The three-toed or three-fingered sloths are
arboreal
Arboreal locomotion is the Animal locomotion, locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. Th ...
neotropical
The Neotropical realm is one of the eight biogeographic realms constituting Earth's land surface. Physically, it includes the tropical terrestrial ecoregions of the Americas and the entire South American temperate zone.
Definition
In bioge ...
mammals .
They are the only members of the
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
''Bradypus'' and the
family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
Bradypodidae. The four living species of three-toed sloths are the
brown-throated sloth
The brown-throated sloth (''Bradypus variegatus'') is a species of three-toed sloth found in the Neotropical realm of Central and South America.
It is the most common of the four species of three-toed sloth, and is found in the forests of South ...
, the
maned sloth
The maned sloth (''Bradypus torquatus'') is a three-toed sloth that is native to South America. It is one of four species of three-toed sloths belonging to the suborder Xenarthra and are placental mammals. They are endemic to the Atlantic coast ...
, the
pale-throated sloth
The pale-throated sloth (''Bradypus tridactylus''), occasionally known as the ai (), is a species of three-toed sloth that inhabits tropical rainforests in northern South America.
It is similar in appearance to, and often confused with, the bro ...
, and the
pygmy three-toed sloth
The pygmy three-toed sloth (''Bradypus pygmaeus''), also known as the monk sloth or dwarf sloth, is a sloth endemic to Isla Escudo de Veraguas, a small island off the Caribbean coast of Panama. The species was first described by Robert P. Ander ...
. In complete contrast to past morphological studies, which tended to place ''Bradypus'' as the
sister group to all other
folivora
Sloths are a group of Neotropical xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their l ...
ns, molecular studies place them nested within the sloth superfamily
Megatherioidea
Sloths are a group of Neotropical xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their li ...
, making them the only surviving members of that radiation.
Extant species
Evolution
A study of mitochondrial
cytochrome b
Cytochrome b within both molecular and cell biology, is a protein found in the mitochondria of eukaryotic cells. It functions as part of the electron transport chain and is the main subunit of transmembrane cytochrome bc1 and b6f complexes.
F ...
and
16S rRNA sequences suggests that ''
B. torquatus'' diverged from ''
B. variegatus'' and ''
B. tridactylus'' about 12 million years ago, while the latter two split 5 to 6 million years ago. The diversification of ''B. variegatus'' lineages was estimated to have started 4 to 5 million years ago.
Relation to the two-toed sloth
Both types of
sloth
Sloths are a group of Neotropical xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their li ...
tend to occupy the same forests; in most areas, a particular species of three-toed sloth and a single species of the somewhat larger and generally faster-moving
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 "la ...
will jointly predominate. Although similar in overall appearance, the two
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 ...
are placed in different families. Recent
phylogenetic
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 ...
analyses support the morphological data from the 1970s and 1980s that the two genera are not closely related and that each adopted their
arboreal
Arboreal locomotion is the Animal locomotion, locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. Th ...
lifestyles independently.
From morphological studies it was unclear from which
ground-dwelling sloth
Ground sloths are a diverse group of extinct sloths in the mammalian superorder Xenarthra. The term is used to refer to all extinct sloths because of the large size of the earliest forms discovered, compared to existing tree sloths. The Caribbea ...
taxa the three-toed sloths evolved or whether they retained their arboreality from the last common ancestor of extant sloths. The two-toed sloths were thought on the basis of morphology to nest phylogenetically within one of the divisions of ground-dwelling
Caribbean sloth
The mammalian order Pilosa, which includes the sloths and anteaters, includes various species from the Caribbean region. Many species of sloths are known from the Greater Antilles, all of which became extinct over the last millennia, but some slot ...
s.
[
]
Understanding of sloth phylogeny has recently been greatly revised by molecular studies, based on
collagen and
mitochondrial DNA
Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial D ...
sequences. These investigations consistently place three-toed sloths within Megatherioidea, close to ''
Megalonyx
''Megalonyx'' (Greek, "large claw") is an extinct genus of ground sloths of the family Megalonychidae, native to North America during the Pliocene and Pleistocene epochs. It became extinct during the Quaternary extinction event at the end of th ...
'',
megatheriid
Megatheriidae is a family of extinct ground sloths that lived from approximately 23 mya—11,000 years ago.
Megatheriids appeared during the Late Oligocene (Deseadan in the SALMA classification), some 29 million years ago, in South Amer ...
s and
nothrotheriids, and two-toed sloths close to
mylodontid
Mylodontidae is a family of extinct South American and North American ground sloths within the suborder Folivora of order Pilosa, living from around 23 million years ago (Mya) to 11,000 years ago. This family is most closely related to another fa ...
s, while moving the Caribbean sloths to a separate,
basal branch of the sloth evolutionary tree.
[
][
]
These results provide further strong support for the long-held belief that arboreality arose separately in the two genera via
convergent paths.
The following sloth family phylogenetic tree is based on collagen and mitochondrial DNA sequence data (see Fig. 4 of Presslee ''et al''., 2019).
Characteristics
Famously slow-moving, a sloth travels at an average speed of .
Three-toed sloths are about the size of a small dog or a large cat, with the head and body having a combined length around and a weight of . Unlike the two-toed sloths, they also have short tails of , and they have three clawed toes on each limb. All sloths have three digits on their
hindlimb
A hindlimb or back limb is one of the paired articulated appendages (limbs) attached on the caudal ( posterior) end of a terrestrial tetrapod vertebrate's torso.http://www.merriam-webster.com/medical/hind%20limb, Merriam Webster Dictionary-Hindl ...
s; the difference is found in the number of digits on the
forelimb
A forelimb or front limb is one of the paired articulated appendages (limbs) attached on the cranial ( anterior) end of a terrestrial tetrapod vertebrate's torso. With reference to quadrupeds, the term foreleg or front leg is often used instead. ...
s; thus they are sometimes referred to as three-fingered sloths. However, sloths are generally regarded as
quadruped
Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...
s.
Behavior
Like 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 "la ...
, three-toed sloths are agile swimmers. They are still slow in trees.
[Three-toed Sloth](_blank)
National Geographic. The muscles that sloths use to grip and produce a pulling motion are much more prominent than those that produce a pushing motion. This means that they struggle to support their body weight when walking on all four limbs, so travelling on the ground is a dangerous and laborious process.
Three-toed sloths are arboreal (tree-dwelling), with a body adapted to hang by their limbs. Large, curved claws and muscles specifically adapted for strength and stamina help sloths to keep a strong grip on tree branches.
The abdominal organs close to their diaphragm (such as their stomach, liver and kidneys) are attached to their lower ribs (or pelvic girdle in the latter case) by fibrinous adhesions, which prevent the weight of these organs from compressing their lungs when hanging, making inhalation easier.
They live high in the canopy, but descend once a week to defecate on the forest floor. During this week-long interval their feces and urine accumulate to about a third of their total body mass.
It takes about a month for a single leaf to pass through their four-chambered stomach and digestive tract.
Although they get most of their fluids from the leaves that they eat, they have been observed drinking directly from rivers. Because of their slow metabolism, they do not need to ingest many leaves on a daily basis, but when ambient temperatures are high, the symbiotic microbes and bacteria present in their gut will break down and ferment food at a faster rate.
Conversely, when temperatures are lower, sloths will consume less, which is opposite to what has been observed in most other mammals.
Only very few species of sloths are found at higher altitudes, and these are found to have thicker coats than those living in lower altitudes. Some of the extinct species of sloths were able to tolerate cooler temperatures, but researchers believe this was probably due to thicker fur, larger size, larger muscle mass and more access to a constant food supply.
Their long, coarse fur often appears greenish, not due to pigment, but to algae growing on it. Sloths' greenish color and their sluggish habits provide an effective camouflage; hanging quietly, sloths resemble a bundle of leaves.
They move between different trees up to four times a day, although they prefer to keep to a particular type of tree, which varies between individuals, perhaps as a means of allowing multiple sloths to occupy overlapping home ranges without competing with each other.
Three-toed sloths are predominantly
diurnal, although they can be active at any hour of the day, while two-toed sloths are
nocturnal
Nocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite.
Nocturnal creatures generally have highly developed sens ...
.
Lifecycle
![Three-toed sloth crossing road in Costa Rica](https://upload.wikimedia.org/wikipedia/commons/4/4e/Three-toed_sloth_crossing_road_in_Costa_Rica.jpg)
Members of this genus tend to live around 25 to 30 years, reaching sexual maturation at three to five years of age.
Three-toed sloths do not have a mating season and breed year round. Females give birth to a single young after a
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 around six months. The offspring cling to their mother's bellies for around nine months. They are
weaned
Weaning is the process of gradually introducing an infant human or another mammal to what will be its adult diet while withdrawing the supply of its mother's milk.
The process takes place only in mammals, as only mammals produce milk. The infan ...
around nine months of age, when the mother leaves her home territory to her offspring and moves elsewhere. Adults are solitary, and mark their territories using anal
scent gland
Scent gland are exocrine glands found in most mammals. They produce semi-viscous secretions which contain pheromones and other semiochemical compounds. These odor-messengers indicate information such as status, territorial marking, mood, and s ...
s and
dung midden
Dung middens, also known as ''dung hills'',''The New Encyclopaedia of Mammals'' D MacDonald 2002 Oxford are piles of dung that mammals periodically return to and build up. They are used as a form of territorial marker. A range of animals are know ...
s.
[
Male three-toed sloths are attracted to females in ]estrus
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 ...
by their screams echoing throughout the canopy. Sloth copulation lasts an average of 25 minutes. Male three-toed sloths are strongly polygamous, and exclude competitors from their territory. Males are also able to compete with one another within small habitable territories.
The home ranges used by wild brown-throated three-toed sloth
The brown-throated sloth (''Bradypus variegatus'') is a species of three-toed sloth found in the Neotropical realm of Central and South America.
It is the most common of the four species of three-toed sloth, and is found in the forests of South ...
s in Costa Rica
Costa Rica (, ; ; literally "Rich Coast"), officially the Republic of Costa Rica ( es, República de Costa Rica), is a country in the Central American region of North America, bordered by Nicaragua to the north, the Caribbean Sea to the no ...
include cacao, pasture, riparian forests
A riparian forest or riparian woodland is a forested or wooded area of land adjacent to a body of water such as a river, stream, pond, lake, marshland, estuary, canal, sink or reservoir.
Etymology
The term riparian comes from the Latin word ' ...
, peri-urban areas and living fence-rows. For the first few months after giving birth, mothers remain at just one or two trees, and guide their young. At about five to seven months of age, when the young have become more independent, mothers expand their resources and leave their young in new areas. During natal dispersion, three-toed sloths prefer tropical forests, often using riparian forest habitat to disperse while avoiding pastures and shade-grown cacao. The home range
A home range is the area in which an animal lives and moves on a periodic basis. It is related to the concept of an animal's territory which is the area that is actively defended. The concept of a home range was introduced by W. H. Burt in 1943. He ...
for mothers are larger than those of young. After separation, only the mothers use the cacao-growing forest, but both use riparian forest. Different types of trees are used by both mother and young, which indicates that this agricultural matrix provides an important habitat type for these animals.
Dentition and skeleton
Three-toed sloths have no incisor
Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, whe ...
or canine teeth, just a set of peg-shaped cheek teeth that are not clearly divided into premolar
The premolars, also called premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mouth ...
s and molars, and lack homology
Homology may refer to:
Sciences
Biology
*Homology (biology), any characteristic of biological organisms that is derived from a common ancestor
* Sequence homology, biological homology between DNA, RNA, or protein sequences
*Homologous chrom ...
with those teeth in other mammals, thus are referred to as molariforms. The molariform dentition in three-toed sloths is simple and can be characterized as dental formula
Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiolo ...
of: .
Three-toed sloths are unusual amongst the mammals
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 ...
in possessing as many as nine 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 ...
, which may be due to mutations in the homeotic genes. All other mammals have seven cervical vertebrae, other than the two-toed sloth and the manatee
Manatees (family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living speci ...
, which have only six.
References
External links
"Three-toed sloth." Passport to Knowledge. 21 Feb. 2009
{{DEFAULTSORT:Three-Toed Sloth
Sloths
Mammals described in 1758
Taxa named by Carl Linnaeus