[3] The two groups of sloths are from different, distantly related families, and are thought to have evolved their morphology via parallel evolution from terrestrial ancestors.

Members of an endemic radiation of Caribbean sloths also formerly lived in the Greater Antilles but became extinct after humans settled the archipelago in the mid-Holocene, around 6,000 years ago.

Sloth, related to slow, literally means "laziness", and their common names in several other languages (e.g. German: Faultier, French: paresseux, Spanish: perezoso, Portuguese: preguiça, Romanian: leneș, Finnish: laiskiainen) also mean "lazy" or similar.

Their slowness permits their low-energy diet of leaves and avoids detection by predatory hawks and cats that hunt by sight.

[4] The shaggy coat has grooved hair that is host to symbiotic green algae which camouflage the animal in the trees and provide it nutrients.

The earliest xenarthrans were arboreal herbivores with sturdy vertebral columns, fused pelvises, stubby teeth, and small brains.

The latter development, about 3 million years ago, allowed megatheriids and nothrotheriids to also invade North America as part of the Great American Interchange.

Additionally, the nothrotheriid Thalassocnus of the west coast of South America became adapted to a semiaquatic and, eventually, perhaps fully aquatic marine lifestyle.

They presumably waded and paddled in the water for short period, but over a span of 4 million years, they eventually evolved into swimming creatures, becoming specialist bottom feeders of seagrasses, similar to the extant sirenians.

[16] Though data has been collected on over 33 different species of sloths by analyzing bone structures, many of the relationships between clades on a phylogenetic tree were unclear.

[20] Ground sloths disappeared from both North and South America shortly after the appearance of humans about 11,000 years ago.

Ground sloth remains found in both North and South America indicate that they were killed, cooked, and eaten by humans.

[24] Brown-throathed and Hoffman's two-toed sloths use their forelimbs as their principal means of propulsion and their skeletal muscle have very high proportions of oxidative slow twitch (Type I) muscle fibers, with high activity of the anaerobic enzyme CK compared to their other metabolic enzymes despite this.

This would be a product of their lower field metabolic rate than other nonhibernating mammals as well suspensory lifestyle and this also would to explain their slow speed of movement.

In most conditions, the fur hosts symbiotic algae, which provide camouflage[26] from predatory jaguars, ocelots,[27] and harpy eagles.

These include biting and blood-sucking flies such as mosquitoes and sandflies, triatomine bugs, lice, ticks and mites.

There are at least five hypotheses: 1) fertilize trees when feces are deposited at the base of the tree;[42] 2) cover feces and avoid predation;[43][44][45] 3) chemical communication between individuals;[46] 4) pick up trace nutrients in their claws, that are then ingested;[47] and 5) favor a mutualistic relationship with populations of fur moths.

Two-toed sloths are omnivorous, with a diverse diet of insects, carrion, fruits, leaves and small lizards, ranging over up to 140 hectares (350 acres).

Three-toed sloths, on the other hand, are almost entirely herbivorous (plant eaters), with a limited diet of leaves from only a few trees,[40] and no other mammal digests its food as slowly.

[40] As much as two-thirds of a well-fed sloth's body weight consists of the contents of its stomach, and the digestive process can take a month or more to complete.

Considering the large energy expenditure and dangers involved in the journey to the ground, this behaviour has been described as a mystery.

[56][57] The average lifespan of two-toed sloths in the wild is currently unknown due to a lack of full-lifespan studies in a natural environment.

[59] Although habitat is limited to the tropical rainforests of Central and South America, in that environment sloths are successful.

On Barro Colorado Island in Panama, sloths have been estimated to constitute 70% of the biomass of arboreal mammals.

Sloths' lower metabolism confines them to the tropics, and they adopt thermoregulation behaviors of cold-blooded animals such as sunning themselves.

[62] The majority of recorded sloth deaths in Costa Rica are due to contact with electrical lines and poachers.