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, making them perfectly suited for an arboreal lifestyle. They are native to the tropical rainforests of Central and South America. With exceptionally low metabolic rates, they can sleep for up to 20 hours a day.

Equipped with long, curved claws on their hands and feet, they 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, twigs, buds, and fruits. Most sloths feature a thick coat of gray to brown hair, often layered with black-and-white fur, which provides effective camouflage in their leafy habitats.

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, 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, short tails, and long limbs. Their hands and feet feature long, curved claws that allow them to cling firmly to a tree branch and also drag themselves effortlessly along the ground. The arms of three-toed sloths are around 50% longer than their legs.

They have small, nearly invisible ears, while their eyes, despite having cone cells for color vision, provide poor eyesight. Hence, their primary source of sensory reception is through tactile receptors in their skin.

All sloths have three toes on each hindlimb; however, two-toed sloths possess only two digits on each forelimb.

Three-Toed Sloths

They have an outer layer of shaggy, long hair, which is typically pale brown to gray in color and covers a dense coat of black-and-white underfur. However, during the rainy season, these animals may appear greenish due to the algal growth on their bodies. It is due to these algae that the outer hair often has many transverse cracks.

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 other three-toed species, the males possess a large patch or speculum in the middle of their back. This patch reveals a black dorsal midline and a white underfur bordering it.

The maned sloth derives its name from the long black hair on the back of its head and neck, which resembles the mane of a lion.

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 grooves on their hair, which generally harbor the algae.

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

These sloths have five or seven vertebrae on their neck.

Taxonomy

These animals are named so for their low metabolism and sluggish movement since the English term ‘sloth’ literally translates to ‘laziness.’ 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 extant 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 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, recent studies reveal that they had already diverged from other placentals around 100 million years ago. These early xenarthrans had small brains, stubby teeth, tough vertical columns, and fused pelvises.
• 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 common ancestor of the two extant genera existed around 28 million years ago. This ancestor gave rise to the modern-day two- and three-toed sloths 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 they continue to remain excellent swimmers.

Distribution and Habitat

Sloths prefer warm, humid conditions and are abundant within 17° of the equator in the tropical rainforests of Central and South America. These animals account for about 70% of the biomass of arboreal mammals on Barro Colorado Island in Panama.

The brown-throated three-toed sloth is the most common of all extant species of sloths. 

• The pale-throated three-toed sloth is found in northern South America, while the maned three-toed sloth is restricted to the Atlantic Forest of southeastern Brazil.
• The pygmy three-toed sloth is exclusively found on Isla Escudo de Veraguas, an island off the coast of Panama.
• The Linnaeus’s two-toed sloth is found in the Guianas, Colombia, Peru, Ecuador, Venezuela, and Brazil (only in the north of the river Amazon).
• One population of Hoffmann’s two-toed sloth ranges from eastern Honduras in the north to western Ecuador in the south, while another inhabits eastern Peru, western Brazil, and northern Bolivia.

Diet

They depend on their sense of smell and touch to find their food. As the name of their order (Foliovora) suggests, these mammals are folivores, consuming leaves, twigs, fruits, and sap. While three-toed sloths are almost entirely herbivorous, feeding on leaves from only a few trees, two-toed sloths are omnivores and have a diverse diet, including insects, small lizards, and decaying carrion.

Behavior

They are primarily solitary and are found to interact only during the breeding season. While both two-toed and three-toed sloths are primarily nocturnal, the latter remain active even during the day.

• When they do move between branches, they are extremely slow-paced, covering barely 4 m (13 ft) per minute. However, under predatory pressure, the speed increases to about 4.5 m (15 ft) per minute. The highest speed attained by sloths on the ground is 3 m (9.8 ft) per minute.
• They typically remain motionless for about 90% of the time and can sleep up to 20 hours a day, depending on the species. They sleep, eat, and even give birth on tree branches.
• They are surprisingly good swimmers and can easily cross rivers and swim between islands by paddling with their long arms. While swimming, they slow their heart rate to about one-third of its normal pace, allowing them to hold their breath for up to 40 minutes underwater.
• About once a week, sloths descend to the ground 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.
• They produce a range of vocalizations, including high-pitched squeaks, shrieks, and bleats.

Lifespan

Studying sloths in the wild over a prolonged period is extremely challenging, which makes it difficult to calculate their exact lifespan. However, it is estimated that two-toed sloths live for approximately 20 years, 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

These animals may breed throughout the year (for example, the maned three-toed sloths) or seasonally (the pale- and brown-throated three-toed sloths). 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.

The females are promiscuous and mate with multiple males during their estrus phase. To gain access to the females, the males compete with each other by biting each other, swiping their claws, or knocking one another off the branch. The dominant male mates with the female, and the entire mating process lasts barely 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. They occasionally give birth to twins, but the mother can only accommodate one baby, and hence, the weakest twin is dropped.

The offspring, weighing between 10 to 17 oz (300 to 400 gm), are born with their eyes open and have sharp claws and well-developed teeth. They instinctively crawl up to the chest of their mother, clinging to her fur for safety. They feed on their mother’s milk and, in about a week, begin sampling leaves from their mother’s mouth.

The young are weaned in 5 to 6 months and attain sexual maturity in 1 or 2 years. Sometimes, young sloths may die after falling from branches due to their mother’s unwillingness to risk their own safety.

Predators 

Their primary enemies include ocelots, jaguars, snakes, harpy eagles, and hawks. 

Parasites

The algae-covered bodies of many 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 commensal and parasitic arthropods. These include moths, mosquitoes, sandflies, triatomine bugs, lice, beetles, ticks, and mites.

Adaptations

• Sloths have a low metabolism, which contributes to their low resting body temperature (and sluggishness), typically ranging from 25 to 35 °C (77 to 95 °F). However, they are heterothermic and can adjust their body temperature according to the ambient temperature. Their temperature can drop as low as 20 °C (68 °F), causing a temporary state of inactivity or torpor. When they become active, their body temperature rises to almost 30 to 34 °C (86 to 93 °F).
• Since leaves make up the majority of their diet, sloths have large, slow-acting, multi-chambered stomachs. These stomachs can account for up to two-thirds of a well-fed sloth’s body weight and host symbiotic bacteria that help break down the hard-to-digest cellulose in leaves. The entire process of digestion often takes more than a month to complete. 
• Although, in most mammals, hair grows towards the extremities, in sloths, it is quite the opposite. This unique adaptation aligns with their upside-down lifestyle, allowing dirt and water droplets to slide off easily.

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.