Shades of Green: Earth's Forests -- Sloths

There can be as many sloths as eight per hectare, which is an extremely high density for such a large, solitary mammal.

Sloths are some of the most arboreal of mammals, spending almost their entire lives in the trees. There are five species total of the "three-toed" and "two-toed" sloths, all inhabiting the tropical forests of Central and South America. Sloths are very hard to see, simply because they move so rarely, and when they do move it is very slowly indeed -- they have been known to take as long as a day in the process of moving from one tree to the next.

A mother sloth and a baby hang upside-down in a tree. Photo courtesy Naomi Woods.

The sloth’s adaptations for life in the forest canopy have only been recently studied. Sloths are almost incapable of moving, or even standing on the ground. They can’t stand upright because their feet are essentially large hooks: their toes have long, curved claws which are perfect for hanging from tree branches, but are useless for standing on. In their forest habitat, however, their hook-like claws provide a far more reliable grip on tree branches than the grasp of a normal "fist".

Sloths' limbs are also designed to suspend the animal's body upside-down from horizontal tree limbs, rather than supporting it right-side-up. These animals are, however, limited to habitats with plenty of thin branches and vines, because their claws can't grasp thicker trees. They move, slowly, through the canopy on networks of liana vines, rather than descending to the ground and then climbing a different tree.

The sloth's apparent laziness is because of its diet. Sloths eat only leaves, which are naturally low in energy and hard to digest. Their seemingly lethargic movements help them conserve the little energy they do get. Their metabolism is a superb adaptation for a leaf eater with such a low energy diet. Their metabolic rate is only half that of similarly sized mammals.

When the sloth is sleeping, its body temperature drops to conserve energy, falling to well below 30 degrees Celsius (a human's body temperature is 37 degrees Celsius, and those of smaller mammals are usually even higher). The sloth's strange appearance and even more unusual behaviors are all adaptations to deal with its low energy diet.

Unlike most mammals living in the warm, humid tropical forests, sloths also have adaptations to stay warm! They are covered in dense hair-like fur. This fur also serves as effective camouflage, because each individual hair contains a groove which holds blue-green algae.

Sloths are not social mammals, and indeed seem to communicate very little with one another. Their main vocalizations are whistle-like sounds, but these are not often heard, perhaps because sloths avoid any behavior that could attract attention to themselves.

During the rainy season, this algae population in a sloth's fur multiplies, giving the sloth a mossy-green color; in the dry season, there is less algae and the sloths turn a more brownish color. In the mornings, sloths can be found hanging from branches high in the forest canopy, sunbathing. Sloths sleep in compact, ball-like positions to conserve heat.

Once spotted by a predator, sloths have few defenses -- certainly they can't escape with any speed! A sloth's sharp claws do offer some protection however, and since they spend most of their time out on thin branches, they are usually out of reach of most predators.

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