Three-toed tree sloths

(Bradypodidae)

Class Mammalia

Order Xenarthra

Family Bradypodidae

Thumbnail description
Arboreal with long coarse shaggy fur and long limbs that appear thick and powerful; both sets of limbs end in three stout hooked claws; small eyes and ears, short snout, peg-like teeth, and a small, stumpy tail

Size
Head and body length 15.8–30.3 in (40–77 cm); tail 1.9–3.5 in (4.7.6–9.0 cm); weight 5.1–12.1 lb (2.3–5.5 kg)

Number of genera, species
1 genus; 4 species

Habitat
Tropical forests

Conservation status
Endangered: 1 species

Distribution
Central and South America

Evolution and systematics

Though members of the order Xenartha have been recorded from Eocene Europe and ground sloths reached North America across the Central American isthmus, there are no fossil records of tree sloths further north than southern Mexico. Bradypus sloths are only distantly related to two-toed Choloepus sloths.

Physical characteristics

Digits on fore- and hindlimbs fused to a mitten-like structure from which only the 3.2–3.9 in (8–10 cm) long claws protrude. These allow branches to be gripped without expending muscular force. The number of digits on the forelimb distinguish Bradypus from Choloepus sloths. They would be better named, three- and two-fingered sloths, since both have three digits on the hindlimbs. There are eight or nine neck vertebrae (most species of mammal, even giraffes, have seven). This allows the head to be turned with a considerable range, an important advantage for an animal with otherwise rather limited flexibility. The testes are internal. There are no incisors or canine teeth and the simple, peg-like incisors lack enamel. Sloths have poor hearing, but fairly good eyesight and smell.

To accommodate a largely suspended, upside-down lifestyle, the fur hangs down from the belly to the back. The underfur is short and fine. The coarse very thick outer fur is grooved along its length, providing attachment for two species of blue-green algae. Along with sebaceous secretions from the sloth, excretions from the algae are food for adults of Cryptoses choloepi, a species of pyralid moth. Adult moths reach densities of up to 132 per sloth. Larval Cryptoses choloepi feed on sloth dung, as do the larvae of three types of beetle (Trichilium spp.) and at least three types of mite (Amblyomma varium and two species of Boophilus). Experimentally, decolonized sloths have been recolonized after 40 days. The algal growth on the hairs is rarely dense enough to make the sloth appear distinctly green. There is no size difference between the sexes.

Distribution

Three-toed sloths live from Honduras to southern Argentina.

Habitat

Various forest types from primary closed canopy to highly disturbed secondary and seasonally dry forests.

Behavior

Feeding, mating and birth all occur in trees. Defecation and urination, however, occurs on the ground and sloths make their way to the ground once or twice a week to eliminate in a hole that is dug by the tail while the sloth clings with its forelegs to the tree trunk or vine above. It is during this process that females of the various moths, beetles and mites that live on the sloth fur will temporarily leave their shaggy host to deposit eggs on its dung. Sloth dung consists of hard rounded pellets about 0.3 in (8 mm) in diameter. About a cupfull are deposited on each occasion. Stereotyped movements of the tail and/or hindlimbs ensure that the hole is covered with leaf litter on completion. The entire process usually takes less than 30 minutes, but many jaguar kills of sloths are reported to occur during this period. Locomotion in trees generally proceeds with the claws used as hooks both in vertical and horizontal progression. Terrestrial movement is a slow flailing crawl with the animal preferring to hook objects with its claws and pull itself forwards. Progress under such conditions has been clocked at 0.25 mph (0.4 km/h). Swimming appears to be much easier, and sloths are frequently encountered crossing rivers. Though movement is generally slow, sloths can move quite quickly if threatened. Despite their general immobility, they are preyed on by large eagles (especially the harpy eagle, Harpia harpyja) and by jaguars. Males advertise presence by wiping pungent smelling secretions from anal glands onto branches. Dung middens also smell strongly and may serve as trysting locations.

Feeding ecology and diet

In all species, the predominant diet is shoots and leaves of forest trees. Sloths feed on Cecropia, the most abundant tree of the Amazonia forest. Regenerating agricultural land river margins and natural gaps may sometimes be important (despite the biting ants that swarm in the tree's hollow stems), but it is never the sole food source. The idea that Bradypus sloths feed only on Cecropia probably arises because an open growth form makes a sloth in a Cecropia easier to see than in almost any other kind of rainforest tree. When feeding, the forelimbs are used to pull leaves slowly towards the mouth. Sloths are highly specialized for an existence that centers around squeezing as much energy as possible out of a low-intake rate of highly indigestible food. This is because leaves eaten by Bradypus sloths are, though energy-rich, also rich in tannins and fiber. Digestion must therefore be a simultaneous process of detoxification and energy extraction. The gut is extensive, making up 30% of the total body weight. Digestion, by bacterial fermentation in a complex multi-chambered stomach, occurs over an extended period to permit maximum absorption of scarce resources. Passage of food through the intestine is also very slow, providing maximum opportunity for the uptake of nutrients and energy. This

means that sloths have little energy to spare and hence move slowly. Their metabolic rates are about half of what would be expected for an animal of their size (sloth adaptations provide plenty of opportunities to contemplate the chicken–and–egg nature of complex physiological and behavioral adaptations). The three-toed sloth has struck a compromise between being large enough to move about efficiently between the canopies of tall trees, the weight of the long gut and big stomach needed for food processing and the need to be light enough to avoid breaking the limbs from which it feeds. In minimizing weight, while maximizing mass, the sloth has compromised on muscles and has the lowest ratio of muscles to skeleton of any comparably sized ground-dwelling mammal. Its muscles are thin and ribbon-like. Much of the sloth's apparent volume comes from its long hair. The compromise works: sloths are the most abundant large mammal in neotropical rainforests (up to 70% of the arboreal mammalian bio-mass), and cropping some 2% of the forest's annual leaf production. It has been suggested that the blue-green algae may also provide some nutrition, being licked directly from the hair or having nutrients absorbed via the skin. Despite great similarities, two species of Bradypus sloth coexist in certain parts of their range. Bradypus and Choloepus sloths can also coexist.

Reproductive biology

Sexual maturity is reached at about three years. One young is produced per year. Nursing requires 6–8 weeks. Weaning occurs as infants first lick leaf fragments from their mother's fur and lips and later sharing the leaves being eaten by the mother. To save energy, sloths barely regulate their body temperature, however pregnant females do invest energy to keep their bodies a few degrees above ambient temperature, the better to develop their embryo. Mating takes 3–5 minutes and may be conducted with the two animals face to face and hanging by their front legs from a branch, or with the female suspended by all four limbs and the male on her back. Sloths may live 10–20 years in the wild and are presumably polygynous.

Conservation status

One species, B. torquatus, is classified as Endangered by the IUCN and the U.S. Department of the Interior, due to its habitat being depleted by lumber extraction and argricultural activities (plantations, cattle pasture).

Significance to humans

Several tropical rainforest species, including sloths, are slowly being recognized for their potential to further human medicine. Sloths are known for their ability to heal quickly, avoid infection, and survive the most severe injuries. Researchers are investigating the basis of this healing response so as to develop improved drugs or treatment methods for severe wounds.

Species accounts

List of Species

Brown-throated three-toed sloth

Bradypus variegatus

taxonomy

Bradypus variegatus Schinz, 1825, Brazil. The genus name Bradypus comes from the Greek bradus for "slow" and pous, for "foot," podos.

other common names

Portuguese: Preguica-de-bentinho (Brazil); Spanish: Perezoso de tres dedos (European Spanish), perico (Bolivia, Colombia, Ecuador), pelejo (Peru).

physical characteristics

Color varies over wide geographical range, long coarse body fur brownish gray with white patches on hindlegs and lower part of back. Extent of white patches highly variable, some populations are nearly all white. Others are a deep foxy red-brown. Fur may have a greenish tinge due to algae. Fur on head shorter and denser. Head with a black "mask" across the eyes extending back to the ear region (ears are hidden in fur and not easily visible). Throat and chest brown. Adult males have a "speculum," a patch of short deep orange fur between the shoulder blades that is bisected by one of more deep brown-black horizontal lines. Distinguished from the two-toed sloth by the number of forefoot claws and a shorter dark muzzle (bigger, paler and more pig-like in Choloepus).

distribution

Southern Mexico to northern Argentina, to elevations of at least 3,610 ft (1,100 m).

habitat

Evergreen and seasonally dry forests. A natural tolerance of disturbance and secondary growth also allows them to survive in isolated trees in deforested pastures, and even in city parks.

behavior

Active at any time of day, though generally more active at night. Drops its body temperature each night (an energy-conserving strategy) and must warm up each morning by basking. This is the time when harpy eagle predation most frequently occurs. When not basking or feeding, likely to be sleeping curled up in a ball in the crook of a tree. Hard to see under such circumstances. Spends up to 18 hours a day asleep to conserve energy. Adults are solitary but home ranges may overlap. Neighbors rarely feed in the same tree, and males may fight each other. Individuals may spend many days in the same tree, and can pass their entire 20- to 30-year life span in home ranges of less than 4.9 acres (2 ha). One of the most common animals in the South American rainforest, Bradypus sloths can occur at densities of six or seven per 2.5 acre (1 ha). When not hanging suspended, may rest in a fork of a branch with head between forelimbs. May be very difficult to see under such conditions. Vocalizations, a shrill whistle and a low reptilian hiss, are given only under duress. The shrill "ai, ai" sounds whistled through the nostrils, are the basis for the name for this animal in the indigenous Guarani language.

feeding ecology and diet

Within home range, a sloth may use up to fifty trees of thirty different species. To avoid toxification by the tannins, phenols and other chemicals in the leaves it ingests, sloths change trees (and tree species) on average once every 1.5 days. Passage of food through the gut is measured in days, rather than the hours usual for most mammals. This is necessary to extract all possible energy from the low-quality forage. Diet preferences are inherited from mother during several months of a "social weaning" process. Since these differ considerably, several sloths may coexist in the same area, but not compete.

reproductive biology

Breeding occurs throughout the year. A single young is born, though twins have been reported once or twice. Gestation is 5 to 6 months. Young weigh 0.44–0.55 lb (0.20–0.25 kg) at birth. Young are weaned within 4 weeks, but are carried by the mother for another five months. Babies are carried resting on their mother's abdomen, graduating to dorsal carriage as they get older and larger. Once the young has learned the location of the trees in the maternal patch, the female leaves, bequeathing the young one all or part of her foraging area. This highly unusual arrangement is thought to minimize energetically wasteful conflict between individuals. Probably polygynous.

conservation status

CITES Appendix II. Not threatened.

significance to humans

Hunted for meat in certain parts of their range.

Pale-throated three-toed sloth

Bradypus tridactylus

taxonomy

Bradypus tridactylus Linnaeus, 1758, Suriname.

other common names

French: Mouton parasseux (French Guiana); Surinamese: Driteenluiaard.

physical characteristics

Back darker, buff to dark brown, with contrasting pale or dark grizzling. Belly paler, off-white to very deep cream. Back and rump with variably sized irregular roundels of cream or dirty orange. Facial area, a cream colored mask extending back to the ears and onto the throat. No black contrasting "mask" as in B. variegatus, though some dark patterning round eyes may occur. Adult males posses a speculum like that of B. variegatus.

distribution

Replaces B. variegatus in eastern Venezuela, the Guyanas, and northeastern Brazil. The two species may coexist in the lower Amazon.

habitat

Lowland rainforest. Less flexible than B. variegatus and rarely recorded from seasonally dry forests or highly disturbed areas.

behavior

Active at any time during day or night. Ecology believed to be very similar to that of B. variagatus. Occurs together with Choloepus didactylus, the two-toed sloth. Resources are partitioned between the two by differing diets, activity patterns and use of different forest strata.

feeding ecology and diet

Believed to be similar to that of B. variegatus.

reproductive biology

Gestation lasts 106 days. In Guyana, births occur only in the rainy season, but elsewhere, reproduction seems flexible and dependant on local conditions. This may be due to the female's ability to halt an embryo's development until conditions are favorable. Female may be sexually receptive while still nursing and can also be both nursing and pregnant at the same time. An interval of seven months between births has been reported under good conditions. Probably polygynous.

conservation status

Not threatened.

significance to humans

Sometimes hunted for meat.

Monk sloth

Bradypus pygmaeus

taxonomy

Bradypus pygmaeus Anderson and Handley, 2001. Known only from Isla Escudo de Veraguas, an island of the Bocas del Toro, off the Caribbean coast of Panama. This recently described species provides a fascinating example of evolution in action. Sloths from the mainland have colonized five of these islands at least four times in the past few thousand years. Each time, they have changed in their isolation, becoming smaller and adapted to their new island homes. The oldest island, Escudo de Varaguas, is 8,900 years old. Only here has the population differentiated enough to be called a new species. Populations on the other islands (1,000 to 5,200 years old) are still many generations away from this.

other common names

English: Dwarf sloth, pygmy three-toed sloth.

physical characteristics

Small (20% less in all measurements) in comparison to other sloths. The speculum is pure orange. The face is tan with a distinctive dark band across the forehead, and a dark throat and an orange wash to the face. Long hair hangs forward from the forehead, giving the impression of a hood. Back with a strong spinal stripe.

distribution

Only found on Escudo de Veraguas island, off the Caribbean coast of western Panama.

habitat

Found only in red mangroves at sea level.

behavior

Not yet studied. Lives only in coastal mangroves.

feeding ecology and diet

Believed to consist entirely of the leaves of red mangroves (Rhizophora mangle).

reproductive biology

Nothing is known. May be polygynous.

conservation status

Nothing known.

significance to humans

Of great interest to evolutionary biologists.

Maned sloth

Bradypus torquatus

taxonomy

Bradypus torquatus Illiger, 1811, Brazil.

other common names

Portuguese: Preguica preta, Preguica de coleira (Brazil).

physical characteristics

Shares the body shape of other sloths. But both head and body are the same color, grizzled tan brown all over. Long black hairs, to 5.9 in (15 cm), fall from the nape, over the neck and shoulders to form the characteristic mane. Speculum absent. In infants and juveniles, the fur is very pale, whitish to pale brown and lacks a mane.

distribution

Forests of the Atlantic Coast of Brazil ("Mata Atlantica"). Distribution very patchy and populations now highly isolated in highly fragmented forests. Few known populations coincide with location of existing protected areas.

habitat

Atlantic Coastal forests of Brazil. Able to survive in secondary forest but prefers vine-rich primary ones.

behavior

May be active during day or night, but are most active during the day. Home ranges average 4.9 acres (2 ha). A new range is often colonized each rainy season.

feeding ecology and diet

Unlike other Bradypus species, B. torquatus adds liana and vine leaves (16%) to a diet of tree leaves. It prefers young leaves (68%), whereas in other three-toed sloths mature leaves dominate. Twenty-one species have been recorded in their diet, with individuals showing strong preferences and eating between seven and twelve species. The chosen trees are actively sought and are not simply the most abundant ones in the forest. Trees of the Moraceae family are highly sought after. Unlike most other tropical trees, Moraceae trees have a continuous production of young leaves, the preferred food of this sloth. The small number of species in the diet may allow the individual sloth's physiology to adapt to detoxifying them. Shares the mechanism of maternal transmission of foraging preferences and foraging home range.

reproductive biology

Single young, weighing 10 oz (300 g) at birth. Reproduction is non-seasonal. Presumably polygynous.

conservation status

Classified by the IUCN as Endangered.

significance to humans

Formerly hunted for food. This species is now one of several Atlantic Coastal Forest endemics being used in public awareness campaigns to promote conservation of this highly threatened ecosystem.

Resources

Books

Eisenberg, J. F., and K. H. Redford. Mammals of the Neotropics. Vol. 3, The Central Tropics: Ecuador, Peru, Bolivia, Brazil. Chicago: University of Chicago Press, 1999.

Janzen, D. H. Costa Rican Natural History. Chicago: University of Chicago Press, 1983.

Reid, F. A. A Field Guide to the Mammals of Central America and Southeast Mexico. Oxford: Oxford University Press, 1997.

Periodicals

Anderson, R. P. and C. O. Handley, Jr. "A new species of three-toed sloth (Mammalia: Xenartha) from Panama, with a review of the genus Bradypus." Proceedings of the Biological Society of Washington 114 (2001): 1–33.

Chiarello, A. G. "Diet of the Atlantic forest maned sloth Bradypus torquatus (Xenartha: Bradypodidae)." Journal of Zoology (London) 246 (1998): 11–19.

Richard-Hansen, C., and E. Tuabe. "Note on the reproductive behavior of the three–toed sloth, Bradypus tridactylus, in French Guiana." Mammalia 246 (1997): 259–263.

Waage, J. K., and G. G. Montgomery. "Cryptoses choloepi: A coprophagous moth that lives on a sloth." Science 193 (1976): 157–158.

Adrian A. Barnett