Sloth - AccessScience from McGraw-Hill Education http://www.accessscience.com/content/sloth/628700 (http://www.accessscience.com/) Article by: Linzey, Donald W. Wytheville Community College, Wytheville, Virginia. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097-8542.628700 (http://dx.doi.org/10.1036/1097-8542.628700) Content Bradypodidae Physiology and ecology Bibliography Megalonychidae Reproduction and development Additional Readings A mammal classified in the order Xenarthra along with anteaters, tamanduas, and armadillos. The sloth differs from all other mammals by having additional articulations (xenarthrales) between their lumbar vertebrae (called xenarthrous vertebrae). They also have a double rather than a single inferior vena cava, the vein that returns blood from the posterior portion of the body to the heart. The dental formula is I 0/0, C 0/0, Pm 0/0, M 5/4 × 2, for a total of 18 teeth which grow throughout life. Females have a primitive, divided uterus and a common urinary and genital tract; males have internal testes. See also: Dentition (/content/dentition/186400) Two extant families of tree sloths (Bradypodidae and Megalonychidae) inhabit the lowland and upland tropical forests of Central and South America. Bradypodidae The family Bradypodidae includes a single genus, the three-toed tree sloth (Bradypus) [see illustration]. This is a slender- bodied animal with a head and body length of 413–700 mm (16–27 in.) and a tail length of 20–90 mm (0.75–3.5 in.) that looks heavier because of its long, thick, coarse hair. The three-toed sloth weighs 2.25–6.20 kg (5–13.5 lb). It has three toes on each front and rear foot. The head is small and round, the tail is short and stumpy, and the eyes and ears are reduced—the animal can neither hear nor see well but depends on its senses of smell and touch. The three species in this genus have 30 ribs and long necks containing nine vertebrae (two more than most mammals). These sloths are drab brown or gray with patches of white. Males have a bright yellow spot, about the size of a silver dollar, on their back. In this spot, the hair is short and glossy. 1 of 4 3/10/2016 9:24 AM Sloth - AccessScience from McGraw-Hill Education http://www.accessscience.com/content/sloth/628700 Three-toed tree sloth (Bradypus tridactylus). (Photo by Dr. Lloyd Glenn Ingles; copyright © California Academy of Sciences) Megalonychidae The family Megalonychidae (two-toed tree sloths) contains a single genus (Choloepus) with two species. Two-toed tree sloths, or unau, with two toes on their front feet and three on their hind feet, are somewhat larger and more active than their three-toed cousins. Adults range 540–740 mm (21–28 in.) in total length and weigh 4.0–8.5 kg (9–18.5 lb). The coloration is grayish brown with a paler face. The large hooklike claws are 8–10 cm (3–4 in.) long. These sloths possess 48 ribs, six to eight vertebrae, and their tail is either absent or vestigial. Physiology and ecology Tree sloths are solitary and spend most of their lives hanging upside down from the upper branches of a tree. They hang by the long, hooked claws at the ends of their toes and slowly move along, hand over hand in the inverted position. They spend their waking hours slowly picking and eating leaves, especially the leaves of the Cecropia tree. Their stomachs contain cellulose-digesting bacteria and may retain food for as long as a month before passing it on to the small intestine. Sloths are so lethargic that two blue-green algae, which look like moss, actually grow in the grooves of their coarse hair. In the rainy season, their fur often has a distinct greenish tinge due to the luxuriant growth of algae; in times of drought, the fur turns yellowish. The greenish color serves to camouflage the animals in the tree canopy. Sloths have a low body temperature of 30–34°C (86–93°F) which fluctuates both daily and seasonally. They regulate their temperature by shifting their position in and out of the sun. Sloths make no attempt to construct a den or shelter in the trees. They sleep and give birth while hanging upside down. They normally sleep about 18 h per day on a vertical branch, keeping a firm hold with all four feet with their head hanging down and resting between the front limbs. Predators include jaguars and eagles, although habitat destruction and excessive hunting may prove to be the most detrimental factors affecting these species. One species (B. torquatus) is classified as endangered by the International Union for Conservation of Nature and Natural Resources (IUCN) and the U.S. Department of the Interior. Reproduction and development Most sloths are believed to breed throughout the year. A single young is born after a gestation ranging from 3.5 to 11.5 months. During its first few months, the young sloth clings flat to the mother's belly. They cease nursing between 4 and 5 2 of 4 3/10/2016 9:24 AM Sloth - AccessScience from McGraw-Hill Education http://www.accessscience.com/content/sloth/628700 weeks of age. Female two-toed sloths reach sexual maturity at 3 years of age, whereas males are sexually mature at 4–5 years of age. Life expectancy in the wild is up to about 12 years, although they have lived up to 31 years in captivity. Donald W. Linzey Bibliography D. Macdonald (ed.), The Encyclopedia of Mammals, Oxford University Press, 2006 R. M. Nowak, Walker's Mammals of the World, 6th ed., The Johns Hopkins University Press, 1999 Additional Readings N. de Moraes-Barros, J. A. B. Silva, and J. S. Morgante, Morphology, molecular phylogeny, and taxonomic inconsistencies in the study of Bradypus sloths (Pilosa, Bradypodidae), J. Mammal., 92(1):86–100, 2011 DOI: 10.1644/10-MAMM-A-086.1 (http://dx.doi.org/10.1644/10-MAMM-A-086.1) J. F. Eisenberg, G. M. McKay, and M. R. Jainudeen, Reproduction by the two-toed sloth Choloepus hoffmanni in captivity, Amer. Soc. Mammal., Abstr. Tech. Pap., 58th Annual Meeting, pp. 41–42, 1978 L. Hautier et al., Skeletal development in sloths and the evolution of mammalian vertebral patterning, Proc. Natl. Acad. Sci. USA, 107(44):18903–18908, 2010 DOI: 10.1073/pnas.1010335107 (http://dx.doi.org/10.1073/pnas.1010335107) C. L. Henderson, Mammals, Amphibians, and Reptiles of Costa Rica: A Field Guide, 2d ed., University of Texas Press, Austin, TX, 2010 F. C. Mendel, Use of hands and feet of three-toed sloths (Bradypus variegatus) during climbing and terrestrial locomotion, J. Mammal., 66:359–366, 1985 F. C. Mendel, Use of hands and feet of two-toed sloths (Choloepus hoffmanni) during climbing and terrestrial locomotion, J. Mammal., 62:413–421, 1981 G. G. Montgomery, The evolution and ecology of armadillos, sloths, and vermilinguas, Smithsonian Institution Press, Washington, DC, 1985 L. Pinder, Body measurements, karyoptype, and birth frequencies of maned sloth (Bradypus torquatus), Mammali, 57:43–48, 1993 J. P. Rafferty (ed.), Rats, Bats, and Xenarthrans, 1st ed., Britannica Educational Publishing, New York, 2011 M. E. Sundquist and G. G. Montgomery, Activity patterns and rates of movement of two-toed and three-toed sloths (Choloepus hoffmanni and Bradypus infuscatus), J. Mammal., 54:946–954, 1973 Animal Diversity Web: Bradypodidae (http://animaldiversity.ummz.umich.edu/site/accounts/information/Bradypodidae.html) Mammal Directory: Sloths (http://www.montclair.edu/csam/prism/rainforest-connection/panama/mammal-directory/sloths/) 3 of 4 3/10/2016 9:24 AM .