Sloth

Sloths are medium-sized mammals best known for their remarkably slow movements, a trait that inspired their name. They are uniquely adapted to life in the trees and are native to the tropical rainforests of Central and South America. With exceptionally low metabolic rates, they spend about 90% of their time resting on trees.

Equipped with long, curved claws on their hands and feet, sloths are often found hanging upside down from the branches of trees. They navigate along the canopy at a leisurely pace of about 40 yards per day. Their diet primarily consists of leaves, but they also consume other plant parts, and even insects, depending on the species. 

They belong to the suborder Folivora, which is part of the order Pilosa, a group that also includes anteaters (suborder Vermilingua). There are six extant species of sloths under two families: Bradypodidae (three-toed sloths) and Choloepodidae (two-toed sloths).

Description

Size

On average, an adult sloth is about 24 to 31 in (60 to 80 cm) long and weighs between 7.9 to 17.0 lb (3.6 to 7.7 kg). However, the now-extinct species Megatherium americanum was one of the largest species of ground sloths to have ever existed, measuring up to 20 ft and weighing around 4 tonnes.

While two-toed sloths are generally larger than their three-toed cousins, the tails of the latter measure about 2.0 to 2.4 in (5 to 6 cm).

Body Plan

Sloths are characterized by their rounded heads, stubby tails, and long, hook-like claws on all four limbs. These claws allow them to latch onto a tree branch and drag themselves on the ground. The arms of three-toed sloths are around 50% longer than their legs, an asymmetry that allows them to suspend themselves from branches with minimal muscular effort. 

All sloths have three toes on each hindlimb. However, while three-toed sloths have three toes on their forelimbs as well, two-toed sloths have only two toes on their forelimbs. This difference in the number of digits of the forelimbs is the primary morphological distinction between the two groups of sloths.

These animals have small, nearly invisible eyes and ears, as well as multiple tactile receptors in their skin. Their eyes provide poor vision, and thus, sloths rely heavily on olfactory and tactile sensory reception.

Three-Toed Sloths

They have a long, shaggy outer coat that is typically pale brown to gray, overlaying a softer, lighter-colored underfur. Most three-toed sloths possess microscopic grooves, also called transverse cracks, that run across the shafts of their outer hair. These cracks support the growth of green algae almost year-round, but especially during the rainy season, when the sloths often appear green due to the increase in algal growth.

These sloths have a perpetually smiling face owing to the coloration and pattern of their short facial hair, as well as facial muscles.

While both sexes look alike in the maned sloth (Bradypus torquatus), in the other three-toed species, the males possess a large patch in the middle of their back. This patch reveals a black dorsal midline and a white underfur bordering it. The maned sloth bears long black hair on the back of its head and neck (resembling the mane of a lion), a feature which gives the species its name.

Unlike most mammals, which typically have seven neck vertebrae, three-toed sloths possess eight or nine. This unique adaptation allows them to rotate their necks up to 270°.

Two-Toed Sloths

Like their three-toed cousins, two-toed sloths also possess an outer layer of thick, long hair, typically gray to brown in color. However, unlike three-toed sloths, they bear longitudinal microscopic algal grooves that run along the length of their body hair.

The fur on the throat of Hoffmann’s two-toed sloth (Choloepus hoffmanni) is pale, while that on Linnaeus’s two-toed sloth (Choloepus didactylus) is dark.

These sloths have five or seven vertebrae on their neck, thereby having less neck flexibility than their three-toed counterparts.

Taxonomy

The term ‘sloth,’ derived from the English word for ‘laziness,’ sheds light upon the sluggish movement and low metabolism of these animals. The name of the suborder, Folivora, comes from Latin, which means leaf-eater.

Sloths belong to the order Pilosa, which also includes the suborder Vermilingua (anteaters). Pilosa is part of the superorder Xenarthra. The six species of sloths are divided into two extant families: Bradypodidae (three-toed sloths) and Choloepodidae (two-toed sloths).

Three-toed sloths belong to the genus Bradypus, which comprises four species: the brown-throated, pale-throated, maned, and pygmy three-toed sloths. Two-toed sloths belong to the genus Choloepus, which includes two species: Linnaeus’s two-toed sloth and Hoffmann’s two-toed sloth.

• Superfamily: Megatherioidea
  • Family: Bradypodidae (Three-toed sloths)
    • Genus: Bradypus
      • Species: Bradypus variegatus (Brown-throated three-toed sloth)
      • Species: Bradypus tridactylus (Pale-throated three-toed sloth)
      • Species: Bradypus torquatus (Maned three-toed sloth)
      • Species: Bradypus pygmaeus (Pygmy three-toed sloth)
  • Family: Megalonychidae (Extinct)
  • Family: Megatheriidae (Extinct)
  • Family: Nothrotheriidae (Extinct)
• Superfamily: Mylodontoidea
  • Family: Choloepodidae (Two-toed sloths)
    • Genus: Choloepus 
      • Species: Choloepus didactylus (Linnaeus’s two-toed sloth)
      • Species: Choloepus hoffmanni (Hoffmann’s two-toed sloth)
  • Family: Mylodontidae (Extinct)
  • Family: Scelidotheriidae (Extinct)
• Family: Megalocnidae (Extinct)

Recent molecular data obtained from the sequencing of mitochondrial DNA and collagen suggest that two-toed sloths are most closely related to mylodontids (family Mylodontidae), whereas three-toed sloths are phylogenetically closest to megatheriids and nothrotheriids (families Megatheriidae and Nothrotheriidae, respectively).

Evolution and Fossil Records

• Although all xenarthrans (an ancient group of placental mammals native to the Americas) are believed to have evolved around 60 million years ago in South America, molecular studies reveal that they had emerged as early as 100 million years ago. Basal xenarthrans, such as Pseudoglyptodon, had small brains, peg-like teeth, stiffened vertical columns, and fused pelvic bones.
• Around 35 million years ago, ground sloths belonging to the family Megalonychidae existed, while around 32 million years ago, the Greater Antilles sloths of the family Megalocnidae thrived. Despite their long existence, both groups eventually faced extinction. The Megalonychidae disappeared approximately 11,000 years ago, whereas the Megalocnidae survived a bit longer and went extinct around 5,000 years ago.
• It is believed that the most recent common ancestor of the two extant genera existed around 28 million years ago, from which the lineages of the modern-day two- and three-toed sloths diverged. These two groups then evolved shared arboreal traits through convergent evolution.
• Having originated in South America, sloths gradually spread to the Caribbean Islands and North America. By the Oligocene Epoch, these animals reached the Greater Antilles, likely through oceanic dispersal.
• Around 9 million years ago, even before the formation of the Isthmus of Panama, members of the megalonychid genus Pliometanastes and those of the mylodontid genus Thinobadistes had colonized North America. Later, with the formation of the isthmus around 3 million years ago, megatheriids and nothrotheriids expanded their range to North America through the Great American Interchange.
• Around this time, members of the nothrotheriid genus Thalassocnus of the west coast of South America gradually adapted to an aquatic lifestyle. In the Late Miocene Epoch, they moved to coastal waters and eventually, in a span of about 4 million years, became expert swimmers, feeding on seagrasses growing on oceanic sediment. However, with time, sloths switched to a completely terrestrial lifestyle, though modern species, especially the three-toed sloths, retain excellent swimming abilities.

Distribution and Habitat

Sloths prefer warm, humid conditions and inhabit tropical rainforests within approximately 17 degrees of the equator, both north and south, across Central and South America. According to a study on Neotropical mammals on Barro Colorado Island, sloths have been estimated to constitute approximately 63% of the total mammalian biomass on this island.

• The brown-throated three-toed sloth is the most common of all extant species of sloths. It ranges from Southern Mexico to Bolivia and northern Brazil.
• The pale-throated three-toed sloth is found in northern parts of South America (typically north of the Amazon River and east of the Andes), while the maned three-toed sloth is restricted to the Atlantic Forest of southeastern Brazil.
• The pygmy three-toed sloth is endemic to Isla Escudo de Veraguas, an island off the coast of Panama.
• The Linnaeus’s two-toed sloth is found in Guyana, Suriname, French Guiana, Colombia, Venezuela, Ecuador, Peru, and Brazil (only in regions north of the Amazon River).
• The Hoffmann’s two-toed sloth has two geographically isolated populations. While one ranges from eastern Honduras to western Ecuador, the other is found in eastern Peru, western Brazil, and northern Bolivia.

Diet

As their suborder name ‘Foliovora’ implies, these mammals are primarily leaf-eaters, with foliage comprising the majority of their diet.

Three-toed sloths mostly consume leaves but may occasionally supplement their diet with buds, flowers, fruits, and tender shoots. In contrast, two-toed sloths are more opportunistic omnivores, supplementing their leaf-based diet with insects, lizards, and even carrion when available.

Behavior

Sloths are primarily solitary animals, coming together only to mate. Two-toed sloths are strictly nocturnal, venturing out only in the dark. However, three-toed sloths are cathemeral, staying active during both day and night, depending on the species. 

• They move extremely slowly between branches, covering barely 4 m (13 ft) per minute. In the presence of a predator, they slightly increase their speed to around 4.5 m (15 ft) per minute. On the ground, they are even slower, dragging themselves along with their claws at a maximum speed of about 3 m (9.8 ft) per minute.
• Sloths spend roughly 90% of their time resting on tree branches. They sleep, eat, and even give birth on tree branches. While sloths typically sleep for about 8 to 10 hours in the wild, some species, like pale-throated three-toed sloths, reportedly sleep for about 16 hours in captivity.
• They are surprisingly good swimmers and can easily cross rivers and swim between islands by paddling with their long arms. 
• They are adept swimmers and can easily cross rivers and swim between islands by paddling with their long arms. Three-toed sloths, in particular, paddle at about 0.81 km/h (0.50 mph), roughly three times their speed on the ground. 
• Most sloths descend from the trees to the ground once a week to urinate and defecate. They keep returning to the same spot for this purpose, digging a hole, expelling their waste, and later covering it with soil. However, the reason why these animals would spend so much energy and make themselves vulnerable to predation remains a mystery. 
• While infants emit high-pitched squeaks or bleats to call out to their mothers, adult sloths may hiss, snort, or growl to alert their conspecifics of danger.

Lifespan

Owing to their reclusive, tree-dwelling lifestyle, sloths are extremely difficult to study in the wild. As a result, their lifespan in natural habitats remains poorly documented.

However, it is estimated that two-toed sloths live for approximately 20 years in captivity, with the oldest living individual, Paula (from a German zoo), recorded to survive for 50 years. In three-toed sloths, the longest-lived individual, Buttercup, reached 27 years at a sloth sanctuary in Costa Rica.

Reproduction and Life Cycle

While two-toed sloths typically breed year-round, some three-toed species, like the brown-throated three-toed sloth, breed seasonally, particularly during the dry season from March to April.

In three-toed sloths, the females emit high-pitched sounds to attract the males, while two-toed sloths do not vocalize at all. Instead, they rub their anal glands against trees to scent-mark and assert their receptivity.

In some species, like the brown-throated three-toed sloth, females are promiscuous and mate with multiple males during their estrus phase. To gain access to the females, the males compete by biting each other, swiping their claws, or knocking one another off the branch. The dominant male mates with the female, and copulation typically lasts less than a minute. When the female is ready to give birth, she descends from a higher canopy to a lower branch. 

While three-toed species have a gestation period of 6 months, in two-toed sloths, it extends up to 12 months, after which a single offspring is born. Though extremely rare, they occasionally give birth to twins, but the mother can only accommodate one baby, and thus, the weakest twin is dropped.

The offspring, weighing between 10 to 17 oz (300 to 400 g), are born with their eyes open and have sharp claws and well-developed teeth. They crawl up to their mother’s chest, clinging to her fur for safety. They initially feed on their mother’s milk but, within a week, begin to sample leaves directly from their mother’s mouth to learn their feeding preferences.

The young are weaned in 5 to 6 months and attain sexual maturity in 1 or 2 years. Sometimes, young sloths fall from the branches and die, as their mothers rarely take the risk to descend and retrieve them.

Predators 

While in the trees, sloths are preyed upon by harpy eagles, spectacled owls, and semi-arboreal snakes such as boa constrictors. On the ground, they face threats from ocelots, pumas, and jaguars. 

Parasites

The bodies of several well-studied sloth species, including the pale-throated three-toed sloth, the brown-throated three-toed sloth, and Linnaeus’s two-toed sloth, serve as hosts to numerous arthropods, like moths, mosquitoes, sandflies, triatomine bugs, lice, beetles, ticks, and mites.

Adaptations

• Sloths have a low metabolism, which contributes to their low resting body temperature, typically ranging from 25 to 35 °C (77 to 95 °F), and reduced activity. However, as heterothermic animals, they can adjust their body temperature according to the ambient conditions, with body temperatures occasionally dropping as low as 20 °C (68 °F), which may induce a temporary state of inactivity or torpor. When they become active again, their body temperature rises to almost 30 to 34 °C (86 to 93 °F).
• Since leaves constitute the bulk of their diet, sloths have large, slow-acting, multi-chambered stomachs. These stomachs, along with the ingested food, can account for between 25% and 67% of a well-fed sloth’s body weight (depending on the intake) and host symbiotic bacteria that break down the hard-to-digest cellulose in leaves. The digestion is exceptionally slow, taking around 20 to 30 days in two-toed sloths and extending up to 50 days in three-toed sloths.
• Unlike most mammals, whose hair grows toward the extremities, sloths exhibit a reversed hair growth pattern. The hair on their belly points upward toward the spine, while that on the limbs directs toward the shoulders and hips rather than the paws or tail. This alignment helps dirt and rainwater slide off easily when the sloth hangs upside down in the trees.
• While swimming, some three-toed sloths experience a drop in their heart rate (diving bradycardia) by about one-third of their normal pace. This adaptation helps them conserve oxygen, allowing them to hold their breath for up to 40 minutes underwater.

Conservation Status

According to the IUCN Red List, the pygmy three-toed sloth is considered Critically Endangered (CR), while the maned three-toed sloth is Vulnerable (VU). The primary causes of the decline in their numbers include deforestation, electrocution from power lines, and trafficking.

Interesting Facts

1. Unlike most mammals, whose muscle mass typically accounts for 40 to 45% of their body weight, sloths have a significantly lower muscle mass, comprising only 25 to 30% of their weight.
2. International Sloth Day is celebrated on the 20th of October every year to raise awareness about these mammals and shed light on the challenges they face in the wild.