Sloth

About the sloth

Sloths are Neotropical mammals that make up the suborder Folivora, a group that includes both the tree-dwelling sloths alive today and a much larger cast of extinct ground sloths, some of which grew to the size of elephants. Living sloths are considered closest kin to anteaters, and together the two groups form the order Pilosa. There are seven living species split between two genera — Bradypus, the three-toed sloths, and Choloepus, the two-toed sloths — and despite the naming, every sloth actually has three toes on each hind limb; only the two-toed group is missing a digit, and only on the forelimbs. The two genera come from separate, distantly related families and are thought to have arrived at similar body plans independently, through parallel evolution from ancestors that lived on the ground.

Sloths spend most of their lives hanging upside down in the rainforest trees of South and Central America. They can run 60 to 80 cm (24 to 31 in) long and weigh anywhere from 3.6 to 7.7 kg (7.9 to 17.0 lb) depending on species, with two-toed sloths running slightly larger than three-toed ones. They have long limbs, rounded heads, and tiny ears; three-toed sloths also carry a stubby 5 to 6 cm (2.0 to 2.4 in) tail. Sloths are unusual among mammals for their neck bones — most mammals have seven cervical vertebrae, but two-toed sloths have five to seven and three-toed sloths have eight or nine, a variation otherwise seen only in manatees.

The shaggy coat itself is a small ecosystem. Its outer hairs grow in the opposite direction from those of most mammals, pointing away from the extremities so they still shed water and cold while the animal hangs upside down. That fur commonly hosts symbiotic green algae, which camouflages the sloth in the canopy, supplies it with nutrients, and in turn feeds sloth moths — some species of which live nowhere else. The fur's ecosystem extends further still, supporting biting and blood-sucking flies, ticks, mites, and a highly specific community of commensal beetles and moths.

Behavior and diet

Sloths move only when they have to, and even then at a crawl. On average they travel about 4 m (13 ft) a minute in the trees, picking up to roughly 4.5 m (15 ft) a minute if a predator forces the issue; on the ground, where they're nearly helpless, their top speed drops to about 3 m (9.8 ft) a minute. That slowness is deliberate: it suits their low-energy leaf diet and helps them go unnoticed by hawks and cats that hunt by sight. Sloth limbs are built for hanging and grasping rather than bearing weight — muscle makes up only 25 to 30 percent of their total body mass, versus 40 to 45 percent in most other mammals — and their long, curved claws let them hang from branches effortlessly. Sloths eat, sleep, and even give birth while hanging, and they sometimes remain suspended from a branch even after death. Wild brown-throated three-toed sloths sleep an average of 9.6 hours a day.

Sloths rely on smell and touch more than sight or hearing, since both senses are relatively poor. Their metabolism runs at roughly half the rate predicted for a placental mammal of their size, with body temperatures that shift with the environment — typically 25 to 35 °C (77 to 95 °F) while active, sometimes dropping as low as 20 °C (68 °F) to induce torpor. That flexible body temperature is paired with strong swimming ability: sloths can paddle across rivers and between islands at speeds up to 13.5 m (44 ft) per minute, and by slowing their heart rate to less than a third of normal, they can hold their breath underwater for up to 40 minutes.

Despite their slow movement, sloths have real strength in their forelimbs. Grip strength in a brown-throated three-toed sloth can equal 99 percent of its body mass on one side alone — well above the grip strength of a black-capped squirrel monkey or common marmoset relative to their own body weight. Their muscle fibers are dominated by slow-twitch, oxidative type I fibers, an adaptation suited to burning energy slowly to match their hanging, low-activity lifestyle.

Range and habitat

Sloths live in the tropical rainforests of South and Central America, spending nearly all their time in the forest canopy rather than on the ground. Both living genera tend to share the same forests, typically with one species of the somewhat smaller three-toed sloth and one species of two-toed sloth predominating together in a given area. Two-toed sloths generally get around better than three-toed sloths when they do need to move between clumps of trees on the ground.

The group's history stretches back roughly 60 million years to South America, where sloths evolved in long isolation before eventually reaching parts of the Caribbean and North America. Ancient sloths were mostly ground-dwelling, and some — like Megatherium — rivaled elephants in size; those larger lineages disappeared around 11,000 years ago, in the same wave of extinction that took most of the Americas' megafauna.

Conservation status

There is no single IUCN Red List status for sloths, because the seven living species are assessed individually rather than as one group.

Historically, sloths' extinct ground-dwelling relatives disappeared following the arrival of humans in the Americas roughly 11,000 years ago, with hunting believed to have contributed alongside climate shifts at the end of the last ice age. A related Caribbean lineage of sloths persisted until about 5,000 years ago before dying out as humans settled those islands.