West Indian sloths and two-toed tree sloths

(Megalonychidae)

Class Mammalia

Order Xenarthra

Family Megalonychidae

Thumbnail description
Small- to medium-sized sloths, from the size of a large domestic cat to a small bear; all species show large caniniform teeth, several molariform teeth, long limbs and strong, curved claws; they have pear-shaped bodies and long, shaggy coats ranging in color from gray to brown, often with lighter fur around the heads and faces

Size
Sloths of the family Megalonychidae range in weight from about the size of a large domestic cat (20 lb [9 kg] for Choloepus) to as large as a large bear; however, none of the West Indian forms grew larger than a black bear

Number of genera, species
1 genera; 2 species; extinct forms grouped with Choloepus include as many as 11 genera, each containing a single species

Habitat
Tropical rainforests and cloud forests

Conservation status
Extinct: all species of West Indian sloths; Data Deficient: 2 species

Distribution
Choloepus occurs in Central America and into South America as far as Brazil. The West Indian sloths may have shared part of that range as well as the islands of the West Indies

Evolution and systematics

Evolved in South America, this diverse order first appears in the fossil record in the Paleocene. It contains sloths and anteaters, the Pilosa, or hairy xenarthrans, and the Cingulata, or plated xenarthrans, which includes the extinct glyptodonts and armadillos. The group is named for the additional articulations between their vertebrae, called xenarthrous processes. The family Megalonychidae includes a single living genus, Choloepus, with two species, and at least 11 extinct genera from the West Indies as well as one from Curaçao. The Megalonychidae appeared first in the fossil record as distinct from the other two families of sloths (the Mylodontidae and the Megatheriidae) in the early Miocene. The early genera were small and differed from the earliest sloth family, the Mylodontidae, in lacking dermal armor. Their long claws were laterally compressed and curved, and the forelimbs were almost as long as the hind limbs. They had caniniform teeth and peg-like molariforms. It has been suggested that their body forms resemble the modern Choloepus, and that they might have been arboreal.

Physical characteristics

West Indian sloths are small to medium sized, and the living Choloepus is the larger of the two tree sloth genera, being the size of a large domestic cat. These animals have slender limbs, pear-shaped bodies, and long claws. Claws in Choloepus are more hook-like than those of the other genera, but all are recurved to some degree. Choloepus and the West Indian sloths have enlarged anterior caniniform teeth, followed by a diastema and a series of molariform teeth separated by small spaces in the jaws. The teeth in sloths emerge as simple rounded cones. The distinctive pattern of cusps and basins that characterize genera individually are formed entirely by wear caused by slight differences in the genus specific pattern of masticatory movements.

Distribution

Choloepus occurs in tropical rainforests and cloud forests up to an altitude of 6,000 ft (1,830 m) in Central America, from Nicaragua south to Brazil in South America. Extinct mega-lonychids

have been found on most of the islands of the West Indies and Curaçao.

Habitat

The living genus, Choloepus, primarily occurs in the tropical rainforests of Central and South America, although some animals survive in Central American cloud forests at altitudes as high as 6,000 ft (1,830 m). The Pleistocene and recent habitats of the West Indian sloths were probably similar to those in which Choloepus lives today, and the present habitats may not have altered much from the time when the sloths were alive.

Behavior

Choloepus is solitary and arboreal. Adults maintain a territory that encompasses a wide variety of tree and vine species. Sloths prefer trees in their home range; they are chosen primarily because of a heavy concentration of lianas in the crowns. Sloths are nocturnal, and use vine-covered trees for resting and sleeping during the day, because predators cannot approach without alerting the sloth. Choloepus is slow moving (as possibly were the extinct sloths), a behavior partially attributable to the low amounts of energy obtainable from the animals' diet as well as a strategy that assists the animals to avoid predators by remaining cryptic. Choloepus has grooves in the outer guard hairs of its coat that house two species of blue green algae that turn the sloths a greenish color, especially during the wet season. It would be reasonable to assume that the fur of some West Indian sloths also housed algae that enhanced their ability to blend into their environment.

Feeding ecology and diet

All sloths are herbivores, and anatomical similarities between Choloepus and West Indian sloths indicate that the latter were probably also folivores, although the leaf species eaten probably differed. Choloepus may also feed on fruits.

Reproductive biology

Living sloths are solitary once weaned, and are polygynous, meeting only to mate. Males do not assist with rearing the young. References suggest a gestation period for Choloepus of 11 months and/or an ability to store sperm; for larger bodied West Indian sloths it was probably longer.

Conservation status

Choloepus is listed as Endangered by the IUCN, mostly due to loss or degradation of the rainforest habitat. Roads also cause mortality because slow-moving sloths are often unable to cross quickly enough to avoid vehicles.

Significance to humans

Choloepus has been used occasionally as food by humans. West Indian sloths disappeared shortly after humans invaded their islands less than 2,000 years ago. They may have also been a source of pelts. Tree sloth pelage is used in a few human societies but, in general, never achieved high fashion status. The claws are sometimes incorporated into jewelry.

Species accounts

List of Species

Lesser Haitian ground sloth

Synocnus comes

subfamily

Megalonychinae

taxonomy

Synocnus comes Paula Couto, 1967

other common names

None known.

physical characteristics

The size of a medium-sized dog, weighing about 50 lb (23 kg). The animal is known only from skeletons from Haiti, but bones recovered indicate that it showed typical sloth body proportions, with a broad trunk, slender limbs, and long claws. In contrast to tree sloths, the ground sloth had a tail long enough to touch the ground. The caudal vertebrae were broad, and this morphology is associated in other extinct sloths with a tail robust enough to serve as a tripodal support to allow the animal to stand bipedally in a fashion similar to tamanduas. Ground sloth has large, triangular caniniform teeth, separated by a diastema from molariforms with sharp cusps and basins. The skull was deep, with a large sagittal crest and the deep mandible allowed large masticatory muscles.

distribution

Known from Haitian cave deposits.

habitat

Resembled Choloepus, with similar habits, but was semi-arboreal.

behavior

Nothing known, although a semi-arboreal ground sloth may have behaved in a manner similar to Choloepus.

feeding ecology and diet

As do their living relatives, ground sloths probably fed on leaves.

reproductive biology

Nothing is known. Probably polygynous.

conservation status

Extinct.

significance to humans

When living, these animals may have been killed for food and pelts. Now they are of interest to students of evolutionary history and ecology.

Resources

Books

Carroll, R. L. Vertebrate Paleontology and Evolution, Ch. XXI. New York: W. H. Freeman and Co., 1998.

Hall, E. R., and H. Kelson. The Mammals of North America. Princeton: Princeton University Press, 1981.

Martin, P. S. Pleistocene Overkill. New York: Columbia University Press, 1984.

McKenna, M. C., and S. K. Bell. Classification of Mammals above the Species Level. New York: Columbia University Press, 1997.

Nowak, R. M. Walker's Mammals of the World. New York: Columbia University Press, 1999.

Webb, S. D. "Late Cenozoic Mammal Dispersal between the Americas." In The Great American Biotic Interchange Topics in Geobiology 4, edited by F. G. Stehli, and S. D. Webb. New York: Plenum Press, 1985.

Virginia L. Naples, PhD