Most

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 ...

s are

viviparous Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ...

, giving birth to live young. However, the five species of monotreme, 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 ...

es and the echidnas, lay eggs. The monotremes have a

sex determination system A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes. In some species there are hermaphr ...

different from that of most other mammals. In particular, the

sex chromosome A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical ...

s of a platypus are more like those of a chicken than those of a therian mammal. 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, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the teats seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly. Viviparous mammals are in the subclass

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 ...

; those living today are in the Marsupialia and

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 ...

infraclasses. A marsupial has 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 The estrous cycle (, originally ) is the set of recurring physiological changes that are induced by reproductive hormones in most mammalian therian females. Estrous cycles start after sexual maturity in females and are interrupted by anestrous p ...

, and gives birth to an underdeveloped ( altricial) newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the 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. ...

. Some placentals, e.g. guinea pig, give birth to fully developed ( precocial) young, usually after long gestation periods, while some others, e.g. mouse, give birth to underdeveloped young.

Maturity and reproductive age

Sexual maturity and thus the earliest age at which mammals can reproduce varies dramatically across species. Members of the rodent family Cricetidae can reach sexual maturity in 1–2 months, e.g. the Norway lemming ('' Lemmus lemmus'') in 39 days. Many

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 ( family Canidae) and bovids ( Bovidae) take about a year to reach maturity while

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) and dolphins ( Delphinidae) require more than 10 years. Some

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 take even longer, with the longest duration being recorded 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, ...

(''Balaena mysticetus''), which reaches maturity at an age of only about 23 years.

Reproductive system

Placental mammals

Male placental mammals

The mammalian male reproductive system contains two main divisions, the

penis A penis (plural ''penises'' or ''penes'' () is the primary sexual organ that male animals use to inseminate females (or hermaphrodites) during copulation. Such organs occur in many animals, both vertebrate and invertebrate, but males do n ...

and the

testicle A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...

s, the latter of which is where sperm are produced. In humans, both of these organs are outside the

abdominal cavity The abdominal cavity is a large body cavity in humans and many other animals that contains many organs. It is a part of the abdominopelvic cavity. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the th ...

, but they can be primarily housed within the abdomen in other animals. For instance, a

dog's penis Canine reproduction is the process of sexual reproduction in domestic dogs, wolves, coyotes and other canine species. Canine sexual anatomy and development Male reproductive system Erectile tissue As with all mammals, a dog's penis is made up of ...

is covered by a penile sheath except when mating. Having the testicles outside the abdomen best facilitates temperature regulation of the sperm, which require specific temperatures to survive. The external location may also cause a reduction in the heat-induced contribution to the spontaneous mutation rate in male germinal tissue. Sperm are the smaller of the two gametes and are generally very short-lived, requiring males to produce them continuously from the time of sexual maturity until death. The produced sperm are stored in the

epididymis The epididymis (; plural: epididymides or ) is a tube that connects a testicle to a vas deferens in the male reproductive system. It is a single, narrow, tightly-coiled tube in adult humans, in length. It serves as an interconnection between the ...

until

ejaculation Ejaculation is the discharge of semen (the ''ejaculate''; normally containing sperm) from the male reproductory tract as a result of an orgasm. It is the final stage and natural objective of male sexual stimulation, and an essential componen ...

. The sperm cells are motile and they swim using tail-like

flagella A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...

to propel themselves towards the

ovum The egg cell, or ovum (plural ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is ...

. The sperm follows temperature gradients ( thermotaxis) and chemical gradients (

chemotaxis Chemotaxis (from '' chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemica ...

) to locate the ovum.

The mammalian female reproductive system likewise contains two main divisions: the vagina and uterus, which act as the receptacle for the sperm, and the ovaries, which produce the female's ova. All of these parts are always internal. The vagina is attached to the uterus through the

cervix The cervix or cervix uteri (Latin, 'neck of the uterus') is the lower part of the uterus (womb) in the human female reproductive system. The cervix is usually 2 to 3 cm long (~1 inch) and roughly cylindrical in shape, which changes during ...

, while the uterus is attached to the ovaries via the

Fallopian tube The fallopian tubes, also known as uterine tubes, oviducts or salpinges (singular salpinx), are paired tubes in the human female that stretch from the uterus to the ovaries. The fallopian tubes are part of the female reproductive system. In ot ...

s. At certain intervals, the ovaries release an ovum, which passes through the fallopian tube into the uterus. If, in this transit, it meets with

sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...

, the egg selects sperm with which to merge; this is termed fertilization. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then

implants Implant can refer to: Medicine *Implant (medicine), or specifically: **Brain implant **Breast implant **Buttock implant **Cochlear implant **Contraceptive implant **Dental implant **Fetal tissue implant **Implantable cardioverter-defibrillator ** ...

itself in the wall of the uterus, where it begins the processes of

embryogenesis 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 sperm ...

and morphogenesis. When developed enough to survive outside the womb, the cervix dilates and contractions of the uterus propel the fetus through the birth canal, which is the vagina. The ova, which are the female sex cells, are much larger than the sperm and are normally formed within the ovaries of the fetus before its birth. They are mostly fixed in location within the ovary until their transit to the uterus, and contain nutrients for the later zygote and

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 ...

. Over a regular interval, in response to hormonal signals, a process of

oogenesis Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage. O ...

matures one ovum which is released and sent down the Fallopian tube. If not fertilized, this egg is released through

menstruation Menstruation (also known as a period, among other colloquial terms) is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. The menstrual cycle is characterized by the rise and fall of hor ...

in humans and other great apes, and reabsorbed in other mammals in the estrus cycle.

=Gestation

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 ...

, called ''pregnancy'' in humans, is the period of time during which the fetus develops, dividing via

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...

inside the female. During this time, the fetus receives all of its nutrition and oxygenated blood from the female, filtered through the placenta, which is attached to the fetus'

via an umbilical cord. This drain of

nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

s can be quite taxing on the female, who is required to ingest slightly higher levels of

calorie The calorie is a unit of energy. For historical reasons, two main definitions of "calorie" are in wide use. The large calorie, food calorie, or kilogram calorie was originally defined as the amount of heat needed to raise the temperature of on ...

s. In addition, certain vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. The length of gestation, called the

gestation period In mammals, pregnancy is the period of reproduction during which a female carries one or more live offspring from implantation in the uterus through gestation. It begins when a fertilized zygote implants in the female's uterus, and ends once it ...

, varies greatly from species to species; it is 40 weeks in humans, 56–60 in

giraffe The giraffe is a large African hoofed mammal belonging to the genus ''Giraffa''. It is the tallest living terrestrial animal and the largest ruminant on Earth. Traditionally, giraffes were thought to be one species, ''Giraffa camelopardalis ...

s and 16 days in hamsters.

=Birth

= Once the fetus is sufficiently developing, chemical signals start the process of birth, which begins with contractions of the uterus and the dilation of the cervix. The fetus then descends to the cervix, where it is pushed out into the vagina, and eventually out of the female. The newborn, which is called an

infant An infant or baby is the very young offspring of human beings. ''Infant'' (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is a formal or specialised synonym for the common term ''baby''. The terms may also be used to ...

in humans, should typically begin respiration on its own shortly after birth. Not long after, the placenta is passed as well.

Monotremes

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, only five species of which exist, all from

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

and New Guinea, are mammals that lay

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...

s. They have one opening for excretion and reproduction called 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 ...

. They hold the eggs internally for several weeks, providing nutrients, and then lay them and cover them like birds. Like marsupial " joeys", monotreme " puggles" are larval and fetus-like, as like them they cannot expand their torso due to the presence of epipubic bones, forcing them to produce undeveloped young.

Marsupials

Marsupials' reproductive systems differ markedly from those of placental mammals,Iowa State University Biology Dept. Discoveries about Marsupial Reproduction Anna King 2001. webpage
(note shows code, html extension omitted) though it is probably the

plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, and ...

condition found in viviparous mammals, including non-placental eutherians.Giallombardo, Andres, 2009 New Cretaceous mammals from Mongolia and the early diversification of Eutheria Ph.D. dissertation, Columbia University, 2009402 pages; AAT 3373736 (abstract) The origin of Placental Mammals, Cimolestidae, Zalambdalestidae During embryonic development, a

choriovitelline placenta A choriovitelline placenta is a placenta formed by the yolk sac and chorion. In a choriovitelline placenta, the yolk sac fuses with the chorion and, subsequently, wrinkles develop that hold the embryo to the uterine wall, thus forming the choriovite ...

forms in all marsupials. In

bandicoots Bandicoots are a group of more than 20 species of small to medium-sized, terrestrial, largely nocturnal marsupial omnivores in the order Peramelemorphia. They are endemic to the Australia–New Guinea region, including the Bismarck Archipelago t ...

, an additional

chorioallantoic 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 matern ...

forms, although it lacks the

chorionic villi Chorionic villi are villi that sprout from the chorion to provide maximal contact area with maternal blood. They are an essential element in pregnancy from a histomorphologic perspective, and are, by definition, a product of conception. Branche ...

found in eutherian placentas.

Gametogenesis

Animals, including mammals, produce gametes (sperm and egg) through meiosis in gonads (testicles in males and ovaries in females). Sperm are produced by the process of

spermatogenesis Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the mitotic division of the stem cells located close to the basement membrane of the tubule ...

and eggs are produced by

. These processes are outlined in the article gametogenesis. During gametogenesis in mammals many genes encoding proteins that take part in DNA repair mechanisms show enhanced or specialized expression These mechanisms include meiotic homologous recombinational repair and mismatch repair.

Outline of reproduction by species

* Carnivora#Reproductive system ** Fossa (animal)#Breeding **

Reproductive behavior of canids Canidae (; from Latin, ''canis'', "dog") is a family (biology), biological family of dog-like carnivorans, colloquially referred to as dogs, and constitutes a clade. A member of this family is also called a canid (). There are three subfamily, ...

Rut (mammalian reproduction) The rut (from the Latin ''rugire'', meaning "to roar") is the mating season of certain mammals, which includes ruminants such as deer, sheep, camels, goats, pronghorns, bison, giraffes and antelopes, and extends to others such as skunks and elepha ...

* Marsupial#Reproduction **

Reproductive behavior of kangaroos Kangaroos are four marsupials from the family Macropodidae (macropods, meaning "large foot"). In common use the term is used to describe the largest species from this family, the red kangaroo, as well as the antilopine kangaroo, eastern ...

*** Red kangaroo#Reproduction *** Eastern grey kangaroo#Reproduction *** Desert rat-kangaroo#Reproduction *** Tree-kangaroo#Reproduction ** Opossum#Reproduction and life cycle ** Koala#Reproduction and development ** Tammar wallaby#Breeding and development ** Southwestern pygmy possum#Reproduction * Marsh rice rat#Reproduction and life cycle

References

{{Animal sexual behavior Reproduction in mammals Fertility