Animal Diversity Web

This fam­ily con­tains 2 species placed in a sin­gle genus, Choloe­pus. Two-toed sloths are found in Cen­tral Amer­ica, through­out north­ern South Amer­ica and in the Ama­zon basin.

Two toed sloths are medium-sized an­i­mals, with a body slightly more than half a meter in length and weight up to around 9 kg. They are cov­ered with long, usu­ally pale gray-brown fur (paler on the head) that takes on a green­ish hue due to sym­bi­otic algae liv­ing on the hairs. Under the coarse outer fur, the pelage grades into a layer of finer, shorter un­der­fur. Ex­ter­nal ears are much re­duced in size. The fore­limbs and hindlimbs are long, with the fore­limbs some­what longer than the hindlimbs (but the dif­fer­ence is not as ex­treme as it is in three-toed sloths, Brady­po­di­dae). The fore­limbs end in two large, curved claws; these are at­tached to dig­its that are en­closed in a web of skin (syn­dacty­lous). The hindlimbs have three claws and are also syn­dacty­lous. The tail is small or ab­sent.

The skulls of these sloths are rel­a­tively short. The ros­trum is dis­tinctly con­vex in pro­file. True tym­panic b u llae are lack­ing. There is, how­ever, an ec­to­tym­panic, and the ptery­goids and al­isphe­noids are swollen, form­ing a sort of in­flated bul­lae. The mandible has a large coro­noid process, and the condy­loid process lies in the same plane as the toothrow. The zy­go­matic arch is in­com­plete, the jugal end­ing in flared dor­sal and ven­tral processes. The pre­max­illa is small.

Mega­lony­chids have 5/4-5 cheek teeth. In­cisors and ca­nines are ab­sent, but the an­te­rior cheek teeth are en­larged, tri­an­gu­lar in cross sec­tion, and ca­nine-like. They are sep­a­rated from the rest of the cheek teeth by a di­astema. The an­te­rior sur­face of the lower ca­nine-like tooth meets the pos­te­rior sur­face of the upper, wear­ing against each other and con­tin­u­ally sharp­en­ing their edges.

These sloths feed mainly on leaves, but also in­clude some fruits, buds, and even small ver­te­brates in their diets. Like three-toed sloths, they move in a very slow and de­lib­er­ate fash­ion (but mega­lony­chids are faster than brady­po­dids). Most of their lives are spent high up in trees, al­though they, like brady­po­dids, come to the ground to defe­cate. They swim well. Two-toed sloths are ca­pa­ble of vig­or­ous self de­fense, using their fore­claws to slash a preda­tor and bit­ing with their en­larged, ca­nine-like cheek teeth.

Two toed sloths are per­haps more het­erother­mic than any other mam­mal. Their body tem­per­a­tures fluc­tu­ate from as low as 24 de­grees C to as high as 33 de­grees C. Un­like three-toed sloths, how­ever, they do not ther­moreg­u­late by bask­ing.

Mega­lony­chids first ap­pear in the fos­sil record in the Early Miocene. They were found in North Amer­ica in the Pleis­tocene, be­com­ing ex­tinct as re­cently as 11,000 years BP (giant ground sloths).

Lit­er­a­ture and ref­er­ences cited

Bar­low, J. C. 1984. Xe­narthrans and pholi­dotes. Pp. 219-239 in An­der­son, S. and J. K. Jones, Jr. (eds). Or­ders and Fam­i­lies of Re­cent Mam­mals of the World. John Wiley and Sons, N.Y. xii+686 pp.

Par­adiso, J. L. 1975. Walker's Mam­mals of the World, Third Edi­tion. Johns Hop­kins Uni­ver­sity Press, Bal­ti­more.

Sav­age, R. J. G. and M. R. Long. 1986. Mam­mal Evo­lu­tion, an Il­lus­trated Guide. Facts of File Pub­li­ca­tions, New York. 259 pp.

Vaughan, T. A. 1986. Mam­mal­ogy. Third Edi­tion. Saun­ders Col­lege Pub­lish­ing, Fort Worth. vii+576 pp.

Wil­son, D. E., and D. M. Reeder. 1993. Mam­mal Species of the World, A Tax­o­nomic and Ge­o­graphic Ref­er­ence. 2nd edi­tion. Smith­son­ian In­sti­tu­tion Press, Wash­ing­ton. xviii+1206 pp.

Con­trib­u­tors

Phil Myers (au­thor), Mu­seum of Zo­ol­ogy, Uni­ver­sity of Michi­gan-Ann Arbor.