genus of mammaliamorphs that appears in the fossil record of the Lufeng Formation of China in the Sinemurian stage of the Early Jurassic period, about 193 million years ago. While sharing many plesiomorphic traits wi ...
'' and living mammals. The earliest known synapsid satisfying Kemp's definitions is ''
Tikitherium
''Tikitherium'' is an extinct genus of mammaliaforms from the Late Triassic. It is thought to be an insectivore and a close relative to Docodonta. ''Tikitherium'' refers to Tiki, the village located near the Tiki Formation where the specimen wa ...
'', dated , so the appearance of mammals in this broader sense can be given this
Late Triassic
The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...
date.
McKenna/Bell classification
In 1997, the mammals were comprehensively revised by
Malcolm C. McKenna and Susan K. Bell, which has resulted in the McKenna/Bell classification. The authors worked together as
paleontologist
Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...
s at the
American Museum of Natural History
The American Museum of Natural History (abbreviated as AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. In Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 26 inter ...
. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa, that reflects the historical genealogy of Mammalia.
[ Their 1997 book, ''Classification of Mammals above the Species Level'',] is a comprehensive work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though molecular genetic data challenge several of the groupings.
In the following list, extinct
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
groups are labelled with a dagger
A dagger is a fighting knife with a very sharp point and usually two sharp edges, typically designed or capable of being used as a thrusting or stabbing weapon.State v. Martin, 633 S.W.2d 80 (Mo. 1982): This is the dictionary or popular-use de ...
(†).
Class Mammalia
* Subclass Prototheria: monotremes: echidnas and the platypus
The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal Endemic (ecology), endemic to Eastern states of Australia, eastern Australia, including Tasmania. The platypu ...
* Subclass Theriiformes: live-bearing mammals and their prehistoric relatives
** Infraclass † Allotheria: multituberculates
** Infraclass †Eutriconodonta
Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia, Africa, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack ''et al.'' in 1973 as a replacement name for the ...
: eutriconodonts
** Infraclass Holotheria
Mammalia is a class of animal within the phylum Chordata. Mammal classification has been through several iterations since Carl Linnaeus initially defined the class. No classification system is universally accepted; McKenna & Bell (1997) and Wilson ...
: modern live-bearing mammals and their prehistoric relatives
*** Superlegion †Kuehneotheria
Kuehneotheriidae is an extinct family of mammaliaforms traditionally placed within 'Symmetrodonta', though now generally considered more basal than true symmetrodonts. All members of Kuehneotheriidae which have been found so far are represented o ...
*** Supercohort Theria
Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes.
Ch ...
: live-bearing mammals
**** Cohort Marsupialia: marsupials
***** Magnorder Australidelphia
Australidelphia is the superorder that contains roughly three-quarters of all marsupials, including all those native to Australasia and a single species — the monito del monte — from South America. All other American marsupials are members of ...
: Australian marsupials and the monito del monte
***** Magnorder Ameridelphia: New World marsupials. Now considered paraphyletic, with shrew opossums being closer to australidelphians.
**** Cohort Placentalia
Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished ...
: placentals
***** Magnorder Xenarthra: xenarthrans
***** Magnorder Epitheria
Epitherians comprise all the placental mammals except the Xenarthra. They are primarily characterized by having a stirrup-shaped stapes in the middle ear, which allows for passage of a blood vessel. This is in contrast to the column-shaped stapes ...
: epitheres
****** Superorder †Leptictida
Leptictida (''leptos iktis'' "small/slender weasel") is a possibly paraphyletic extinct order of eutherian mammals. Their classification is contentious: according to cladistic studies, they may be (distantly) related to Euarchontoglires (rodents ...
****** Superorder Preptotheria
Preptotheria is a superorder of placental mammals proposed by McKenna & Bell in their classification of mammals.
Classification
The Linnean taxonomy of Preptotheria according to the scheme of McKenna & Bell (1997):
Cohort Placentalia
Magnorde ...
******* Grandorder Anagalida
Anagaloidea is an extinct order of placental mammals that first appeared during the Paleocene epoch.
Taxonomy
According to the traditional (morphological) view, Anagaloidea is part of the superorder Anagalida, along with the elephant shrews, rod ...
: lagomorphs
The lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families: the Leporidae ( hares and rabbits) and the Ochotonidae (pikas). The name of the order is derived from the Ancient Greek ''lagos'' (λα ...
, rodent
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are na ...
s and elephant shrews
******* Grandorder Ferae
Ferae ( , , "wild beasts") is a mirorder of placental mammalsMalcolm C. McKenna, Susan K. Bell: ''Classification of Mammals: Above the Species Level'' in Columbia University Press, New York (1997), 631 Seiten. that groups together clades Pan-Ca ...
: carnivora
Carnivora is a Clade, monophyletic order of Placentalia, placental mammals consisting of the most recent common ancestor of all felidae, cat-like and canidae, dog-like animals, and all descendants of that ancestor. Members of this group are f ...
ns, pangolin
Pangolins, sometimes known as scaly anteaters, are mammals of the order Pholidota (, from Ancient Greek ϕολιδωτός – "clad in scales"). The one extant family, the Manidae, has three genera: '' Manis'', '' Phataginus'', and '' Smut ...
s, †creodont
Creodonta ("meat teeth") is a former order of extinct carnivorous placental mammals that lived from the early Paleocene to the late Miocene epochs in North America, Europe, Asia and Africa. Originally thought to be a single group of animals ance ...
s and relatives
******* Grandorder Lipotyphla: insectivorans
******* Grandorder Archonta
The Archonta are a now-abandoned group of mammals, considered a superorder in some classifications, which consists of these orders:
*Primates
* Plesiadapiformes (extinct primate-like archontans)
* Scandentia (treeshrews)
* Dermoptera (colugos)
W ...
: bat
Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...
s, primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
s, colugos and treeshrew
The treeshrews (or tree shrews or banxrings) are small mammals native to the tropical forests of South and Southeast Asia. They make up the entire order Scandentia, which split into two families: the Tupaiidae (19 species, "ordinary" treeshrews) ...
s (now considered paraphyletic, with bats being closer to other groups)
******* Grandorder Ungulata
Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, ...
: ungulates
******** Order Tubulidentata '' incertae sedis'': aardvark
******** Mirorder Eparctocyona
Eparctocyona is a clade of placental mammals comprising the artiodactyls (even-toed ungulates), cetacea
Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aq ...
: † condylarths, whale
Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...
s and artiodactyls (even-toed ungulates)
******** Mirorder † Meridiungulata: South American ungulates
******** Mirorder Altungulata
Altungulata or Pantomesaxonia (''sensu'' and later authors) is an invalid clade (mirorder) of ungulate mammals comprising the perissodactyls, hyracoids, and tethytheres (sirenians, proboscideans, and related extinct taxa).
The name "Pantomesaxo ...
: perissodactyls (odd-toed ungulates), elephant
Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantidae an ...
s, manatees and hyrax
Hyraxes (), also called dassies, are small, thickset, herbivorous mammals in the order Hyracoidea. Hyraxes are well-furred, rotund animals with short tails. Typically, they measure between long and weigh between . They are superficially simil ...
es
Molecular classification of placentals
As of the early 21st century, molecular studies based on DNA analysis have suggested new relationships among mammal families. Most of these findings have been independently validated by retrotransposon
Retrotransposons (also called Class I transposable elements or transposons via RNA intermediates) are a type of genetic component that copy and paste themselves into different genomic locations (transposon) by converting RNA back into DNA through ...
presence/absence data. Classification systems based on molecular studies reveal three major groups or lineages of placental mammals— Afrotheria, Xenarthra and Boreoeutheria—which diverged in the Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
. The relationships between these three lineages is contentious, and all three possible hypotheses have been proposed with respect to which group is basal. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria
Epitherians comprise all the placental mammals except the Xenarthra. They are primarily characterized by having a stirrup-shaped stapes in the middle ear, which allows for passage of a blood vessel. This is in contrast to the column-shaped stapes ...
(basal Xenarthra) and Exafroplacentalia
Exafroplacentalia or Notolegia is a clade of placental mammals proposed in 2001 on the basis of molecular research.
Exafroplacentalia places Xenarthra as a sister group to the Boreoeutheria (comprising Laurasiatheria and Euarchontoglires), thus ...
(basal Afrotheria). Boreoeutheria in turn contains two major lineages— Euarchontoglires and Laurasiatheria.
Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on the type of DNA used (such as nuclear
Nuclear may refer to:
Physics
Relating to the nucleus of the atom:
* Nuclear engineering
*Nuclear physics
*Nuclear power
*Nuclear reactor
*Nuclear weapon
*Nuclear medicine
*Radiation therapy
*Nuclear warfare
Mathematics
*Nuclear space
*Nuclear ...
or mitochondrial
A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is use ...
) and varying interpretations of paleogeographic
Palaeogeography (or paleogeography) is the study of historical geography, generally physical landscapes. Palaeogeography can also include the study of human or cultural environments. When the focus is specifically on landforms, the term paleo ...
data.[
The cladogram above is based on Tarver ''et al.''. (2016)
]
Evolution
Origins
Synapsida, a clade that contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod (~323 million to ~300 million years ago), when they split from the reptile lineage. Crown group mammals evolved from earlier mammaliaforms
Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recen ...
during the Early Jurassic. The cladogram takes Mammalia to be the crown group.
Evolution from older amniotes
The first fully terrestrial vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
s were amniote
Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are disti ...
s. Like their amphibious early tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct theraps ...
predecessors, they had lungs and limbs. Amniotic eggs, however, have internal membranes that allow the developing embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while amphibian
Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
s generally need to lay their eggs in water.
The first amniotes apparently arose in the Pennsylvanian subperiod of the Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
. They descended from earlier reptiliomorph amphibious tetrapods, which lived on land that was already inhabited by insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
s and other invertebrates as well as fern
A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except t ...
s, moss
Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) '' sensu stricto''. Bryophyta (''sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and hor ...
es and other plants. Within a few million years, two important amniote lineages became distinct: the synapsids, which would later include the common ancestor of the mammals; and the sauropsid
Sauropsida ("lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia. Sauropsida is the sister taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early syna ...
s, which now include turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked tu ...
s, lizard
Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia alt ...
s, snake
Snakes are elongated, Limbless vertebrate, limbless, carnivore, carnivorous reptiles of the suborder Serpentes . Like all other Squamata, squamates, snakes are ectothermic, amniote vertebrates covered in overlapping Scale (zoology), scales. Ma ...
s, crocodilian
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...
s and dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s (including bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
s). Synapsids have a single hole (temporal fenestra
An infratemporal fenestra, also called the lateral temporal fenestra or simply temporal fenestra, is an opening in the skull behind the orbit in some animals. It is ventrally bordered by a zygomatic arch. An opening in front of the eye sockets, ...
) low on each side of the skull. Primitive synapsids included the largest and fiercest animals of the early Permian
The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...
such as Dimetrodon
''Dimetrodon'' ( or ,) meaning "two measures of teeth,” is an extinct genus of non-mammalian synapsid that lived during the Cisuralian (Early Permian), around 295–272 million years ago (Mya). It is a member of the family Sphenacodontid ...
. Nonmammalian synapsids were traditionally – and incorrectly – called "mammal-like reptiles" or pelycosaurs; we now know they were neither reptiles nor part of reptile lineage.[
]Therapsid
Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more ...
s, a group of synapsids, evolved in the Middle Permian
The Guadalupian is the second and middle series/epoch of the Permian. The Guadalupian was preceded by the Cisuralian and followed by the Lopingian. It is named after the Guadalupe Mountains of New Mexico and Texas, and dates between 272.95 ± 0. ...
, about 265 million years ago, and became the dominant land vertebrates. They differ from basal eupelycosaurs in several features of the skull and jaws, including: larger skulls and incisors which are equal in size in therapsids, but not for eupelycosaurs.[ The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very similar to their early synapsid ancestors and ending with ]probainognathia
Probainognathia is one of the two major subgroups of the clade Eucynodontia, the other being Cynognathia. The earliest forms were carnivorous and insectivorous, though some groups eventually also evolved herbivorous diets. The earliest and most ...
n cynodont
The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...
s, some of which could easily be mistaken for mammals. Those stages were characterized by:
* The gradual development of a bony secondary palate.
* Abrupt acquisition of endothermy among Mammaliamorpha
Mammaliamorpha is a clade of cynodonts. It contains the clades Tritylodontidae and Mammaliaformes, as well as a few genera that do not belong to either of these groups. The family Tritheledontidae has also been placed in Mammaliamorpha by some ph ...
, thus prior to the origin of mammals by 30-50 millions of years '.
* Progression towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was slow and erratic: for example, all herbivorous nonmammaliaform therapsids retained sprawling limbs (some late forms may have had semierect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semisprawling hindlimbs. In fact, modern monotremes still have semisprawling limbs.
* The dentary gradually became the main bone of the lower jaw which, by the Triassic, progressed towards the fully mammalian jaw (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of reptiles).
First mammals
The Permian–Triassic extinction event
The Permian–Triassic (P–T, P–Tr) extinction event, also known as the Latest Permian extinction event, the End-Permian Extinction and colloquially as the Great Dying, formed the boundary between the Permian and Triassic geologic periods, as ...
about 252 million years ago, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of carnivorous therapsids. In the early Triassic, most medium to large land carnivore niches were taken over by archosaur
Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avian d ...
s which, over an extended period (35 million years), came to include the crocodylomorphs
Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction.
During Mesozoic and early Cenozoic times, cr ...
, the pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...
s and the dinosaurs; however, large cynodonts like ''Trucidocynodon
''Trucidocynodon'' is an extinct genus of ecteniniid cynodonts from Upper Triassic of Brazil. It contains a single species, ''Trucidocynodon riograndensis''. Fossils of ''Trucidocynodon'' were discovered in Santa Maria Formation outcrops in Pal ...
'' and traversodontids
Traversodontidae is an extinct family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and eastern North America. ...
still occupied large sized carnivorous and herbivorous niches respectively. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well.
The first mammals (in Kemp's sense) appeared in the Late Triassic epoch (about 225 million years ago), 40 million years after the first therapsids. They expanded out of their nocturnal insectivore
A robber fly eating a hoverfly
An insectivore is a carnivorous animal or plant that eats insects. An alternative term is entomophage, which can also refer to the human practice of eating insects.
The first vertebrate insectivores wer ...
niche from the mid-Jurassic onwards; The Jurassic ''Castorocauda
''Castorocauda'' is an extinct, semi-aquatic, superficially otter-like genus of docodont mammaliaforms with one species, ''C. lutrasimilis''. It is part of the Yanliao Biota, found in the Daohugou Beds of Inner Mongolia, China dating to the M ...
'', for example, was a close relative of true mammals that had adaptations for swimming, digging and catching fish. Most, if not all, are thought to have remained nocturnal (the nocturnal bottleneck
The nocturnal bottleneck hypothesis is a hypothesis to explain several mammalian traits. In 1942, Gordon Lynn Walls described this concept which states that placental mammals were mainly or even exclusively nocturnal through most of their evolu ...
), accounting for much of the typical mammalian traits. The majority of the mammal species that existed in the Mesozoic Era
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising ...
were multituberculates, eutriconodonts and spalacotheriid
Spalacotheriidae is a family of extinct mammals belonging to the paraphyletic group ' Symmetrodonta'. They lasted from the Early Cretaceous to the Campanian in North America, Europe, Asia and North Africa.
Spalacotheriids are characterised by ha ...
s. The earliest known metatherian
Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as well ...
is ''Sinodelphys
''Sinodelphys'' is an extinct eutherian from the Early Cretaceous, estimated to be 125 million years old. It was discovered and described in 2003 in rocks of the Yixian Formation in Liaoning Province, China, by a team of scientists including Zhe ...
'', found in 125 million-year-old Early Cretaceous
The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pro ...
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
in China's northeastern Liaoning Province
Liaoning () is a coastal provinces of China, province in Northeast China that is the smallest, southernmost, and most populous province in the region. With its capital at Shenyang, it is located on the northern shore of the Yellow Sea, and i ...
. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.
The oldest known fossil among the Eutheria ("true beasts") is the small shrewlike ''Juramaia sinensis
''Juramaia'' is an extinct genus of very basal eutherian mammal known from the Late Jurassic ( Oxfordian stage) deposits of western Liaoning, China. It is a small shrew-like mammal with a body length of approximately 70–100 mm, making it ...
'', or "Jurassic mother from China", dated to 160 million years ago in the late Jurassic. A later eutherian relative, ''Eomaia
''Eomaia'' ("dawn mother") is a genus of extinct fossil mammals containing the single species ''Eomaia scansoria'', discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower ...
'', dated to 125 million years ago in the early Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage. In particular, the epipubic bones extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, other nontherian mammals and ''Ukhaatherium
''Ukhaatherium'' is a now extinct species of mammal that lived during the upper Cretaceous about 84 to 72 million years ago in today's East Asia. It is known above all from the fossil locality Ukhaa Tolgod, Mongolia. An adult ''Ukhaatherium'' h ...
'', an early Cretaceous animal in the eutherian order Asioryctitheria
Asioryctitheria ("Asian digging beasts") is an extinct order of early eutherians.
Skull structure
With the exception of '' Prokennalestes'', these advanced forms lacked a Meckelian groove. Furthermore, they were equipped with double-rooted can ...
. This also applies to the multituberculates. They are apparently an ancestral feature, which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles during locomotion, reducing the amount of space being presented, which placentals require to contain their fetus
A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal deve ...
during gestation periods. A narrow pelvic outlet indicates that the young were very small at birth and therefore pregnancy
Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins.
Pregnancy usually occurs by sexual intercourse, but ca ...
was short, as in modern marsupials. This suggests that the placenta was a later development.
One of the earliest known monotremes was '' Teinolophos'', which lived about 120 million years ago in Australia. Monotremes have some features which may be inherited from the original amniotes such as the same orifice to urinate, defecate and reproduce (cloaca
In animal anatomy, a cloaca ( ), plural cloacae ( or ), is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles and birds, a ...
)—as lizards and birds also do— and they lay eggs which are leathery and uncalcified.
Earliest appearances of features
''Hadrocodium
''Hadrocodium wui'' is an extinct mammaliaform that lived during the Sinemurian stage of the Early Jurassic approximately in the Lufeng Formation of the Lufeng Basin in what is now the Yunnan province in south-western China
(, paleocoordinates ...
'', whose fossils date from approximately 195 million years ago, in the early Jurassic
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
, provides the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones; there is no space in the jaw for the articular, a bone involved in the jaws of all early synapsids.
The earliest clear evidence of hair or fur is in fossils of ''Castorocauda
''Castorocauda'' is an extinct, semi-aquatic, superficially otter-like genus of docodont mammaliaforms with one species, ''C. lutrasimilis''. It is part of the Yanliao Biota, found in the Daohugou Beds of Inner Mongolia, China dating to the M ...
'' and ''Megaconus
''Megaconus'' is an extinct genus of allotherian mammal from the Middle Jurassic Tiaojishan Formation of Inner Mongolia, China. The type and only species, ''Megaconus mammaliaformis'' was first described in the journal ''Nature'' in 2013. ''Meg ...
'', from 164 million years ago in the mid-Jurassic. In the 1950s, it was suggested that the foramina (passages) in the maxilla
The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...
e and premaxilla
The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has b ...
e (bones in the front of the upper jaw) of cynodonts were channels which supplied blood vessels and nerves to vibrissae (whiskers
Vibrissae (; singular: vibrissa; ), more generally called Whiskers, are a type of stiff, functional hair used by mammals to touch, sense their environment. These hairs are finely specialised for this purpose, whereas other types of hair are c ...
) and so were evidence of hair or fur; it was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae, as the modern lizard ''Tupinambis
''Tupinambis'' is a lizard genus which belongs to the family Teiidae and contains eight described species. These large lizards are commonly referred to as tegus (''teiús'' in Portuguese). '' T. merianae'' (Argentine black and white tegu), '' T. ...
'' has foramina that are almost identical to those found in the nonmammalian cynodont '' Thrinaxodon''. Popular sources, nevertheless, continue to attribute whiskers to ''Thrinaxodon''. Studies on Permian coprolites suggest that non-mammalian synapsids
Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the Sauropsida, sauropsids, the group that inc ...
of the epoch already had fur, setting the evolution of hairs possibly as far back as dicynodont
Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typicall ...
s.
When endothermy first appeared in the evolution of mammals is uncertain, though it is generally agreed to have first evolved in non-mammalian therapsids.[ Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals,] but there is evidence that some of their ancestors, perhaps including ancestors of the therians, may have had body temperatures like those of modern therians. Likewise, some modern therians like afrotheres and xenarthrans have secondarily developed lower body temperatures.
The evolution of erect limbs in mammals is incomplete—living and fossil monotremes have sprawling limbs. The parasagittal (nonsprawling) limb posture appeared sometime in the late Jurassic or early Cretaceous; it is found in the eutherian ''Eomaia'' and the metatherian ''Sinodelphys'', both dated to 125 million years ago. Epipubic
Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials, monotremes and fossil mammals like multituberculates, and even basal eutherians (the ancestors of placental mammals, who lack them).
They first occ ...
bones, a feature that strongly influenced the reproduction of most mammal clades, are first found in Tritylodontidae
Tritylodontidae ("three-knob teeth", named after the shape of their cheek teeth) is an extinct family of small to medium-sized, highly specialized mammal-like cynodonts, bearing several mammalian traits like erect limbs, endothermy and detail ...
, suggesting that it is a synapomorphy between them and mammaliformes
Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent comm ...
. They are omnipresent in non-placental mammaliformes, though '' Megazostrodon'' and ''Erythrotherium
''Erythrotherium'' (meaning "red beast") is an extinct genus of basal mammaliaforms from the Late Triassic to Lower Jurassic. It is related to ''Morganucodon''. Only one species is recorded, ''Erythrotherium parringtoni'', from Red Beds, Stormber ...
'' appear to have lacked them.
It has been suggested that the original function of lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...
(milk
Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digestion, digest solid food. Immune factors and immune ...
production) was to keep eggs moist. Much of the argument is based on monotremes, the egg-laying mammals. In human females, mammary glands become fully developed during puberty, regardless of pregnancy.
Rise of the mammals
Therian mammals took over the medium- to large-sized ecological niches in the Cenozoic
The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...
, after the Cretaceous–Paleogene extinction event
The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...
approximately 66 million years ago emptied ecological space once filled by non-avian dinosaurs and other groups of reptiles, as well as various other mammal groups, and underwent an exponential increase in body size (megafauna
In terrestrial zoology, the megafauna (from Greek μέγας ''megas'' "large" and New Latin ''fauna'' "animal life") comprises the large or giant animals of an area, habitat, or geological period, extinct and/or extant. The most common threshold ...
). Then mammals diversified very quickly; both birds and mammals show an exponential rise in diversity. For example, the earliest known bat dates from about 50 million years ago, only 16 million years after the extinction of the non-avian dinosaurs.
Molecular phylogenetic studies initially suggested that most placental orders diverged about 100 to 85 million years ago and that modern families appeared in the period from the late Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
through the Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
. However, no placental fossils have been found from before the end of the Cretaceous. The earliest undisputed fossils of placentals comes from the early Paleocene
The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...
, after the extinction of the non-avian dinosaurs. In particular, scientists have identified an early Paleocene animal named ''Protungulatum donnae
''Protungulatum'' ('first ungulate') is a extinct genus of pan-euungulate mammals within extinct family Protungulatidae, and also one of the earliest known placental mammals in the fossil record, that lived in North America from the Late Creta ...
'' as one of the first placental mammals. however it has been reclassified as a non-placental eutherian. Recalibrations of genetic and morphological diversity rates have suggested a Late Cretaceous
The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...
origin for placentals, and a Paleocene origin for most modern clades.
The earliest known ancestor of primates is ''Archicebus achilles
''Archicebus'' is a genus of fossil primates that lived in the early Eocene forests (~55.8–54.8 million years ago) of what is now Jingzhou in the Hubei Province in central China, discovered in 2003. The only known species, ''A. achil ...
'' from around 55 million years ago. This tiny primate weighed 20–30 grams (0.7–1.1 ounce) and could fit within a human palm.
Anatomy
Distinguishing features
Living mammal species can be identified by the presence of sweat gland
Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin that produce sweat. Sweat glands are a type of exocrine gland, which are glands that produce and secrete substances onto an epithelial sur ...
s, including those that are specialized to produce milk to nourish their young. In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.
Many traits shared by all living mammals appeared among the earliest members of the group:
* Jaw joint – The dentary (the lower jaw bone, which carries the teeth) and the squamosal (a small cranial
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
bone) meet to form the joint. In most gnathostomes
Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...
, including early therapsids, the joint consists of the articular (a small bone at the back of the lower jaw) and quadrate
Quadrate may refer to:
* Quadrate bone
* Quadrate (heraldry)
* Quadrate lobe of liver
* Quadrate tubercle
The quadrate tubercle is a small tubercle found upon the upper part of the femur. It serves as a point of insertion of the quadratus femori ...
(a small bone at the back of the upper jaw).[
* Middle ear – In crown-group mammals, sound is carried from the ]eardrum
In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear
The outer ear, external ear, or auris externa is the extern ...
by a chain of three bones, the malleus, the incus
The ''incus'' (plural incudes) or anvil is a bone in the middle ear. The anvil-shaped small bone is one of three ossicles in the middle ear. The ''incus'' receives vibrations from the ''malleus'', to which it is connected laterally, and transmit ...
and the stapes. Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapsids.
* Tooth replacement – Teeth can be replaced once (diphyodonty A diphyodont is any animal with two ss of teeth, initially the ''deciduous'' set and consecutively the ''permanent'' set. Most mammals are diphyodonts—as to chew their food they need a strong, durable and complete set of teeth.
Diphyodonts contra ...
) or (as in toothed whales and murid rodents) not at all ( monophyodonty). Elephants, manatees, and kangaroos continually grow new teeth throughout their life (polyphyodont
A polyphyodont is any animal whose teeth are continually replaced. In contrast, diphyodonts are characterized by having only two successive sets of teeth.
Polyphyodonts include most toothed fishes, many reptiles such as crocodiles and geckos, and ...
y).
* Prismatic enamel – The enamel coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the dentin
Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by ena ...
to the tooth's surface.
* Occipital condyles – Two knobs at the base of the skull fit into the topmost neck vertebra
In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In ...
; most other tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct theraps ...
s, in contrast, have only one such knob.
For the most part, these characteristics were not present in the Triassic ancestors of the mammals. Nearly all mammaliaforms possess an epipubic bone, the exception being modern placentals.[
]
Sexual dimorphism
On average, male mammals are larger than females, with males being at least 10% larger than females in over 45% of investigated species. Most mammalian orders also exhibit male-biased sexual dimorphism
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
, although some orders do not show any bias or are significantly female-biased ( Lagomorpha). Sexual size dimorphism increases with body size across mammals (Rensch's rule
Rensch's rule is a biological rule on allometrics, concerning the relationship between the extent of sexual size dimorphism and which sex is larger. Across species within a lineage, size dimorphism increases with increasing body size when the male ...
), suggesting that there are parallel selection pressures on both male and female size. Male-biased dimorphism relates to sexual selection on males through male–male competition for females, as there is a positive correlation between the degree of sexual selection, as indicated by mating systems, and the degree of male-biased size dimorphism. The degree of sexual selection is also positively correlated with male and female size across mammals. Further, parallel selection pressure on female mass is identified in that age at weaning is significantly higher in more polygynous
Polygyny (; from Neoclassical Greek πολυγυνία (); ) is the most common and accepted form of polygamy around the world, entailing the marriage of a man with several women.
Incidence
Polygyny is more widespread in Africa than in any ...
species, even when correcting for body mass. Also, the reproductive rate is lower for larger females, indicating that fecundity selection selects for smaller females in mammals. Although these patterns hold across mammals as a whole, there is considerable variation across orders.
Biological systems
The majority of mammals have seven cervical vertebrae
In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In ...
(bones in the neck). The exceptions are the manatee and the two-toed sloth
''Choloepus'' is a genus of xenarthran mammals of Central and South America within the monotypic family Choloepodidae, consisting of two-toed sloths, sometimes also called two-fingered sloths. The two species of ''Choloepus'' (which means "lame ...
, which have six, and the three-toed sloth which has nine. All mammalian brains possess a neocortex
The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, sp ...
, a brain region unique to mammals. Placental brains have a corpus callosum, unlike monotremes and marsupials.
The lung
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...
s of mammals are spongy and honeycombed. Breathing is mainly achieved with the diaphragm
Diaphragm may refer to:
Anatomy
* Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen
* Pelvic diaphragm or pelvic floor, a pelvic structure
* Urogenital diaphragm or triangular ligament, a pelvic structure
Other
* Diap ...
, which divides the thorax from the abdominal cavity, forming a dome convex to the thorax. Contraction of the diaphragm flattens the dome, increasing the volume of the lung cavity. Air enters through the oral and nasal cavities, and travels through the larynx, trachea and bronchi, and expands the alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* ...
. Relaxing the diaphragm has the opposite effect, decreasing the volume of the lung cavity, causing air to be pushed out of the lungs. During exercise, the abdominal wall contracts
A contract is a legally enforceable agreement between two or more parties that creates, defines, and governs mutual rights and obligations between them. A contract typically involves the transfer of goods, services, money, or a promise to tran ...
, increasing pressure on the diaphragm, which forces air out quicker and more forcefully. The rib cage is able to expand and contract the chest cavity through the action of other respiratory muscles. Consequently, air is sucked into or expelled out of the lungs, always moving down its pressure gradient.[ This type of lung is known as a bellows lung due to its resemblance to blacksmith bellows.]
The mammalian heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...
has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. The heart has four valves, which separate its chambers and ensures blood flows in the correct direction through the heart (preventing backflow). After gas exchange
Gas exchange is the physical process by which gases move passively by Diffusion#Diffusion vs. bulk flow, diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liqui ...
in the pulmonary capillaries (blood vessels in the lungs), oxygen-rich blood returns to the left atrium via one of the four pulmonary veins. Blood flows nearly continuously back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period, the left atrium will contract, pumping blood into the ventricle. The heart also requires nutrients and oxygen found in blood like other muscles, and is supplied via coronary arteries
The coronary arteries are the arterial blood vessels of coronary circulation, which transport oxygenated blood to the heart muscle. The heart requires a continuous supply of oxygen to function and survive, much like any other tissue or organ of ...
.
The integumentary system
The integumentary system is the set of organs forming the outermost layer of an animal's body. It comprises the skin and its appendages, which act as a physical barrier between the external environment and the internal environment that it serves ...
(skin) is made up of three layers: the outermost epidermis
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...
, the dermis
The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...
and the hypodermis. The epidermis is typically 10 to 30 cells thick; its main function is to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is 15 to 40 times thicker than the epidermis. The dermis is made up of many components, such as bony structures and blood vessels. The hypodermis is made up of adipose tissue, which stores lipids and provides cushioning and insulation. The thickness of this layer varies widely from species to species;[ ]marine mammal
Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their reli ...
s require a thick hypodermis (blubber
Blubber is a thick layer of vascularized adipose tissue under the skin of all cetaceans, pinnipeds, penguins, and sirenians.
Description
Lipid-rich, collagen fiber-laced blubber comprises the hypodermis and covers the whole body, except for pa ...
) for insulation, and right whale
Right whales are three species of large baleen whales of the genus ''Eubalaena'': the North Atlantic right whale (''E. glacialis''), the North Pacific right whale (''E. japonica'') and the Southern right whale (''E. australis''). They are clas ...
s have the thickest blubber at . Although other animals have features such as whiskers, feathers, setae
In biology, setae (singular seta ; from the Latin word for "bristle") are any of a number of different bristle- or hair-like structures on living organisms.
Animal setae
Protostomes
Annelid setae are stiff bristles present on the body. Th ...
, or cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
that superficially resemble it, no animals other than mammals have hair
Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals.
The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and f ...
. It is a definitive characteristic of the class, though some mammals have very little.
Herbivores have developed a diverse range of physical structures to facilitate the consumption of plant material. To break up intact plant tissues, mammals have developed teeth
A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...
structures that reflect their feeding preferences. For instance, frugivore
A frugivore is an animal that thrives mostly on raw fruits or succulent fruit-like produce of plants such as roots, shoots, nuts and seeds. Approximately 20% of mammalian herbivores eat fruit. Frugivores are highly dependent on the abundance an ...
s (animals that feed primarily on fruit) and herbivores that feed on soft foliage have low-crowned teeth specialized for grinding foliage and seed
A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...
s. Grazing
In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to roam around and consume wild vegetations in order to convert the otherwise indigestible (by human gut) cellulose within grass and other ...
animals that tend to eat hard, silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
-rich grasses, have high-crowned teeth, which are capable of grinding tough plant tissues and do not wear down as quickly as low-crowned teeth. Most carnivorous mammals have carnassialiforme teeth (of varying length depending on diet), long canines and similar tooth replacement patterns.
The stomach of even-toed ungulates (Artiodactyla) is divided into four sections: the rumen
The rumen, also known as a paunch, is the largest stomach compartment in ruminants and the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals. The rumen's microbial favoring environment allow ...
, the reticulum, the omasum and the abomasum (only ruminants have a rumen). After the plant material is consumed, it is mixed with saliva in the rumen and reticulum and separates into solid and liquid material. The solids lump together to form a bolus
Bolus may refer to:
Geography
* Bolus, Iran, a village in Ardabil Province, Iran
* Bolus, or Baulus, an Anatolian village on the site of ancient Berissa
Medicine
* Bolus (digestion), a ball-shaped mass moving through the digestive tract
* Bolus ...
(or cud
Cud is a portion of food that returns from a ruminant's stomach to the mouth to be chewed for the second time. More precisely, it is a bolus of semi-degraded food regurgitated from the reticulorumen of a ruminant. Cud is produced during the phy ...
), and is regurgitated. When the bolus enters the mouth, the fluid is squeezed out with the tongue and swallowed again. Ingested food passes to the rumen and reticulum where cellulolytic microbe
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s (bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
, protozoa
Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
and fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
) produce cellulase
Cellulase (EC 3.2.1.4; systematic name 4-β-D-glucan 4-glucanohydrolase) is any of several enzymes produced chiefly by fungi, bacteria, and protozoans that catalyze cellulolysis, the decomposition of cellulose and of some related polysaccharide ...
, which is needed to break down the cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
in plants. Perissodactyls, in contrast to the ruminants, store digested food that has left the stomach in an enlarged cecum, where it is fermented by bacteria. Carnivora have a simple stomach adapted to digest primarily meat, as compared to the elaborate digestive systems of herbivorous animals, which are necessary to break down tough, complex plant fibers. The caecum is either absent or short and simple, and the large intestine is not sacculated or much wider than the small intestine.
The mammalian excretory system involves many components. Like most other land animals, mammals are ureotelic, and convert ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
into urea
Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid.
Urea serves an important r ...
, which is done by the liver
The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
as part of the urea cycle
The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions that produces urea (NH2)2CO from ammonia (NH3). Animals that use this cycle, mainly amphibians and mammals, are called ureotelic.
The urea cycle converts highl ...
. Bilirubin
Bilirubin (BR) (Latin for "red bile") is a red-orange compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates. This catabolism is a necessary process in the body's clearance of waste products that arise from the ...
, a waste product derived from blood cell
A blood cell, also called a hematopoietic cell, hemocyte, or hematocyte, is a cell produced through hematopoiesis and found mainly in the blood. Major types of blood cells include red blood cells (erythrocytes), white blood cells (leukocytes), ...
s, is passed through bile
Bile (from Latin ''bilis''), or gall, is a dark-green-to-yellowish-brown fluid produced by the liver of most vertebrates that aids the digestion of lipids in the small intestine. In humans, bile is produced continuously by the liver (liver bile ...
and urine
Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excretion, excreted from the body through the urethra.
Cel ...
with the help of enzymes excreted by the liver. The passing of bilirubin via bile through the intestinal tract
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...
gives mammalian feces
Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...
a distinctive brown coloration. Distinctive features of the mammalian kidney include the presence of the renal pelvis
The renal pelvis or pelvis of the kidney is the funnel-like dilated part of the ureter in the kidney. It is formed by the covnvergence of the major calyces, acting as a funnel for urine flowing from the major calyces to the ureter. It has a mucous ...
and renal pyramid
The renal medulla is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which then splits up to form the segmental arteries whi ...
s, and of a clearly distinguishable cortex and medulla, which is due to the presence of elongated loops of Henle
In the kidney, the loop of Henle () (or Henle's loop, Henle loop, nephron loop or its Latin counterpart ''ansa nephroni'') is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its ...
. Only the mammalian kidney has a bean shape, although there are some exceptions, such as the multilobed reniculate kidney The reniculate kidney is a multilobed kidney found in marine and aquatic mammals such as pinnipeds (seals, sea lions and walruses) and cetaceans (dolphins and whales) but absent in terrestrial mammals except bears. Kidneys of this morphology
Morph ...
s of pinnipeds, cetacea
Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel them ...
ns and bears.[ Most adult placental mammals have no remaining trace of the ]cloaca
In animal anatomy, a cloaca ( ), plural cloacae ( or ), is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles and birds, a ...
. In the embryo, the embryonic cloaca
The cloaca is a structure in the development of the urinary and reproductive organs.
The hind-gut is at first prolonged backward into the body-stalk as the tube of the allantois; but, with the growth and flexure of the tail-end of the embryo, ...
divides into a posterior region that becomes part of the anus, and an anterior region that has different fates depending on the sex of the individual: in females, it develops into the vestibule that receives the urethra
The urethra (from Greek οὐρήθρα – ''ourḗthrā'') is a tube that connects the urinary bladder to the urinary meatus for the removal of urine from the body of both females and males. In human females and other primates, the urethra con ...
and vagina
In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen ...
, while in males it forms the entirety of the penile urethra
The spongy urethra (cavernous portion of urethra, penile urethra) is the longest part of the male urethra, and is contained in the corpus spongiosum of the penis.
It is about 15 cm long, and extends from the termination of the membranous po ...
.[ However, the ]tenrec
A tenrec is any species of mammal within the afrotherian family Tenrecidae endemic to Madagascar. Tenrecs are wildly diverse; as a result of convergent evolution some resemble hedgehogs, shrews, opossums, rats, and mice. They occupy aquatic, a ...
s, golden moles, and some shrew
Shrews (family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to different fa ...
s retain a cloaca as adults. In marsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally. Monotremes, which translates from Greek into "single hole", have a true cloaca.
Sound production
As in all other tetrapods, mammals have a larynx
The larynx (), commonly called the voice box, is an organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal inlet is about ...
that can quickly open and close to produce sounds, and a supralaryngeal vocal tract
The vocal tract is the cavity in human bodies and in animals where the sound produced at the sound source (larynx in mammals; syrinx (biology), syrinx in birds) is filtered.
In birds it consists of the Vertebrate trachea, trachea, the Syrinx (bio ...
which filters this sound. The lungs and surrounding musculature provide the air stream and pressure required to phonate
The term phonation has slightly different meanings depending on the subfield of phonetics. Among some phoneticians, ''phonation'' is the process by which the vocal folds produce certain sounds through quasi-periodic vibration. This is the defini ...
. The larynx controls the pitch and volume
Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). The de ...
of sound, but the strength the lungs exert to exhale
Exhalation (or expiration) is the flow of the breath out of an organism. In animals, it is the movement of air from the lungs out of the airways, to the external environment during breathing.
This happens due to elastic properties of the lungs, ...
also contributes to volume. More primitive mammals, such as the echidna, can only hiss, as sound is achieved solely through exhaling through a partially closed larynx. Other mammals phonate using vocal fold
The human voice consists of sound made by a human being
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedality, bipedalism and exceptional cognitive skills due to a large and compl ...
s. The movement or tenseness of the vocal folds can result in many sounds such as purr
A purr is a tonal fluttering sound made by some species of felids and two species of genets. It varies in loudness and tone among species and in the same animal. Felids are a family of mammals that belong to the order Carnivora and are infor ...
ing and screaming
A scream is a loud speech production, vocalization in which air is passed through the vocal cords with greater force than is used in regular or close-distance vocalisation. This can be performed by any creature possessing lungs, including human ...
. Mammals can change the position of the larynx, allowing them to breathe through the nose while swallowing through the mouth, and to form both oral and nasal
Nasal is an adjective referring to the nose, part of human or animal anatomy. It may also be shorthand for the following uses in combination:
* With reference to the human nose:
** Nasal administration, a method of pharmaceutical drug delivery
** ...
sounds; nasal sounds, such as a dog whine, are generally soft sounds, and oral sounds, such as a dog bark, are generally loud.[
Some mammals have a large larynx and thus a low-pitched voice, namely the ]hammer-headed bat
The hammer-headed bat ('), also known as hammer-headed fruit bat and big-lipped bat, is a megabat widely distributed in West and Central Africa. It is the only member of the genus ''Hypsignathus'', which is part of the tribe Epomophorini along w ...
(''Hypsignathus monstrosus'') where the larynx can take up the entirety of the thoracic cavity while pushing the lungs, heart, and trachea into the abdomen
The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso. ...
. Large vocal pads can also lower the pitch, as in the low-pitched roars of big cats. The production of infrasound is possible in some mammals such as the African elephant (''Loxodonta'' spp.) and baleen whale
Baleen whales (systematic name Mysticeti), also known as whalebone whales, are a parvorder of carnivorous marine mammals of the infraorder Cetacea (whales, dolphins and porpoises) which use keratinaceous baleen plates (or "whalebone") in their ...
s. Small mammals with small larynxes have the ability to produce ultrasound
Ultrasound is sound waves with frequency, frequencies higher than the upper audible limit of human hearing range, hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hea ...
, which can be detected by modifications to the middle ear and cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
. Ultrasound is inaudible to birds and reptiles, which might have been important during the Mesozoic, when birds and reptiles were the dominant predators. This private channel is used by some rodents in, for example, mother-to-pup communication, and by bats when echolocating. Toothed whales also use echolocation, but, as opposed to the vocal membrane that extends upward from the vocal folds, they have a melon
A melon is any of various plants of the family Cucurbitaceae with sweet, edible, and fleshy fruit. The word "melon" can refer to either the plant or specifically to the fruit. Botanically, a melon is a kind of berry, specifically a "pepo". The ...
to manipulate sounds. Some mammals, namely the primates, have air sacs attached to the larynx, which may function to lower the resonances or increase the volume of sound.
The vocal production system is controlled by the cranial nerve nuclei in the brain, and supplied by the recurrent laryngeal nerve
The recurrent laryngeal nerve (RLN) is a branch of the vagus nerve (cranial nerve X) that supplies all the intrinsic muscles of the larynx, with the exception of the cricothyroid muscles. There are two recurrent laryngeal nerves, right and le ...
and the superior laryngeal nerve, branches of the vagus nerve
The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and right ...
. The vocal tract is supplied by the hypoglossal nerve
The hypoglossal nerve, also known as the twelfth cranial nerve, cranial nerve XII, or simply CN XII, is a cranial nerve that innervates all the extrinsic and intrinsic muscles of the tongue except for the palatoglossus, which is innervated by ...
and facial nerves. Electrical stimulation of the periaqueductal gray (PEG) region of the mammalian midbrain
The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation. The name comes from the Greek ''mesos'', " ...
elicit vocalizations. The ability to learn new vocalizations is only exemplified in humans, seals, cetaceans, elephants and possibly bats; in humans, this is the result of a direct connection between the motor cortex, which controls movement, and the motor neuron
A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectl ...
s in the spinal cord.[
]
Fur
The primary function of the fur of mammals is thermoregulation. Others include protection, sensory purposes, waterproofing, and camouflage. Different types of fur serve different purposes:[
* Definitive – which may be ]shed
A shed is typically a simple, single-story roofed structure that is used for hobbies, or as a workshop in a back garden or on an allotment. Sheds vary considerably in their size and complexity of construction, from simple open-sided ones de ...
after reaching a certain length
* Vibrissae – sensory hairs, most commonly whisker
Vibrissae (; singular: vibrissa; ), more generally called Whiskers, are a type of stiff, functional hair used by mammals to sense their environment. These hairs are finely specialised for this purpose, whereas other types of hair are coarser ...
s
* Pelage – guard hairs, under-fur, and awn hair
Awn hairs are the intermediate hairs in a mammal's coat. They are shorter than the guard hairs and longer than the down hairs. They help with insulation and protect the down hairs underneath. Most of the visible coat is made of this kind of hair ...
* Spines – stiff guard hair used for defense (such as in porcupine
Porcupines are large rodents with coats of sharp spines, or quills, that protect them against predation. The term covers two families of animals: the Old World porcupines of family Hystricidae, and the New World porcupines of family, Erethizont ...
s)
* Bristle
A bristle is a stiff hair or feather (natural or artificial), either on an animal, such as a pig, a plant, or on a tool such as a brush or broom.
Synthetic types
Synthetic materials such as nylon are also used to make bristles in items such as br ...
s – long hairs usually used in visual signals. (such as a lion's mane)
* Velli – often called "down fur" which insulates newborn mammals
* Wool
Wool is the textile fibre obtained from sheep and other mammals, especially goats, rabbits, and camelids. The term may also refer to inorganic materials, such as mineral wool and glass wool, that have properties similar to animal wool.
As ...
– long, soft and often curly
Thermoregulation
Hair length is not a factor in thermoregulation: for example, some tropical mammals such as sloths have the same length of fur length as some arctic mammals but with less insulation; and, conversely, other tropical mammals with short hair have the same insulating value as arctic mammals. The denseness of fur can increase an animal's insulation value, and arctic mammals especially have dense fur; for example, the musk ox has guard hairs measuring as well as a dense underfur, which forms an airtight coat, allowing them to survive in temperatures of .[ Some desert mammals, such as camels, use dense fur to prevent solar heat from reaching their skin, allowing the animal to stay cool; a camel's fur may reach in the summer, but the skin stays at .][ ]Aquatic mammal
Aquatic and semiaquatic mammals are a diverse group of mammals that dwell partly or entirely in bodies of water. They include the various marine mammals who dwell in oceans, as well as various freshwater species, such as the European otter. Th ...
s, conversely, trap air in their fur to conserve heat by keeping the skin dry.[
]
Coloration
Mammalian coats are colored for a variety of reasons, the major selective pressures including camouflage
Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the ...
, sexual selection
Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
, communication, and thermoregulation. Coloration in both the hair and skin of mammals is mainly determined by the type and amount of melanin
Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
; eumelanin
Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amin ...
s for brown and black colors and pheomelanin for a range of yellowish to reddish colors, giving mammals an earth tone. Some mammals have more vibrant colors; certain monkeys such mandrills and vervet monkeys, and opossums such as the Mexican mouse opossums and Derby's woolly opossums, have blue skin due to structural coloration, light diffraction in collagen fibers. Many sloths appear green because their fur hosts green algae; this may be a symbiosis, symbiotic relation that affords camouflage
Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the ...
to the sloths.
Camouflage is a powerful influence in a large number of mammals, as it helps to conceal individuals from predators or prey. In arctic and subarctic mammals such as the arctic fox (''Alopex lagopus''), collared lemming (''Dicrostonyx groenlandicus''), stoat (''Mustela erminea''), and snowshoe hare (''Lepus americanus''), seasonal polyphenism, seasonal color change between brown in summer and white in winter is driven largely by camouflage. Some arboreal mammals, notably primates and marsupials, have shades of violet, green, or blue skin on parts of their bodies, indicating some distinct advantage in their largely arboreal habitat due to convergent evolution.
Aposematism, warning off possible predators, is the most likely explanation of the black-and-white pelage of many mammals which are able to defend themselves, such as in the foul-smelling skunk and the powerful and aggressive honey badger. Coat color is sometimes sexual dimorphism, sexually dimorphic, as in Sexual dimorphism in non-human primates#Pelage color and markings, many primate species. Differences in female and male coat color may indicate nutrition and hormone levels, important in mate selection. Coat color may influence the ability to retain heat, depending on how much light is reflected. Mammals with a darker colored coat can absorb more heat from solar radiation, and stay warmer, and some smaller mammals, such as voles, have darker fur in the winter. The white, pigmentless fur of arctic mammals, such as the polar bear, may reflect more solar radiation directly onto the skin.[ The dazzling black-and-white striping of zebras appear to provide some protection from biting flies.]
Reproductive system
Mammals are solely Gonochorism, gonochoric (an animal is born with either male or female genitalia, as opposed to hermaphrodites where there is no such schism). In male placentals, the penis is used both for urination and copulation. Depending on the species, an erection may be fueled by blood flow into vascular, spongy tissue or by muscular action. A penis may be contained in a Penile sheath, prepuce when not erect, and some placentals also have a penis bone (baculum). Marsupials typically have forked penises, while the echidna penis generally has four heads with only two functioning. The testes of most mammals descend into the scrotum which is typically posterior to the penis but is often anterior in marsupials. Female mammals generally have a clitoris, labia majora and labia minora on the outside, while the internal system contains paired oviducts, 1–2 uteri, 1–2 Cervix, cervices and a vagina
In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen ...
. Marsupials have two lateral vaginas and a medial vagina. The "vagina" of monotremes is better understood as a "urogenital sinus". The uterine systems of placental mammals can vary between a duplex, where there are two uteri and cervices which open into the vagina, a bipartite, where two uterine horns have a single cervix that connects to the vagina, a bicornuate, which consists where two uterine horns that are connected distally but separate medially creating a Y-shape, and a simplex, which has a single uterus.
The ancestral condition for mammal reproduction is the birthing of relatively undeveloped, either through direct vivipary or a short period as soft-shelled eggs. This is likely due to the fact that the torso could not expand due to the presence of epipubic bones. The oldest demonstration of this reproductive style is with ''Kayentatherium'', which produced undeveloped perinates, but at much higher litter sizes than any modern mammal, 38 specimens. Most modern mammals are viviparity, viviparous, giving birth to live young. However, the five species of monotreme, the platypus and the four species of echidna, lay eggs. The monotremes have a sex determination system different from that of most other mammals. In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.
Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. Marsupials have a short gestation
Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregna ...
period, typically shorter than its estrous cycle and generally giving birth to a number of undeveloped newborns that then undergo further development; in many species, this takes place within a pouch-like sac, the Pouch (marsupial), marsupium, located in the front of the mother's abdomen
The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso. ...
. This is the Symplesiomorphy, plesiomorphic condition among viviparous mammals; the presence of epipubic bones in all non-placental mammals prevents the expansion of the torso needed for full pregnancy. Even non-placental eutherians probably reproduced this way. The placentals give birth to relatively complete and developed young, usually after long gestation periods. They get their name from the placenta
The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mater ...
, which connects the developing fetus to the uterine wall to allow nutrient uptake. In placental mammals, the epipubic is either completely lost or converted into the baculum; allowing the torso to be able to expand and thus birth developed offspring.
The mammary gland
A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in primat ...
s of mammals are specialized to produce milk, the primary source of nutrition for newborns. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly. Compared to placental mammals, the milk of marsupials changes greatly in both production rate and in nutrient composition, due to the underdeveloped young. In addition, the mammary glands have more autonomy allowing them to supply separate milks to young at different development stages. Lactose is the main sugar in placental mammal milk while monotreme and marsupial milk is dominated by oligosaccharides. Weaning is the process in which a mammal becomes less dependent on their mother's milk and more on solid food.
Endothermy
Nearly all mammals are endothermy, endothermic ("warm-blooded"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in weather and climates too cold for ectothermic ("cold-blooded") reptiles and insects. Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles. Small insectivorous mammals eat prodigious amounts for their size. A rare exception, the naked mole-rat produces little metabolic heat, so it is considered an operational poikilotherm. Birds are also endothermic, so endothermy is not unique to mammals.
Species lifespan
Among mammals, species maximum lifespan varies significantly (for example the shrew
Shrews (family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to different fa ...
has a lifespan of two years, whereas the oldest bowhead whale
The bowhead whale (''Balaena mysticetus'') is a species of baleen whale belonging to the family Balaenidae and the only living representative of the genus ''Balaena''. They are the only baleen whale endemic to the Arctic and subarctic waters, ...
is recorded to be 211 years). Although the underlying basis for these lifespan differences is still uncertain, numerous studies indicate that the ability to DNA repair, repair DNA damage is an important determinant of mammalian lifespan. In a 1974 study by Hart and Setlow, it was found that DNA excision repair capability increased systematically with species lifespan among seven mammalian species. Species lifespan was observed to be robustly correlated with the capacity to recognize DNA double-strand breaks as well as the level of the DNA repair protein Ku80. In a study of the cells from sixteen mammalian species, genes employed in DNA repair were found to be Downregulation and upregulation, up-regulated in the longer-lived species. The cellular level of the DNA repair enzyme poly ADP ribose polymerase was found to correlate with species lifespan in a study of 13 mammalian species. Three additional studies of a variety of mammalian species also reported a correlation between species lifespan and DNA repair capability.
Locomotion
Terrestrial
Most vertebrates—the amphibians, the reptiles and some mammals such as humans and bears—are plantigrade, walking on the whole of the underside of the foot. Many mammals, such as cats and dogs, are digitigrade, walking on their toes, the greater stride length allowing more speed. Digitigrade mammals are also often adept at quiet movement. Some animals such as horses are unguligrade, walking on the tips of their toes. This even further increases their stride length and thus their speed. A few mammals, namely the great apes, are also known to Knuckle-walking, walk on their knuckles, at least for their front legs. Giant anteaters and platypuses are also knuckle-walkers. Some mammals are bipedalism, bipeds, using only two limbs for locomotion, which can be seen in, for example, humans and the great apes. Bipedal species have a larger field of vision than quadrupeds, conserve more energy and have the ability to manipulate objects with their hands, which aids in foraging. Instead of walking, some bipeds hop, such as kangaroos and kangaroo rats.
Animals will use different gaits for different speeds, terrain and situations. For example, horses show four natural gaits, the slowest horse gait is the Horse gait#Walk, walk, then there are three faster gaits which, from slowest to fastest, are the Trot (horse gait), trot, the canter and the Horse gait#Gallop, gallop. Animals may also have unusual gaits that are used occasionally, such as for moving sideways or backwards. For example, the main gait (human), human gaits are bipedal walking and running, but they employ many other gaits occasionally, including a four-legged crawling (human), crawl in tight spaces. Mammals show a vast range of gaits, the order that they place and lift their appendages in locomotion. Gaits can be grouped into categories according to their patterns of support sequence. For quadrupeds, there are three main categories: walking gaits, running gaits and leaping gaits. Walking is the most common gait, where some feet are on the ground at any given time, and found in almost all legged animals. Running is considered to occur when at some points in the stride all feet are off the ground in a moment of suspension.[
]
Arboreal
Arboreal animals frequently have elongated limbs that help them cross gaps, reach fruit or other resources, test the firmness of support ahead and, in some cases, to brachiation, brachiate (swing between trees). Many arboreal species, such as tree porcupines, silky anteaters, spider monkeys, and Phalangeriformes, possums, use prehensile tails to grasp branches. In the spider monkey, the tip of the tail has either a bare patch or adhesive pad, which provides increased friction. Claws can be used to interact with rough substrates and reorient the direction of forces the animal applies. This is what allows squirrels to climb tree trunks that are so large to be essentially flat from the perspective of such a small animal. However, claws can interfere with an animal's ability to grasp very small branches, as they may wrap too far around and prick the animal's own paw. Frictional gripping is used by primates, relying upon hairless fingertips. Squeezing the branch between the fingertips generates frictional force that holds the animal's hand to the branch. However, this type of grip depends upon the angle of the frictional force, thus upon the diameter of the branch, with larger branches resulting in reduced gripping ability. To control descent, especially down large diameter branches, some arboreal animals such as squirrels have evolved highly mobile ankle joints that permit rotating the foot into a 'reversed' posture. This allows the claws to hook into the rough surface of the bark, opposing the force of gravity. Small size provides many advantages to arboreal species: such as increasing the relative size of branches to the animal, lower center of mass, increased stability, lower mass (allowing movement on smaller branches) and the ability to move through more cluttered habitat.[ Size relating to weight affects gliding animals such as the sugar glider. Some species of primate, bat and all species of sloth achieve passive stability by hanging beneath the branch. Both pitching and tipping become irrelevant, as the only method of failure would be losing their grip.][
]
Aerial
Bats are the only mammals that can truly fly. They fly through the air at a constant speed by moving their wings up and down (usually with some fore-aft movement as well). Because the animal is in motion, there is some airflow relative to its body which, combined with the velocity of the wings, generates a faster airflow moving over the wing. This generates a lift force vector pointing forwards and upwards, and a drag force vector pointing rearwards and upwards. The upwards components of these counteract gravity, keeping the body in the air, while the forward component provides thrust to counteract both the drag from the wing and from the body as a whole.
The wings of bats are much thinner and consist of more bones than those of birds, allowing bats to maneuver more accurately and fly with more lift and less drag. By folding the wings inwards towards their body on the upstroke, they use 35% less energy during flight than birds. The membranes are delicate, ripping easily; however, the tissue of the bat's membrane is able to regrow, such that small tears can heal quickly. The surface of their wings is equipped with touch-sensitive receptors on small bumps called Merkel cells, also found on human fingertips. These sensitive areas are different in bats, as each bump has a tiny hair in the center, making it even more sensitive and allowing the bat to detect and collect information about the air flowing over its wings, and to fly more efficiently by changing the shape of its wings in response.
Fossorial and subterranean
A fossorial (from Latin ''fossor'', meaning "digger") is an animal adapted to digging which lives primarily, but not solely, underground. Some examples are badgers, and naked mole-rats. Many rodent
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are na ...
species are also considered fossorial because they live in burrows for most but not all of the day. Species that live exclusively underground are subterranean, and those with limited adaptations to a fossorial lifestyle sub-fossorial. Some organisms are fossorial to aid in temperature regulation while others use the underground habitat for protection from predators or for food storage.[Damiani, R, 2003, Earliest evidence of cynodont burrowing, The Royal Society Publishing, Volume 270, Issue 1525]
Fossorial mammals have a fusiform body, thickest at the shoulders and tapering off at the tail and nose. Unable to see in the dark burrows, most have degenerated eyes, but degeneration varies between species; pocket gophers, for example, are only semi-fossorial and have very small yet functional eyes, in the fully fossorial marsupial mole the eyes are degenerated and useless, Talpa (genus), talpa moles have vestigial eyes and the cape golden mole has a layer of skin covering the eyes. External ears flaps are also very small or absent. Truly fossorial mammals have short, stout legs as strength is more important than speed to a burrowing mammal, but semi-fossorial mammals have cursorial legs. The front paws are broad and have strong claws to help in loosening dirt while excavating burrows, and the back paws have webbing, as well as claws, which aids in throwing loosened dirt backwards. Most have large incisors to prevent dirt from flying into their mouth.
Many fossorial mammals such as shrews, hedgehogs, and moles were classified under the now obsolete order Insectivora.
Aquatic
Fully aquatic mammals, the cetaceans and sirenians, have lost their legs and have a tail fin to propel themselves through the water. Flipper movement is continuous. Whales swim by moving their tail fin and lower body up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering. Their skeletal anatomy allows them to be fast swimmers. Most species have a dorsal fin to prevent themselves from turning upside-down in the water. The flukes of sirenians are raised up and down in long strokes to move the animal forward, and can be twisted to turn. The forelimbs are paddle-like flippers which aid in turning and slowing.
Semi-aquatic mammals, like pinnipeds, have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. The elbows and ankles are enclosed within the body. Pinnipeds have several adaptions for reducing Drag (physics), drag. In addition to their streamlined bodies, they have smooth networks of Muscle fascicle, muscle bundles in their skin that may increase laminar flow and make it easier for them to slip through water. They also lack Arrector pili muscle, arrector pili, so their fur can be streamlined as they swim.[ They rely on their fore-flippers for locomotion in a wing-like manner similar to penguins and sea turtles.] Fore-flipper movement is not continuous, and the animal glides between each stroke. Compared to terrestrial carnivorans, the fore-limbs are reduced in length, which gives the locomotor muscles at the shoulder and elbow joints greater mechanical advantage;[ the hind-flippers serve as stabilizers.] Other semi-aquatic mammals include beavers, hippopotamuses, otters and platypuses. Hippos are very large semi-aquatic mammals, and their barrel-shaped bodies have wikt:graviportal, graviportal skeletal structures, adapted to carrying their enormous weight, and their specific gravity allows them to sink and move along the bottom of a river.
Behavior
Communication and vocalization
Many mammals communicate by vocalizing. Vocal communication serves many purposes, including in mating rituals, as alarm signal, warning calls, to indicate food sources, and for social purposes. Males often call during mating rituals to ward off other males and to attract females, as in the roar (vocalization), roaring of lions and red deer. The whale song, songs of the humpback whale may be signals to females; they have different dialects in different regions of the ocean. Social vocalizations include the territory (animal), territorial calls of gibbons, and the use of frequency in greater spear-nosed bats to distinguish between groups. The vervet monkey gives a distinct alarm call for each of at least four different predators, and the reactions of other monkeys vary according to the call. For example, if an alarm call signals a python, the monkeys climb into the trees, whereas the eagle alarm causes monkeys to seek a hiding place on the ground. Prairie dogs similarly have complex calls that signal the type, size, and speed of an approaching predator. Elephants communicate socially with a variety of sounds including snorting, screaming, trumpeting, roaring and rumbling. Some of the rumbling calls are infrasound, infrasonic, below the hearing range of humans, and can be heard by other elephants up to away at still times near sunrise and sunset.
file:Killer whale.ogg, left, Orca calling including occasional echolocation clicks
Mammals signal by a variety of means. Many give visual Anti-predator adaptation, anti-predator signals, as when deer and gazelle stotting, stot, honest signal, honestly indicating their fit condition and their ability to escape, or when white-tailed deer and other prey mammals flag with conspicuous tail markings when alarmed, informing the predator that it has been detected. Many mammals make use of scent-marking, sometimes possibly to help defend territory, but probably with a range of functions both within and between species. Microbats and toothed whales including oceanic dolphins vocalize both socially and in echolocation.
Feeding
To maintain a high constant body temperature is energy expensive—mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different species have since adapted to meet their dietary requirements in a variety of ways. Some eat other animals—this is a carnivore, carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants, which contain complex carbohydrates such as cellulose. An herbivorous diet includes subtypes such as granivory (seed eating), folivory (leaf eating), frugivory (fruit eating), nectarivory (nectar eating), gummivory (gum eating) and mycophagy (fungus eating). The digestive tract of an herbivore is host to bacteria that ferment these complex substances, and make them available for digestion, which are either housed in the multichambered stomach or in a large cecum. Some mammals are coprophagous, consuming feces
Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...
to absorb the nutrients not digested when the food was first ingested.[ An omnivore eats both prey and plants. Carnivorous mammals have a simple digestive system, digestive tract because the proteins, lipids and minerals found in meat require little in the way of specialized digestion. Exceptions to this include ]baleen whale
Baleen whales (systematic name Mysticeti), also known as whalebone whales, are a parvorder of carnivorous marine mammals of the infraorder Cetacea (whales, dolphins and porpoises) which use keratinaceous baleen plates (or "whalebone") in their ...
s who also house gut flora in a multi-chambered stomach, like terrestrial herbivores.
The size of an animal is also a factor in determining diet type (Allen's rule). Since small mammals have a high ratio of heat-losing surface area to heat-generating volume, they tend to have high energy requirements and a high metabolism, metabolic rate. Mammals that weigh less than about are mostly insectivorous because they cannot tolerate the slow, complex digestive process of an herbivore. Larger animals, on the other hand, generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (carnivores that feed on larger vertebrates) or a slower digestive process (herbivores). Furthermore, mammals that weigh more than usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).
Some mammals are omnivores and display varying degrees of carnivory and herbivory, generally leaning in favor of one more than the other. Since plants and meat are digested differently, there is a preference for one over the other, as in bears where some species may be mostly carnivorous and others mostly herbivorous. They are grouped into three categories: mesocarnivore, mesocarnivory (50–70% meat), hypercarnivore, hypercarnivory (70% and greater of meat), and hypocarnivore, hypocarnivory (50% or less of meat). The dentition of hypocarnivores consists of dull, triangular carnassial teeth meant for grinding food. Hypercarnivores, however, have conical teeth and sharp carnassials meant for slashing, and in some cases strong jaws for bone-crushing, as in the case of hyenas, allowing them to consume bones; some extinct groups, notably the Machairodontinae, had saber-shaped maxillary canine, canines.
Some physiological carnivores consume plant matter and some physiological herbivores consume meat. From a behavioral aspect, this would make them omnivores, but from the physiological standpoint, this may be due to zoopharmacognosy. Physiologically, animals must be able to obtain both energy and nutrients from plant and animal materials to be considered omnivorous. Thus, such animals are still able to be classified as carnivores and herbivores when they are just obtaining nutrients from materials originating from sources that do not seemingly complement their classification. For example, it is well documented that some ungulates such as giraffes, camels, and cattle, will gnaw on bones to consume particular minerals and nutrients. Also, cats, which are generally regarded as obligate carnivores, occasionally eat grass to regurgitate indigestible material (such as hairballs), aid with hemoglobin production, and as a laxative.
Many mammals, in the absence of sufficient food requirements in an environment, suppress their metabolism and conserve energy in a process known as hibernation. In the period preceding hibernation, larger mammals, such as bears, become polyphagic to increase fat stores, whereas smaller mammals prefer to collect and stash food. The slowing of the metabolism is accompanied by a decreased heart and respiratory rate, as well as a drop in internal temperatures, which can be around ambient temperature in some cases. For example, the internal temperatures of hibernating arctic ground squirrels can drop to , however the head and neck always stay above . A few mammals in hot environments aestivate in times of drought or extreme heat, for example the fat-tailed dwarf lemur (''Cheirogaleus medius'').
Intelligence
In intelligent mammals, such as primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...
s, the cerebrum is larger relative to the rest of the brain. Intelligence itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rat IQ, Rats, for example, are considered to be highly intelligent, as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh biome, habitat. In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain smaller than a cat, which must think to outwit its prey.
Tool use by animals may indicate different levels of learning and Animal cognition, cognition. The Tool use by sea otters, sea otter uses rocks as essential and regular parts of its foraging behaviour (smashing abalone from rocks or breaking open shells), with some populations spending 21% of their time making tools. Other tool use, such as chimpanzees using twigs to "fish" for termites, may be developed by Observational learning, watching others use tools and may even be a true example of animal teaching. Tools may even be used in solving puzzles in which the animal appears to experience a Eureka effect, "Eureka moment". Other mammals that do not use tools, such as dogs, can also experience a Eureka moment.
Brain size was previously considered a major indicator of the intelligence of an animal. Since most of the brain is used for maintaining bodily functions, greater ratios of Brain-to-body mass ratio, brain to body mass may increase the amount of brain mass available for more complex cognitive tasks. Allometric analysis indicates that mammalian brain size scales at approximately the or exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalization quotient, encephalisation quotient that can be used as another indication of animal intelligence. Sperm whales have the largest brain mass of any animal on earth, averaging and in mature males.
Self-awareness appears to be a sign of abstract thinking. Self-awareness, although not well-defined, is believed to be a precursor to more advanced processes such as metacognition, metacognitive reasoning. The traditional method for measuring this is the mirror test, which determines if an animal possesses the ability of self-recognition. Mammals that have passed the mirror test include Asian elephants (some pass, some do not); chimpanzees; bonobos; orangutans; human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s, from 18 months (mirror stage); bottlenose dolphins killer whales; and false killer whales.[
]
Social structure
Eusociality is the highest level of social organization. These societies have an overlap of adult generations, the division of reproductive labor and cooperative caring of young. Usually insects, such as bees, ants and termites, have eusocial behavior, but it is demonstrated in two rodent species: the naked mole-rat and the Damaraland mole-rat.
Presociality is when animals exhibit more than just sexual interactions with members of the same species, but fall short of qualifying as eusocial. That is, presocial animals can display communal living, cooperative care of young, or primitive division of reproductive labor, but they do not display all of the three essential traits of eusocial animals. Humans and some species of Callitrichidae (marmosets and tamarins) are unique among primates in their degree of cooperative care of young. Harry Harlow set up an experiment with rhesus monkeys, presocial primates, in 1958; the results from this study showed that social encounters are necessary in order for the young monkeys to develop both mentally and sexually.
A fission-fusion society is a society that changes frequently in its size and composition, making up a permanent social group called the "parent group". Permanent social networks consist of all individual members of a community and often varies to track changes in their environment. In a fission–fusion society, the main parent group can fracture (fission) into smaller stable subgroups or individuals to adapt to Social environment, environmental or social circumstances. For example, a number of males may break off from the main group in order to hunt or forage for food during the day, but at night they may return to join (fusion) the primary group to share food and partake in other activities. Many mammals exhibit this, such as primates (for example orangutans and spider monkeys), elephants, spotted hyenas, lions, and dolphins.
Solitary animals defend a territory and avoid social interactions with the members of its species, except during breeding season. This is to avoid resource competition, as two individuals of the same species would occupy the same niche, and to prevent depletion of food. A solitary animal, while foraging, can also be less conspicuous to predators or prey.
In a dominance hierarchy, hierarchy, individuals are either dominant or submissive. A despotic hierarchy is where one individual is dominant while the others are submissive, as in wolves and lemurs, and a pecking order is a linear ranking of individuals where there is a top individual and a bottom individual. Pecking orders may also be ranked by sex, where the lowest individual of a sex has a higher ranking than the top individual of the other sex, as in hyenas. Dominant individuals, or alphas, have a high chance of reproductive success, especially in harems
Harem (Persian: حرمسرا ''haramsarā'', ar, حَرِيمٌ ''ḥarīm'', "a sacred inviolable place; harem; female members of the family") refers to domestic spaces that are reserved for the women of the house in a Muslim family. A hare ...
where one or a few males (resident males) have exclusive breeding rights to females in a group. Non-resident males can also be accepted in harems, but some species, such as the common vampire bat (''Desmodus rotundus''), may be more strict.
Some mammals are perfectly Monogamy in animals, monogamous, meaning that they pair bond, mate for life and take no other partners (even after the original mate's death), as with wolves, Eurasian beavers, and otters. There are three types of polygamy: either one or multiple dominant males have breeding rights (polygyny in animals, polygyny), multiple males that females mate with (polyandry), or multiple males have exclusive relations with multiple females (polygynandry). It is much more common for polygynous mating to happen, which, excluding lek mating, leks, are estimated to occur in up to 90% of mammals. Lek mating occurs when males congregate around females and try to attract them with various courtship displays and vocalizations, as in harbor seals.
All higher mammals (excluding monotremes) share two major adaptations for care of the young: live birth and lactation. These imply a group-wide choice of a degree of parental care. They may build nests and dig burrows to raise their young in, or feed and guard them often for a prolonged period of time. Many mammals are K-selected, and invest more time and energy into their young than do r-selected animals. When two animals mate, they both share an interest in the success of the offspring, though often to different extremes. Mammalian females exhibit some degree of maternal aggression, another example of parental care, which may be targeted against other females of the species or the young of other females; however, some mammals may "aunt" the infants of other females, and care for them. Mammalian males may play a role in child rearing, as with tenrecs, however this varies species to species, even within the same genus. For example, the males of the southern pig-tailed macaque (''Macaca nemestrina'') do not participate in child care, whereas the males of the Japanese macaque (''M. fuscata'') do.
Humans and other mammals
In human culture
Non-human mammals play a wide variety of roles in human culture. They are the most popular of pets, with tens of millions of dogs, cats and other animals including rabbits and mice kept by families around the world. Mammals such as mammoths, horses and deer are among the earliest subjects of art, being found in Upper Paleolithic cave paintings such as at Lascaux. Major artists such as Albrecht Dürer, George Stubbs and Edwin Landseer are known for their portraits of mammals. Many species of mammals have been hunted for sport and for food; deer and wild boar are especially popular as game (hunting), game animals. Mammals such as horse racing, horses and greyhound racing, dogs are widely raced for sport, often combined with gambling, betting on the outcome. There is a tension between the role of animals as companions to humans, and their existence as individuals with animal rights, rights of their own. Mammals further play a wide variety of roles in literature, film, mythology, and religion.
Uses and importance
The domestication of mammals was instrumental in the Neolithic Revolution, Neolithic development of agriculture and of civilization
A civilization (or civilisation) is any complex society characterized by the development of a state, social stratification, urbanization, and symbolic systems of communication beyond natural spoken language (namely, a writing system).
Ci ...
, causing farmers to replace hunter-gatherers around the world. This transition from hunting and gathering to pastoralism, herding flocks and agriculture, growing crops was a major step in human history. The new agricultural economies, based on domesticated mammals, caused "radical restructuring of human societies, worldwide alterations in biodiversity, and significant changes in the Earth's landforms and its atmosphere... momentous outcomes".
Domestication, Domestic mammals form a large part of the livestock raised for meat
Meat is animal flesh that is eaten as food. Humans have hunted, farmed, and scavenged animals for meat since prehistoric times. The establishment of settlements in the Neolithic Revolution allowed the domestication of animals such as chic ...
across the world. They include (2009) around 1.4 billion cattle, 1 billion sheep, 1 billion domestic pigs, and (1985) over 700 million rabbits. Working animal, Working domestic animals including cattle and horses have been used for work and transport from the origins of agriculture, their numbers declining with the arrival of mechanised transport and agricultural machinery. In 2004 they still provided some 80% of the power for the mainly small farms in the third world, and some 20% of the world's transport, again mainly in rural areas. In mountainous regions unsuitable for wheeled vehicles, pack animals continue to transport goods. Mammal skins provide leather
Leather is a strong, flexible and durable material obtained from the tanning, or chemical treatment, of animal skins and hides to prevent decay. The most common leathers come from cattle, sheep, goats, equine animals, buffalo, pigs and hogs, ...
for shoes, clothing and upholstery. Wool
Wool is the textile fibre obtained from sheep and other mammals, especially goats, rabbits, and camelids. The term may also refer to inorganic materials, such as mineral wool and glass wool, that have properties similar to animal wool.
As ...
from mammals including sheep, goats and alpacas has been used for centuries for clothing.
Mammals serve a major role in science as animal model, experimental animals, both in fundamental biological research, such as in genetics, and in the development of new medicines, which must be tested exhaustively to demonstrate their Pharmacovigilance, safety. Millions of mammals, especially mice and rats, are used in animal testing, experiments each year. A knockout mouse is a genetically modified mouse with an inactivated gene, replaced or disrupted with an artificial piece of DNA. They enable the study of sequencing, sequenced genes whose functions are unknown. A small percentage of the mammals are non-human primates, used in research for their similarity to humans.
Despite the benefits domesticated mammals had for human development, humans have an increasingly detrimental effect on wild mammals across the world. It has been estimated that the mass of all ''wild'' mammals has declined to only 4% of all mammals, with 96% of mammals being humans and their livestock now (see figure). In fact, terrestrial wild mammals make up only 2% of all mammals.
Hybrids
Hybrids are offspring resulting from the breeding of two genetically distinct individuals, which usually will result in a high degree of heterozygosity, though hybrid and heterozygous are not synonymous. The deliberate or accidental hybridizing of two or more species of closely related animals through captive breeding is a human activity which has been in existence for millennia and has grown for economic purposes. Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids. Natural hybrids will occur in hybrid zones, where two populations of species within the same genera or species living in the same or adjacent areas will interbreed with each other. Some hybrids have been recognized as species, such as the red wolf (though this is controversial).
Artificial selection, the deliberate selective breeding of domestic animals, is being used to breeding back, breed back Holocene extinction, recently extinct animals in an attempt to achieve an animal breed with a phenotype that resembles that extinct wildtype ancestor. A breeding-back (intraspecific) hybrid may be very similar to the extinct wildtype in appearance, ecological niche and to some extent genetics, but the initial gene pool of that wild type is lost forever with its extinction
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
. As a result, bred-back breeds are at best vague look-alikes of extinct wildtypes, as Heck cattle are of the aurochs.
Purebred wild species evolved to a specific ecology can be threatened with extinction through the process of genetic pollution, the uncontrolled hybridization, introgression genetic swamping which leads to homogenization or Fitness (biology), out-competition from the heterosis, heterosic hybrid species. When new populations are imported or selectively bred by people, or when habitat modification brings previously isolated species into contact, extinction in some species, especially rare varieties, is possible. Interbreeding can swamp the rarer gene pool and create hybrids, depleting the purebred gene pool. For example, the endangered wild water buffalo is most threatened with extinction by genetic pollution from the Water buffalo, domestic water buffalo. Such extinctions are not always apparent from a morphology (biology), morphological standpoint. Some degree of gene flow is a normal evolutionary process, nevertheless, hybridization threatens the existence of rare species.
Threats
The loss of species from ecological communities, defaunation, is primarily driven by human activity.[ This has resulted in empty forests, ecological communities depleted of large vertebrates.] In the Quaternary extinction event, the mass die-off of megafaunal variety coincided with the appearance of humans, suggesting a human influence. One hypothesis is that humans hunted large mammals, such as the woolly mammoth, into extinction. The 2019 ''Global Assessment Report on Biodiversity and Ecosystem Services'' by IPBES states that the total Biomass (ecology), biomass of wild mammals has declined by 82 percent since the beginning of human civilization. Wild animals make up just 4% of mammalian Biomass (ecology), biomass on earth, while humans and their domesticated animals make up 96%.
Various species are predicted to List of critically endangered species, become extinct in the near future, among them the rhinoceros, giraffes, and species of primates and pangolins. According to the WWF's 2020 ''Living Planet Report'', vertebrate wildlife populations have declined by 68% since 1970 as a result of human activities, particularly overconsumption, population growth and intensive farming, which is evidence that humans have triggered a sixth mass extinction event. Hunting alone threatens hundreds of mammalian species around the world. Scientists claim that the growing demand for meat
Meat is animal flesh that is eaten as food. Humans have hunted, farmed, and scavenged animals for meat since prehistoric times. The establishment of settlements in the Neolithic Revolution allowed the domestication of animals such as chic ...
is contributing to biodiversity loss as this is a significant driver of deforestation
Deforestation or forest clearance is the removal of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. The most concentrated d ...
and habitat destruction
Habitat destruction (also termed habitat loss and habitat reduction) is the process by which a natural habitat becomes incapable of supporting its native species. The organisms that previously inhabited the site are displaced or dead, thereby ...
; species-rich habitats, such as significant portions of the Amazon rainforest, are being converted to agricultural land for meat production. Another influence is over-hunting and Species affected by poaching, poaching, which can reduce the overall population of game animals, especially those located near villages, as in the case of peccary, peccaries. The effects of poaching can especially be seen in the ivory trade with African elephants. Marine mammals are at risk from entanglement from fishing gear, notably Cetacean bycatch, cetaceans, with discard mortalities ranging from 65,000 to 86,000 individuals annually.
Attention is being given to endangered species globally, notably through the Convention on Biological Diversity, otherwise known as the Rio Accord, which includes 189 signatory countries that are focused on identifying endangered species and habitats. Another notable conservation organization is the IUCN, which has a membership of over 1,200 governmental and Non-governmental organization, non-governmental organizations.
List of recently extinct mammals, Recent extinctions can be directly attributed to human influences. The IUCN characterizes 'recent' extinction as those that have occurred past the cut-off point of 1500, and around 80 mammal species have gone extinct since that time and 2015. Some species, such as the Père David's deer are extinct in the wild, and survive solely in captive populations. Other species, such as the Florida panther, are Ecological extinction, ecologically extinct, surviving in such low numbers that they essentially have no impact on the ecosystem. Other populations are only Local extinction, locally extinct (extirpated), still existing elsewhere, but reduced in distribution,[ as with the extinction of gray whales in the Atlantic Ocean, Atlantic.]
Notes
See also
* List of mammal genera – living mammals
* List of mammalogists
* List of monotremes and marsupials
* List of placental mammals
* List of prehistoric mammals
* List of threatened mammals of the United States
* Lists of mammals by population, Lists of mammals by population size
* Lists of mammals by region
* Mammals described in the 2000s
* Mammals in culture
* Small mammal
References
Further reading
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External links
Biodiversitymapping.org – All mammal orders in the world with distribution maps
Paleocene Mammals
a site covering the rise of the mammals, paleocene-mammals.de
Evolution of Mammals
a brief introduction to early mammals, enchantedlearning.com
European Mammal Atlas EMMA
from Societas Europaea Mammalogica, European-mammals.org
Marine Mammals of the World
– An overview of all marine mammals, including descriptions, both fully aquatic and semi-aquatic, noaa.gov
Mammalogy.org
The American Society of Mammalogists was established in 1919 for the purpose of promoting the study of mammals, and this website includes a mammal image library
{{Authority control
Mammals,
Bathonian first appearances
Extant Middle Jurassic first appearances
Taxa named by Carl Linnaeus