Saturday, 4 December 2010

Miscellaneous Mammals4: Linnes Two-toed Sloth

Sharing and enclosure in Twilight World with some Dourocouli monkeys are Bristol Zoo’s pair of Linne’s Two-Toed sloth, Choloepus didactylus. This is the commonest species of sloth to be seen in zoos, followed by its close relative Hoffman’s Two-toed Sloth C.hoffmanni.

Both species have multiple subspecies, and few if any in captivity are diagnosed as to which they belong to. For this reason the sloth’s in captivity are primarily display and research subjects, rather than captive breeding and reintroduction candidates. In any case, both species are widespread and are listed as of Least Concern by the IUCN.

Living sloth’s are in 2 families – Choloepus in Megalonychidae, the other family , the Bradypodidae, contains the Three-toed Sloths, Bradypus, which have 4 species, Bradypodid sloths are much harder to maintain in captivity and so are not usually kept in zoos outside their native range. This is particularly unfortunate as two species, B.pygmaeus and B. torquatus, are Critically Endangered. B.pygmaeus is known only from a single island with a total area of under 5 square kilometres. B.torquatus, like other animals of the Brazilian Atlantic Rain Forest, is threatened by habitat destruction.

Sloths harbour several different species of algae in their fur, which help with camouflage. Sloth hair is specialised to provide anchorage for the algae, with the two families having distinctly different hair structure. They are also hosts to a variety of arthropods, including several species of sloth moth Cryptoses. These have larvae which feed on sloth dung, the adults flying until they locate a sloth, where they remain for the remainder of their lives. Some other species may even lay eggs in the sloth’s fur, with the larvae feeding on the algae.

The two living families of sloths are not closely related, and Choloepus is actually much closer to the giant ground sloths that became extinct at the end of the last glaciation. In addition to the more well known fossil ground sloths, there was a moderate radiation of various megalonychid sloths in the Caribbean and Antilles , all of which seem to have become extinct by 5,000 years ago, almost certainly as a result of human activities. Whether Choloepus is descended from terrestrial ancestors, making it in effect a miniature giant ground sloth, is uncertain, but some of the Caribbean ground sloths certainly seem to have been at least partly arboreal, so it is not impossible.

With the distant evolutionary relationship between the living species, it becomes harder to generalise about them. It has been suggested that the symbiotic algae in sloth hair do more than provide camouflage, and the failure to maintain Bradypus sloths outside their native range is partly due to the loss of their symbiotes. It may also be that their need for behavioural thermoregulation means that an incorrect enclosure design makes it impossible for them to regulate their body temperature in a natural manner, with inevitable results.

Sloths have very slow metabolisms, and consequently have trouble maintaining a constant body temperature. Bradypus sloth’s habitually climb into open canopy in order to thermoregulate, especially in the early morning. This makes them vulnerable to Harpy Eagles, and sloth’s comprise at least a third of the diet of the eagle.

Choloepus appears to be strictly nocturnal in the wild, whereas many if not all species of Bradypus are active both day and night. Choloepus appear to feed on a wider variety of foliage, which makes them far easier to maintain. Bradypus feeds largely, though not exclusively, on Cecropia leaves, and is very reluctant to switch to new diets in captivity

In captivity Choloepus species will feed on almost anything (including animal protein) and are fairly regularly bred. Needless to say, this does not happen quickly – at Bristol we have had various births over the years but no successful rearing as yet. One birth occurred over a year after the male in the enclosure died, so pregnancy in Choloepus is at least 12 months and probably more. Once born, a baby will spend at least 6 months riding around on its mother’s belly before becoming independent.

.The chief conservation activity for sloths is habitat protection. As they are dependent on evergreen forest for the most part, and are very vulnerable to hunters, protection of the forest from fires or poaching is the best way forward to protect these ancient animals.

Next week, our other edentates, the yellow armadillo.

(images from wikipedia)

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