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Despite the superficial resemblance between the sloth genera Choloepus and Bradypus, the two actually differ in more than just the number of their toes. In the past they were recognized as a single family, Bradypodidae, but in modern times Choloepus, the two-toed sloth, has been found to be more closely related to the extinct ground sloths than either group is to Bradypus, the three-toed sloth. Currently ground sloths and Choloepus species are included together in the taxon Megalonychidae.
Megalonychids are united by a number of cranial characteristics that have been laid out by Anderson and Jones (1984), including the presence of a distinct rostrum with a pre-nasal bone, inflated pterygoid and alisphenoid, a condyloid process that is wider than long, and mandibles with an elongate symphysis and conspicuous coronoid. Additionally, Megalonychids lack tympanic bullae and a complete jugal but do posses an ectotympanic. The pterygoids do not meet at the midline. The post-cranial features include a variable number of cervical vertebrae ranging from 5-7 (and occaisionally 8), 18-27 thoracolumbar vertebrae, fused acromion and coracoid processes, an entepicodylar foramen, and a lack of a third trochanter. Choloepus possesses three digits on the hindlimb, and not surprisingly given its common name, two on the forelimb. The digits on each limb are tightly bound together until the long, recurved claws (Anderson and Jones, 1984).
The genus Choloepus also possesses a collection of unique dental characteristics. The teeth are subcylindrical in shape, persistently growing, and follow the dental formula 5/4. The basic tooth structure consists of a central axis of vasodentine enveloped by a thin layer of dentine and then a thicker, outer layer of cement (Anderson and Jones 1984). The anterior most teeth on the maxilla do appear to be larger and caniniform relative to the other teeth and are separated from the more molariform elements by a diastema. Choloepus has a unique dental feature among extant mammals whereby the anterior of the lower canines occlude with the posterior of the upper canines resulting in self-sharpening of these teeth (Anderson and Jones 1984).
Choloepus hoffmanni is a curious looking mammal with a convex face, inconspicuous ears, and no tail. Adults usually reach a length between 540 and 740 mm and weigh anywhere from 4 to 8.5 kg on average (Nowak 1991). The coat consists of both short fur and longer guard hairs, and the hairs themselves have a unique structure that is not seen in any other modern mammals. The specialized morphology encourages the growth of algae, which camouflages the sloth from predators and may actually provide nutrients either through skin absorption or when the coat is licked (Nowak, 1991).
The two-toed sloth is nocturnal in nature and is rarely seen engaging in its arboreal activities during the day, remaining inactive even at dawn and dusk (Mendel 1985). Almost all life processes are carried out in trees while suspended upside down, including mating and birth. This species climbs down to the ground for one activity only, elimination, and with the sloth’s slow metabolism it only occurs once a week (Nowak 1991). Perhaps due to the sluggishness of both metabolism and habit, Choloepus has the most variable temperature of any mammal, ranging from 24 to 33 °C, depending on the environment. As a result of the inability to maintain a constant body temperature, all species of this genus are restricted to equatorial habitats with C. hoffmanni specifically found in tropical forests from Nicaragua to Peru and central Brazil (Nowak 1991).
Similar to Bradypus, Choloepus is a generalized herbivore, but tends to be more active than the three-toed sloth. They will feed on more variable food sources, including leaves, twigs, fruit, and occasionally insects (Nowak 1991). While females will most often forage and socialize in groups, males are generally solitary. Interestingly, population studies have found a sex ratio in C. hoffmanni of approximately 11 females to 1 male (Nowak 1991). After mating there is a gestation period of around 11.5 months, and after birth the single young does not stray from the mother for the first 5 months. The two form a close relationship with the offspring associating with the mother for another two years or more (Nowak 1991).
Additional Information on the Skull
Click on the thumbnails below for labeled images of the skull in standard anatomical views.
Anderson, S. and J. K. Jones, Jr. 1984. Orders and Families of Recent Mammals of the World. John Wiley and Sons, New York. 686pp.
Mendel, F.C. 1985. Adaptations for Suspensory Behavior in the Limbs of Two-toed Sloths. in The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas. G. Gene Montogomery, ed. Smithsonian Institution Press, Washington and London. 451 pp.
Nowak, R. 1991. Walker's Mammals of the World, 5th edition. The John's Hopkins University Press, Baltimore and London. 642pp.
Choloepus hoffmanni species account on the Animal Diversity Web (University of Michigan)
Sloth page from xenarthra.org
See movies of sloths on junglewalk.com
The Sloth Web Site
Choloepus bibliography on Sloth-World.org