Bradypus variegatus is one of just a handful of extant sloth species. The three-toed sloth, as it is commonly called, has small eyes, unnoticeable ears, and forelimbs that are about one and a half times longer then its hind limbs, giving this mammal a comic appeal. The short, blunt tail, which according to Nowak (1991) resembles 'an amputated limb', can only add to this species' curious appearance. Adults generally reach a length of 413 to 700 mm and attain weights between 2.25 and 5.5 kg (Nowak 1991).
The coat of Bradypus consists of two types of hair, the first a short fine fur and the other long, thick guard hairs that are grooved and appear to encourage algal growth. As with Choleopus, Nowak (1991) suspects that the algae provides camouflage to the sloth as well as nutrients.
Like all living sloths, B. variegatus is noted for its very slow movement and strictly arboreal lifestyle. All daily life activities are carried out while upside down, including eating, sleeping, mating, and giving birth. Only urination and defecation is done on the ground and the sloth need to descend from the trees just once a week to do so. A sluggish metabolism probably contributes to the lessened elimination needs, and as a result of the same slow, life processes Bradypus, like most other sloths, cannot maintain a constant body temperature (Nowak 1991). This condition leads to a dependence on a consistently warm habitat such as is found in the tropical forests of Honduras to northern Argentina, where Bradypus variegatus lives (Nowak 1991).
Overall, species in the genus Bradypus tend to be less active then the ecologically similar two-toed sloth. Individuals rarely even cross over to a neighboring tree in the course of one day, preferring to move less than a few meters (Nowak 1991). Bradypus variegatus is a more specialized forager than Choleopus hoffmanni, feeding almost exclusively on twigs and leaves from Cecropia trees (Nowak 1991). In addition, three-toed sloths are less social than other species, preferring to feed, sleep, and travel alone rather than in groups. Males and females probably only pair for mating, and a one to one sex ratio has been observed in the wild (Nowak 1991). There is usually only a single young after a gestation period of 5-6 months, and while the offspring will no longer nurse after about one month, it still relies on its mother for another five (Nowak 1991).
Species in the genus Bradypus belong to the family Bradypodidae, which at one time included Choleopus, but is now restricted to three-toed sloths. This family can be diagnosed by cranial characteristics, including a short rostrum, dorso-ventrally flattened skull, tympanic bullae, mandibles with conspicuous coronoid processes, a jugal ending posteriorly in long, flared ventral and dorsal processes, and a condyloid process that is longer than it is wide. Additionally, the pterygoids do not meet at the midline(Anderson and Jones 1984). The simple, peg-like teeth are very similar in structure to those of the family Megalonychidae (two-toed sloths and ground sloths) in that they are composed of a central core of vasodentine that is wrapped by a thin layer of dentine and then a thick outer layer of cement (Anderson and Jones 1984). The teeth are also continuously growing. In contrast to megalonychids, bradypodids have very small or even absent anterior maxillary teeth (caniniforms) and when present, there is no gap separating them from molariform elements (Anderson and Jones 1984). The dental formula is 5/4.
Post-cranial characteristics of Bradypus include hands and feet with three toes each, the digits tightly bound up until the strongly recurved claws, 18-27 thoracolumbar vertebrae, and 8 or 9 cervical vertebrae (Anderson and Jones 1984). They also lack a third trochanter and entepicondylar foramen. The extra cervical vertebrae give Bradypus increased flexibility, allowing the sloth to turn its head 270 degrees, or nearly three-quarters of the way around!
Literature
Anderson, S. and J. K. Jones, Jr. 1984. Orders and Families of Recent Mammals of the World. John Wiley and Sons, New York. 686pp.
Nowak, R. 1991. Walker's Mammals of the World, 5th edition. The John's Hopkins University Press, Baltimore and London. 642pp.
Links
Bradypus variegatus species account on the Animal Diversity Web (University of Michigan)
Sloth page from xenarthra.org
The Sloth Web Site
Bradypus bibliography on Sloth-World.org
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