Digits on fore- and hindlimbs fused to a mitten-like structure from which only the 3.2-3.9 in (8-10 cm) long claws protrude. These allow branches to be gripped without expending muscular force. The number of digits on the forelimb distinguish Bradypus from Choloepus sloths. They would be better named, three- and two-fingered sloths, since both have three digits on the hindlimbs. There are eight or nine neck vertebrae (most species of mammal, even giraffes, have seven). This allows the head to be turned with a considerable range, an important advantage for an animal with otherwise rather limited flexibility. The testes are internal. There are no incisors or canine teeth and the simple, peg-like incisors lack enamel. Sloths have poor hearing, but fairly good eyesight and smell.
To accommodate a largely suspended, upside-down lifestyle, the fur hangs down from the belly to the back. The underfur is short and fine. The coarse very thick outer fur is grooved along its length, providing attachment for two species of blue-green algae. Along with sebaceous secretions from the sloth, excretions from the algae are food for adults of Cryptoses choloepi, a species of pyralid moth. Adult moths reach densities of up to 132 per sloth. Larval Cryptoses choloepi feed on sloth dung, as do the larvae of three types of beetle (Trichilium spp.) and at least three types of mite (Amblyomma varium and two species of Boophilus). Experimentally, decolonized sloths have been recolonized after 40 days. The algal growth on the hairs is rarely dense enough to make the sloth appear distinctly green. There is no size difference between the sexes.
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