Bradypus variegatus, Three-toed Sloth skull

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Bradypus variegatus, commonly known as the three-toed sloth, is one of only a few surviving species in this unique group. This mammal stands out with its small eyes and ears that are almost invisible, along with forelimbs that are about 1.5 times longer than its hind limbs, giving it an amusing appearance. The short, blunt tail resembles an amputated limb, as described by Nowak (1991), adding to the species' intriguing look. Adults typically grow between 413 and 700 mm in length and weigh between 2.25 and 5.5 kg (Nowak, 1991). The coat of Bradypus consists of two types of hair: short, fine fur and long, thick guard hairs that are grooved and appear to encourage algal growth. According to Nowak (1991), the algae may provide camouflage for the sloth as well as nutrients, just like in Choloepus. Like all living sloths, Bradypus variegatus is known for its extremely slow movement and entirely arboreal lifestyle. All daily activities are carried out while hanging upside down, including eating, sleeping, mating, and giving birth. Only urination and defecation occur on the ground, and the sloth needs to descend from the trees just once a week to do so. A sluggish metabolism probably contributes to reduced elimination needs, and as a result of this slow lifestyle, Bradypus cannot maintain a constant body temperature (Nowak, 1991). This condition leads to dependence on consistently warm habitats such as those found in tropical forests across Honduras to northern Argentina, where B. variegatus lives (Nowak, 1991). Overall, species within the genus Bradypus tend to be less active than their ecologically similar two-toed sloth counterparts. Individuals rarely cross over to a neighboring tree within one day, preferring to move less than a few meters (Nowak, 1991). Bradypus variegatus is a more specialized forager than Choloepus 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). Typically, there is only one young after a gestation period of 5-6 months, and while the offspring no longer nurses after about one month, it still relies on its mother for another five (Nowak, 1991). This specimen, a male skull, was collected by the Ollala brothers on June 6, 1931, in Igarape Brabo, Rio Tapajos, Para, Brazil. It is housed in the Department of Mammalogy at the American Museum of Natural History. It was made available to The University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Robert Fajardo of Harvard Medical School and Dr. Timothy Rowe of The University of Texas at Austin. Funding for image processing was provided by a National Science Foundation Digital Libraries Initiative grant to Dr. Rowe.