Copyright @1997 Michael L.Smith
San Francisco State University
Geography 316: Biogeography
The Biogeography of the
Brown-throated Three-toed sloth (Bradypus variegatus)
Copyright @1997 Michael L.Smith
By: Christine Chan, student in Geography 316, Fall 1999
Kingdom: Animalia
Phylum: Chordata
Class:
Mammalia
Order :
Endentata = Xenarthra
or Suborder: Xenarthra
Family: Bradypodidae
Genus: Bradypus
Species: Bradypus variegatus
Description of Species
The
brown-throated three-toed sloth (Bradypus variegatus) can be identified and distinguished
from other members of its genus by the brown color of its fur on the sides of its face and
throat, prominent dark brown forehead, suborbital stripe outlining the ocular area of the
face and shorter mandibular spout (Wetzel and Koch 1973; Wetzel and Avila-Pires 1980).
Listed below is the average size and
weight of an adult Bradypus variegatus (the geographic range of animals studied is from
Nicaragua to Brazil), (Wetzel and Koch 1973; Wetzel and Avila-Pires 1980):
Average total body length (108 animals studied)—578 mm (length ranging between
420-800mm).
Average length of tail (101 animals studied)—58 mm (length ranging
between 38-90mm).
Average length of hind foot (101 animals studied)—122 mm (length
ranging between 90-180 mm).
Average length of ear ( 41 animals studied)—13 mm ( length ranging
between 8-22 mm).
Average weight (25 animals studied)—4.34 kg (weight ranging between
2.25-5.50 kg).
Bradypus variegatus is a remarkably slow moving,
nocturnal and diurnal mammal. Its long, coarse, thick hair is brownish/grey in color and
takes on a greenish tinge in the rainier season when microscopic blue-green algae forms on
its back (Beebe, 1926). This additional color may act as camouflage protecting the sloth
from the sight of predators below. Under its long top hair, another layer of soft,
shorter fur can be found (Walker 1991). The fur of the family Bradypodidae is thought to
be very useful, thus helping provide protection from ants and other meat eaters on land as
well as against hungry fish when in the water (Bebee 1926; Krieg 1939). Unlike many
mammals, sloth hair points downward from its stomach towards its back. This helps the
sloth shed the rain while in an up-side down position (Beebe 1926). Males can be
distinguished from females by a white or orange patch of fur with a black stripe called a
dorsal marking located between their shoulders (Goffart 1971).
Bradypus variegatus’s small,
round head is slightly darker than the rest of its body and its eyes and ears are likewise
very small. Its forearms are longer than its back legs and both front and back legs
support three closely united toes that extend to claws. These claws are very sharp and
when prompted the sloth responds by slashing out at its enemy and inflicting deep wounds
(Walker 1975). It is thought that with a poorly developed sense of sight (Bebee 1926) and
hearing, the genus Bradypus relies almost entirely on its sense of smell and touch to find
food (Walker 1975), although more recent studies contradict the idea that this genus has
poor eyesight (Mendel 1985, Piggins & Muntz 1985).
Habitat
Bradypus
variegatus spends most of its life in the middle layers and the tops of trees where it
hangs upside down from branches or sits in the forks between tree limbs. The genus
Bradypus are said to be great sitters and when sitting, they sometimes cross their
fore-arms over their chests and rest their heads upon them (Goffart 1971). Sloths are
known to sleep or rest up to twenty hours a day (Bebee 1926). The habitat selection of the
genus Bradypus involves many conditions such as the most recent history of feeding and
activity, the physiological state of the animal and plant it uses for food, social history
and possibly that of its ancestors, composition and diversity of plant species where the
animal lives; and seasonal variations in the availability and composition of the different
plants species the animal uses (Montgomery and Sunquist, 1978). The trees within its
habitat must be close enough together for the sloth to proceed around the forest canopy
when needed. The crowns of the trees in tropical forests are often thick with interlocking
lianas and other vegetation and provide strong footholds for sloth travel, sleeping,
mating and eating. When looking for food, all members of the sloth family progress in an
up-side down position through the tree canopies although they also climb vertically (Bebee
1926).
Copyright @ 1997 Micheal L.
Smith
Bradypus variegatus prefers trees with large crowns and
selects them based on the amount of time the crowns are exposed to sun (Montgomery and
Sunquist 1978). This behavior is related to its variable body temperature in which the
genus Bradypus thermoregulates its body by moving into the trees when its hot and
increasing its exposure to the sun when its cold (Montgomery and Sunquist 1978). It favors
lower elevation humid forest conditions and is strictly arboreal. On occasion, Bradypus
variegatus does leave the trees and crawls along the forest floor or swims in the flooded
forest to find other trees for food. It has difficulty crawling but swims well (
Rodrigues; Britton 1941; Worman 1946; Tirler 1966), although it has never been known to
dive into the water. Little change in climatic conditions and high light intensity are
both factors that restrict Bradypus variegatus to this particular habitat.
Natural History
Limited
to equatorial habitats of constant high temperature climates and almost daily rainfall,
Bradypus variegatus’s low and unstable body temperature varies depending on the
temperature of its surroundings (Prosser and Brown 1961). With a body temperature as low
as 34ºC in Bradypus (Goffart 1971) and commonly varying 10ºC during the day (Irving et
al., 1942; McNab, 1978), continued exposure to the sun may result in death (Britton and
Atkinson, 1938). On a cloudy day Bradypus variegatus’s body temperature does not
exceed 5º above the air temperature (McNab 1978; Montgomery and Sunquist 1978). Because
its body temperature is lower then most mammals studied, it barely keeps warm even in the
tropics (Goffart 1971). One reason why Bradypus variegatus is not able to control its body
temperature and is restricted to tropical climates is its reduced skeletal muscles which
only contributes 25% -30 % of its overall weight (Goffart 1971). This small muscle mass
effects its metabolism as Bradypus variegatus turns food into energy at half the rate of
other species its size (Cavendish 1997). Their nerves and muscles function slower than
most mammals as well (Goffart 1971).
Bradypus is peaceful and shy. Because
the animal has little knowledge of fear, it can be found close to human dwellings (Goffart
1971). Its predators include jaguars, Harpy Eagles and anacondas but, because of its
arboreal lifestyle, it lives relatively free from danger. Both two-toed and three-toed
sloths are known to reduce their activity level around sunrise, the usual hunting time for
the Harpy Eagle (Sunquist and Montgomery 1973). Because the three-toed sloth can not stand
on level surfaces, they are rarely seen on the ground (Walker 1975). However, it usually
descends to the forest floor once a week to defecate (Montgomery and Sunquist 1978). For
Bradypus variegatus, moving across the forest floor is difficult and a rare occurrence. It
must drag itself along by grasping onto objects or toeholds and pulling with its long
claws (Walker 1975).
Copyright @ 1997 Micheal L. Smith
Most often a solitary animal (Goffart 1971),
Bradypus variegatus can occasionally be found living in pairs. According to Bebee (1926),
when the male and female Bradypus are found living together, they seem to do so
grudgingly. The family Bradypodidae’s home range is usually less than two hectares
(Montgomery and Sunquist 1978). Although normally very quiet, the female is said to call
out with an a-ee sound when attempting to attract a male (Goffart 1971). The male responds
to this call by approaching the female slowly and quietly (Krieg 1961; Tiler 1966). Its
courtship lacks emotion and ends quickly, the males leaving after a few hours (Bebee
1926). Conflicting information prevails as Bradypus is thought to engage in a ventre a
ventre embrace while mating (Britton 1941) and is also described as mating from behind
(Tirler 1966). Generally, the sloth has one offspring per year and the gestation period is
estimated between four and six months (Montgomery and Sunquist 1975). Thus, the female
sloth spends about half of its adult life pregnant.
The female never builds a nest or
shelter for its young (Goffart 1971). Instead, the infant remains attached to its
mother’s stomach where the first three to four weeks are spent nursing (Montgomery
and Sunquist 1975). During these first months the young inherit their mother’s
preference in tree species therefore, eliminating the need for competition among different
sloths (Montgomery and Sunquist, 1975). Once fully weaned, approximately six months, the
mother abruptly leaves her baby and shifts her range of territory slightly also reducing
competition between her and her baby (Montgomery and Sunquist, 1975).
Bradypus variegatus eats tree leaves,
shoots and other foliage. According to Francois Bourliere’s Mammals of the World
(1955), and Marshall Cavendish’s Encyclopedia of Mammals (1997), the leaves of the
Cecropia tree comprise most of its diet. This information is also said to be attributed to
the higher visibility of the Cecropia tree than of other potential food sources. These
trees are typically located near clearings, riverbanks, and the forest’s edge. While
liana leaves are also said to constitute most of their diet (Eisenberg and Thorington
1973; Montgomery and Sunquist 1978), it should be noted that all leaves are not potential
food for Bradypus variegatus. The leaves “food potential” depends on the degree
of maturity and whether or not they are digestible by the animal (Westoby, 1974).
According to Montgomery and Sunquist (1975), young tree leaves are more easily digestible
than older ones. When eating, the genus bradypus will slowly pull the foliage to its mouth
with its claws (Walker, 1991) and subsequently gets most of its water supply through the
leaves it eats.
Bradypus (three-toed sloth) is very
different from its relative Choloepus (two-toed sloth) even though they live in
microsymmetry with each other(Montgomery and Sunquist, 1970). Aside from the fact that the
two-toed sloth has two claws on its front limbs and the three-toed sloth has three,
differences can be found in the number of vertebrae, their behavior, and their habits
(Pocock 1924; Krumbiegel 1941). While the three-toed sloth is said to eat mainly the
leaves of the Cecropia tree or lianas leaves, the two-toed sloth will eat a variety of
foods in its habitat. The arms of the three-toed sloth are also visibly longer than that
of the two-toed sloth. And unlike the three-toed sloth which has nine vertebrae, ideal for
flexibility (Walker 1991), the two-toed sloth has a short neck with only six cervical
vertebrae and a limited range of movement (Goffart 1971). The two-toed sloth is also
visibly larger than the three-toed sloth. Finally, the two-toed sloth is nocturnal where
as the three-toed sloth is active during the day and at night.
Sloths and other members of their
order were among the first mammals of South America. South American fossils date as far
back as the Paleocene epoch and the order Xenarthra are said to form an exclusively New
World order (Walker 1991). In Grimek (1975), Thenius is described as saying that the
Xenarthra order arose in South America and while remaining there for most of its history,
emerged into many different groups. At some point during the middle of the Pliocene epoch,
giant ground sloths began to invade North America. They were followed in the Pleistocene
epoch (Ice Age) by their relatives the armadillos and the glyptodonts. The tree sloth of
today moved no further north than northern Mexico. Although their migration was
successful, the degree of their specialization did not favor the sloths existence in areas
where there were changing climate conditions. As a result, Bradypus variegatus is
restricted to the non-seasonal climate conditions of tropical regions.
Evolution
Although
fossil records are known with resembling characteristics similar to tree sloths,
conclusive evidence as to the ancestors of present day sloths is still not available
(Miller 1939). This may be because animals living in trees are seldom preserved (Romer
1966). Because tree sloths have no fossil genera assigned to them, all ground sloths are
assumed to be fossils, while tree sloth are recent (Webb 1985). As stated by Webb (1985),
armadillos were probably the ultimate ancestors of the sloth. He also noted that if this
were true, then all sloths must have evolved from terrestrial ancestors. Webb (1985)
recognized that it had not yet been determined when tree sloths became arboreal and in
which branches of their family tree.
According to Goffart (1971), sloths
belong to the order Edentates with a suborder Xenarthra. Flower (1888) stated that
it is probable all New World Endentates came from one common stock, evolving in the
Pliocene and the Pleistocene period. The Endentates center of evolution was isolated to
South America during their early geologic history (Walker 1975). Although the suborder
Xenarthra identifies its member as possessing extraarticulations between the lower dorsal
and the lumbar vertebrae, they are poorly developed in Bradypus (Frenchkop 1953). The
suborder Xenarthra includes the family Bradypodidae (sloth), Myrmecophagidae (ant-eaters),
and the Dasypodidae (armadillos) (Goffart 1971). Sarich (1985) indicated that at least 75
to 85 million years ago sloths, anteaters, and armadillos diverged from each other.
There appears to be disagreements as
to the most accurate identification of the above mentioned order. In Walker (1991),
Xenarthra is considered the correct name for the order suggesting that Endentata is
incorrect. The order Endentata included Xenarthra as well as the ancestral suborder
Palaeanodonta. Recent identification of this order is based on information provided by
Glass (1985), who explains that Palaeanodonta are not ancestral to Xenarthra but to
Pholidota. Glass (1985) suggests that the name Edentata be synonymous with Pholidota. The
order Xenarthra has also been used by Barlow (1984), Engelmann (1985), and Wetzel (1985a)
but not by Corbet and Hill (1986).
Wetzel (1985) acknowledges the fact
that very little taxonomic attention has been given to the order Xenarthra and that the
species within this order lack uniform support by the scientific community. Traditionally,
the family Bradypodidae consists of two genera: Cholopus (Illiger 1811) and Bradypus
(Linnaeus 1758). However, studies written by Wetzel (1985) can be found which suggests
that the family Bradypodidae includes two subgenus: Scaeopus (Peters) which recognizes one
species named Bradypus torquatus (maned sloth); and subgenus Bradypus L. which
contain two species Bradypus tridactylus L. (pale-throated three-toed sloth) and Bradypus
variegatus (brown-throated three-toed sloth). Many synonyms have been used for B.
variegatus over the years and include: B. cuculliger (Moeller 1975), Arctopithecus
boliviensis (Gray 1871); as B. boliviensis (Cabrera 1958; Bucher 1980; Mares
1981), Arctopithecus griseus (Gray 1871), Bradypus tridactylus (Moeller 1975), and
Bradypus infuscatus (Wagler 1831; Cabrera 1959; Baker 1974). The two-toed sloth, a close
relative of the three-toed sloth is identified as part of the family Megalonychidae, which
recognizes two species Cholepus hoffmanni (Peters 1859), and Choloepus didactylus (Linne
1758).
According to Walker (1991), new
evidence suggests that the three-toed and the two-toed sloth depict convergent surviving
lines that produced two different groups of ground sloths from different ancestral stock.
A few million years ago a member of the Xenarthra order, the giant ground sloth, began
migrating into North America from its origins in South America (Grzimek 1975). Three
ancestral ground sloth families are said to have once existed; Megatheriidae the South
American giant ground sloth whose ancestors were arboreal, and the Megalonychidae and
Mylondontidae which were always terrestrial ground sloths. Although the Megatheriidae
family is said to have been arboreal, all three families are said to have once been fully
terrestrial and obtained sizes ranging from an averaged size dog to an elephant (Walker
1991). Evidently, Mylondontidae left more fossil evidence in South America during the late
Eocene epoch through the Oligocene epoch than did the other sloth groups (Webb 1985).
Recent radiocarbon dating of a giant ground sloth specimen determined that the genus
Mylodon (Owen 1840) dates back to as little as 13,500 years ago (Walker 1991). It is
suggested by Webb (1985), that the genus Choloepus evolved from Megalonychidae, and the
genus Bradypus resembles the long extinct ground sloth family Megatheriidae. As stated by
(Webb 1985), early ancestors of the Megatheriidae were arboreal. However, according to
Engelmann (1985), the relationships between the extinct ground sloths and the living
genera of tree sloths remains unclear. Engelmann (1985) suggests that any of the following
relationships mentioned could also be possible: Guth (1961), who suggested that
Bradypus could be more closely related to megatheres while Choloepus could be more closely
related to mylodonts; and Patterson and Pascual (1968) who suggested that while Bradypus
has special affinities megatheriidae, Choloepus could be more closely related to
megalonychids. This origin goes back much further than Sarich’s (1985), who suggested
that Choloepus and Bradypus diverged about 35 million years ago around the late Oligocene
epoch. Support for the early separation of lineage’s by Sarich involves studies of
protein taxonomy. According to Webb (1985), one perspective indicates that the giant
ground sloth of the late Cenozoic era were highly specialized end members of the
phyllophagous xenarthrans while recent tree sloth’s are believed to be more
representative of Miocene stages of sloth evolution.
Within the order Xenarthra, sloths
are perhaps the most highly diversified animal group, mostly of which are represented in
fossil record (Wetzel 1985). Because of this, the sample sloth phylogeny is fairly
incomplete with respect to its representation in taxa as well as nature of material used
for some taxa. This makes it difficult to determine relationships within the Phyllophaga.
Engelmann (1985), suggests that the relationships presented be used as framwork for
further investigation and study.
From G. Gene Montgomery's The Evolution and Ecology of Armadillos, Sloths,
and Vermilinguas
Distribution
The species, Bradypus variegatus, is endemic to the
tropical and subtropical regions of the American continent (Grasse 1955). These animals
are best suited for highly humid and woody areas, (Britton 1941), near rivers (Goldman
1920; Bebee 1926), and in open jungles (Seitz 1889; Menegaux 1980). Their continuous
distribution can range from Honduras in Central America into South America (roughly
between 15ºN and 25ºS Latitude). In South America, Bradypus variegatus can be found in
coastal Ecuador, through Columbia and Venezuela (except for Llanos, and the Orinoco river
delta), continuing to through the forested areas of Ecuador, Peru, Bolivia, through-out
Brazil (except Amp?), and extending to the northern portion of Argentina (Cabrera, 1958).
Bradypus variegatus can be found living in the trees of neotropical deciduous forests
(Eisenberg and Thorington, 1973; Montgomery and Sunquist, 1975, 1978) as well as tropical
rainforests.
Map of Distribution:
From G. Gene Montgomery's, The evolution and Ecology of Armadillos,
Sloths, and Vermilinguas
Current Status
Although some members of the sloth family are fewer in
numbers than others, all species of Bradypus are potentially endangered by habitat
destruction. Classified as endangered by the IUCN and the USDI, Bradypus torquatus’s
home range along the Atlantic’s coastal forest of Brazil is rapidly being destroyed
for charcoal production and lumber extractions. Plantation and cattle pastures follow the
clear cutting with little hope for habitat recovery. As mentioned by Walker (1991), B.
variegatus boliviensis, a subspecies of Bradypus variegatus has already been listed on
appendix 2 of the CITIES.
Bibliography
Goffart M., 1971. Function and Form in the Sloth. Oxford:
Pergamon Press.
Greene, H.W. 1989 “Agonistic Behavior by the
Three-toed Sloth, Badypus variegatus.” Biotropica 21.4
369-372.
Micheal L. Smith and Cocori.com SA. 1995-1999. Cocori
Complete Costa Rica Home Page. http://www.cocori.com
www.cocori.com/photo/imfauna/sloth
3t1.htm
www.cocori.com/photo/imfauna/bcup.htm
www.cocori.com/photo/imfauna/slotha.htm
Montgomery, Gene G., ed. and Sunquist M.E. 1978. The Ecology of
Arboreal Folivores. Washington D.C. Smithsonian Institution Press.
Montgomery, Gene G., ed. 1985. The Evolution and Ecology of Armadillos, Sloths,
and Vermilinguas. Washington and London: Smithsonian Institution Press.
Nagy K.A. and Montgomery G.G. 1980. “Field Metabolic Rate, Water Flux, and Food
Consumption in the Three-toed Sloth.” Journal of Mammalogy
61.3 465-472.
Walker, Earnest P., 1975. Mammals of the World. Vol.
I, 3rd ed. Baltimore and London: The John Hopkins University Press.
Walker, Earnest P., 1991. Mammals of the World. Vol.
I, 5th ed. Baltimore and London: The John Hopkins University Press, 515-520.
send comments to bholzman@sfsu.edu
Geog 316 homepage Back to Geography home page Back to SFSU homepage