San Francisco State University
Department of Geography

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 34C in Bradypus (Goffart 1971) and commonly varying 10C 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 5 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 15N and 25S 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.
 

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