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The Sloth: Uniquely Adapted to Its Environment

by Richard Garrigues

Every creature is adapted to its environment and to the requirements of its particular lifestyle (carnivore, herbivore, omnivore, etc.). Yet few tropical rain forest mammals seem more uniquely adapted than do the sloths, or perezosos, as they are known here in Costa Rica.

Named for their slow-paced actions, sloths are admittedly not as exciting to watch as, say, a troop of foraging white-faced monkeys or an inquisitive family of coatimundis. In fact, despite being one of the most numerous mammals in most forested lowland areas throughout Costa Rica, sloths are not all that easy to see. They make no noise that might attract our attention, their motions through the trees (when they move at all) are slow enough so as not to cause branches to bounce and sway, and their fur even has a greenish appearance that helps to camouflage them among the leaves that are their source of nourishment.

These are not evolutionary adaptations designed to thwart tourists from viewing them, but rather the result of different methods of staying hidden from potential predators that include wild cats and some of the larger hawks and eagles. The green color is not part of the animal's natural coloration but is caused by algae that grow on the sloth's fur.

The characteristic slow motion of sloths is the result of having sacrificed much of their muscle mass for the sake of being relatively lightweight for an animal of their size (from about 1 to 2 feet in body length). The advantage of being lightweight means that sloths can move farther out on the limbs of trees to reach the youngest, most tender (and easily digestible) leaves without much risk of breaking the branch. At the same time maintaining a relatively large size has the advantage of allowing sloths to conserve body heat more efficiently than a smaller animal could. Sloths also operate at a much lower normal body temperature than most other mammals in an effort to conserve as much energy as possible.

The need to conserve energy stems from a low-energy diet that consists exclusively of leaves of rain forest trees and vines. No other food in the rain forest is as plentiful or as easy to harvest. And few are as difficult to digest. In fact, the sloth only manages to subsist on leaves because of an association with specialized bacteria that live in the animal's gut and help to break down the cellulose and lignin contained in leaves. Without the digestive assistance of these bacteria, a sloth would starve to death even with a full belly of leaves.

In order to balance their diet, as well as to avoid build up of toxic levels of chemicals that many leaves are laced with, sloths change trees every day or so, and each individual feeds on an average of 25 to 30 different tree species. This knowledge, obtained from sloth studies in Panama, totally dispels the beliefs that a sloth spends its entire life in one tree or that they only eat the leaves of Cecropia trees. Although given their relatively open structure and apparent palatability, cecropias are as good a place as any to start your search for the elusive perezoso.

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