Sloth

The suborder Folivora encompasses sloths, a Neotropical assemblage of xenarthran mammals, which includes both extant arboreal tree sloths and extinct terrestrial ground sloths. Characterized by their deliberate locomotion, arboreal sloths inhabit the tropical rainforests of South and Central America, where they spend the majority of their existence suspended inverted from trees. Within the xenarthran order Pilosa, sloths are recognized as the closest relatives of anteaters.

Sloths are a Neotropical group of xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their lives hanging upside down in the trees of the tropical rainforests of South America and Central America. Sloths are considered to be most closely related to anteaters, together making up the xenarthran order Pilosa.

Currently, six extant sloth species are recognized, distributed across two genera: Bradypus (three-toed sloths) and Choloepus (two-toed sloths). Notwithstanding this conventional nomenclature, all sloth species possess three digits on each hind limb; however, two-toed sloths are distinguished by having only two digits on each forelimb. These two distinct sloth lineages originate from separate, distantly related families, and their shared morphology is hypothesized to have arisen through parallel evolution from terrestrial progenitors. Beyond the extant species, numerous ground sloth species, some attaining the colossal size of elephants (e.g., Megatherium), populated both North and South America throughout the Pleistocene Epoch. Nevertheless, these megafaunal sloths underwent extinction during the Quaternary extinction event approximately 12,000 years ago, coinciding with the disappearance of most large fauna across the Americas. While this extinction event temporally correlates with human arrival, climatic shifts have also been proposed as a contributing factor. Furthermore, an endemic radiation of Caribbean sloths previously inhabited the Greater Antilles, but these species vanished following human colonization of the archipelago during the mid-Holocene, approximately 6,000 years ago.

The nomenclature 'sloth' derives from their exceptionally low metabolic rate and characteristically deliberate movements. The term sloth, etymologically linked to slow, literally translates to 'laziness,' a connotation reflected in their common names across various languages (e.g., German: Faultier, French: paresseux, Spanish: perezoso, Portuguese: preguiça, Romanian: leneș, Finnish: laiskiainen), which similarly denote 'lazy' or analogous concepts. This reduced pace facilitates their folivorous, low-energy diet and serves as a defense mechanism against detection by visually-oriented predators such as hawks and felines. While largely incapacitated on terrestrial surfaces, sloths exhibit proficiency in swimming. Their shaggy pelage features grooved hairs that host symbiotic green algae, which not only provide camouflage within arboreal environments but also supply the animal with supplementary nutrients. These algae additionally sustain sloth moths, certain species of which are obligate symbionts exclusively found on sloths.

Taxonomy and Evolutionary History

Sloths are classified within the superorder Xenarthra, a clade of placental mammals hypothesized to have originated in South America approximately 60 million years ago. A particular study indicates that xenarthrans diverged from other placental mammalian lineages approximately 100 million years ago. This superorder also encompasses anteaters and armadillos. Early xenarthrans were characterized as arboreal herbivores, possessing robust vertebral columns, fused pelves, rudimentary dentition, and relatively small brains. Taxonomically, sloths are assigned to the suborder Folivora, which is part of the order Pilosa. These designations are derived from the Latin terms 'leaf eater' and 'hairy,' respectively. Pilosa represents one of the most diminutive orders within the Mammalia class; its sole other suborder comprises the anteaters.

The suborder Folivora is categorized into a minimum of eight families; however, only two of these families contain extant species, with the remaining lineages being entirely extinct (†):

• †Megalocnidae: This family represents the Greater Antilles sloths, a basal lineage that emerged approximately 32 million years ago and became extinct around 5,000 years ago.
• Superfamily Megatherioidea
  • Bradypodidae, comprising the three-toed sloths, includes four extant species:
    • The brown-throated three-toed sloth (Bradypus variegatus) stands as the most prevalent extant sloth species, inhabiting the Neotropical forests of South and Central America.
    • The pale-throated three-toed sloth (Bradypus tridactylus) is found in the tropical rainforests of northern South America. This species bears a strong morphological resemblance to, and is frequently misidentified as, the brown-throated three-toed sloth, which possesses a considerably broader geographical range. Genetic analyses suggest that these two species diverged approximately six million years ago.
    • The maned three-toed sloth (Bradypus torquatus) is currently restricted to the Atlantic Forest biome in southeastern Brazil.
    • The critically endangered pygmy three-toed sloth (Bradypus pygmaeus) is an endemic species, exclusively inhabiting the diminutive Isla Escudo de Veraguas, situated off the Panamanian coast.
  • The extinct Megalonychidae family comprised ground sloths that thrived for approximately 35 million years before their extinction around 11,000 years ago. Historically, this taxonomic group was believed to encompass both extant two-toed sloths and the extinct sloths of the Greater Antilles.
  • The Megatheriidae, an extinct family of ground sloths, persisted for approximately 23 million years, becoming extinct around 11,000 years ago. This family is notable for containing the largest known sloth species.
  • The extinct Nothrotheriidae family consisted of ground sloths that inhabited Earth from roughly 11.6 million to 11,000 years ago. In addition to terrestrial forms, this family also included Thalassocnus, a genus characterized by either semiaquatic or fully aquatic adaptations.
• The superfamily Mylodontoidea encompasses
  • Choloepodidae, the family of two-toed sloths, which currently includes two extant species. Among these is
    • Linnaeus's two-toed sloth (Choloepus didactylus), indigenous to Venezuela, the Guianas, Colombia, Ecuador, Peru, and the Brazilian territories situated north of the Amazon River.
    • Hoffmann's two-toed sloth (Choloepus hoffmanni) resides in tropical forest environments. Its geographical distribution is disjunct, characterized by two distinct ranges separated by the Andes mountain range. One population extends from eastern Honduras southward to western Ecuador, while the other occupies eastern Peru, western Brazil, and northern Bolivia.
  • The extinct Mylodontidae family comprised ground sloths that were present for approximately 23 million years, ultimately disappearing around 11,000 years ago.
  • Collagen sequence data suggests that the extinct Scelidotheriidae group is phylogenetically more distant from Mylodon than Choloepus. Consequently, this group has been re-elevated to the taxonomic rank of a full family.

Evolution

The most recent common ancestor of the two extant sloth genera is estimated to have lived approximately 28 million years ago. The morphological resemblances between two-toed and three-toed sloths exemplify convergent evolution towards an arboreal lifestyle, representing "one of the most striking examples of convergent evolution known among mammals." Ancient Xenarthra exhibited a considerably greater diversity of species and a broader geographical distribution compared to their modern counterparts. Historically, sloths were predominantly terrestrial, with some species, such as Megatherium, attaining sizes comparable to elephants.

Sloths originated in South America during an extended period of geographical isolation, subsequently dispersing to several Caribbean islands and North America. It is hypothesized that oceanic dispersal, facilitated by swimming, allowed pilosans to reach the Greater Antilles by the Oligocene epoch. Furthermore, the megalonychid Pliometanastes and the mylodontid Thinobadistes are believed to have colonized North America approximately 9 million years ago, predating the formation of the Isthmus of Panama. The emergence of the Isthmus about 3 million years ago subsequently enabled megatheriids and nothrotheriids to also migrate into North America as participants in the Great American Interchange. Concurrently, the nothrotheriid Thalassocnus, native to the west coast of South America, adapted to a semiaquatic and, ultimately, potentially fully aquatic marine existence. In the regions of Peru and Chile, Thalassocnus began inhabiting coastal environments during the late Miocene. Initially, these sloths presumably waded and briefly paddled in water; however, over a period of 4 million years, they evolved into proficient swimmers, becoming specialized bottom feeders of seagrasses, analogous to modern sirenians.

Both extant types of arboreal sloths frequently cohabit the same forest ecosystems. In most regions, a specific species of the comparatively smaller and typically slower-moving three-toed sloth (Bradypus) and a single species of the two-toed variety will collectively dominate. Historically, morphological comparisons led to the hypothesis that two-toed sloths were phylogenetically nested within one of the lineages of the extinct Greater Antilles sloths. Despite the accumulation of data from bone structure analysis across more than 33 different sloth species, numerous inter-clade relationships within the phylogenetic tree remained ambiguous. A significant portion of the morphological evidence supporting the hypothesis of diphyly has been derived from studies of inner ear structures.

Recent molecular analyses, utilizing collagen and mitochondrial DNA sequences, support the diphyly (convergent evolution) hypothesis, yet have challenged certain morphological inferences. These investigations consistently position two-toed sloths proximal to mylodontids and three-toed sloths within the Megatherioidea clade, alongside Megalonyx, megatheriids, and nothrotheriids. Consequently, the formerly recognized family Megalonychidae is rendered polyphyletic, as both two-toed sloths and Greater Antilles sloths are now phylogenetically separated from Megalonyx. Greater Antilles sloths are currently classified within a distinct, basal lineage of the sloth phylogenetic tree.

Phylogeny

The subsequent phylogenetic tree illustrating sloth family relationships is derived from collagen and mitochondrial DNA sequence analyses.

Extinctions

Marine sloths inhabiting the Pacific coast of South America faced extinction at the close of the Pliocene epoch. This event coincided with the closure of the Central American Seaway, which induced a cooling trend in coastal waters. This cooling led to the demise of extensive seagrass beds, a primary food source, and likely exacerbated thermoregulation challenges for these sloths, given their inherently slow metabolic rates.

Terrestrial sloths vanished from both North and South America approximately 11,000 years ago, shortly after the arrival of human populations. Empirical evidence suggests that human predation was a contributing factor to the extinction of the American megafauna. Fossilized remains of ground sloths discovered across both continents indicate their consumption by humans, evidenced by signs of hunting and cooking. Concurrently, climatic shifts associated with the termination of the last glacial period might have also played a role, although prior interglacial transitions did not correlate with comparable extinction magnitudes.

Megalocnus and several other Caribbean sloth species persisted until approximately 5,000 years ago, significantly later than the mainland extinction of ground sloths. Their eventual disappearance coincided with the human colonization of the Greater Antilles.

Biology

Morphology and anatomy

Sloths typically measure between 60 and 80 cm (24 to 31 in) in length and exhibit a species-dependent weight range of 3.6 to 7.7 kg (7.9 to 17.0 lb). Generally, two-toed sloths are marginally larger than their three-toed counterparts. Characteristically, sloths possess elongated limbs and spherical crania adorned with diminutive ears. Three-toed sloths are further distinguished by a vestigial tail, approximately 5 to 6 cm (2.0 to 2.4 in) in length.

Sloths represent an anomaly among mammalian species due to their deviation from the typical count of seven cervical vertebrae. Two-toed sloths possess between five and seven, whereas three-toed sloths exhibit eight or nine. Manatees are the only other mammals known to deviate from the septal cervical count, typically having six.

Physiology

Sloths possess color vision but exhibit limited visual acuity and impaired auditory perception. Consequently, they primarily depend on olfaction and tactile senses for foraging.

Sloths are characterized by exceptionally low metabolic rates, often less than half of what is anticipated for mammals of comparable size, and correspondingly low body temperatures. During periods of activity, their temperature ranges from 30 to 34 °C (86 to 93 °F), decreasing further during rest. As heterothermic organisms, their body temperature fluctuates with environmental conditions, typically spanning 25 to 35 °C (77 to 95 °F), with the capacity to descend to 20 °C (68 °F), thereby inducing a state of torpor.

Brown-throated and Hoffman's two-toed sloths primarily utilize their forelimbs for locomotion. Their skeletal muscles exhibit a high proportion of oxidative slow-twitch (Type I) fibers. Paradoxically, despite this, they demonstrate elevated activity of the anaerobic enzyme creatine kinase (CK) relative to other metabolic enzymes, although overall CK activity in sloths is lower compared to other animal species. The prevalence of these muscle fiber types is hypothesized to be an adaptation for slower energy consumption. Concurrently, their predominantly anaerobic muscle metabolism, particularly ATP production via the CK pathway, may facilitate faster and more economical energy generation. These physiological characteristics are likely a consequence of their lower field metabolic rate compared to other non-hibernating mammals, coupled with their arboreal, suspensory lifestyle, which collectively accounts for their characteristically slow movement. In the brown-throated sloth, forelimb muscle mass constitutes merely 5.1 percent of its total body weight.

Sloth fur exhibits a unique growth pattern, with its outer hairs developing in a direction contrary to that observed in most other mammalian species. While typical mammalian hair growth is directed towards the extremities, sloths' fur grows away from these points. This adaptation is crucial for elemental protection, given their prolonged inverted posture with limbs positioned above their bodies. Under typical environmental conditions, the fur serves as a host for symbiotic algae, which confers camouflage against predators such as jaguars, ocelots, and harpy eagles. Consequently, the algal presence transforms sloth fur into a micro-ecosystem, supporting numerous species of both commensal and parasitic arthropods. A substantial diversity of arthropod species is associated with sloths. This assemblage encompasses biting and hematophagous flies, including mosquitoes and sandflies, alongside triatomine bugs, lice, ticks, and mites. Furthermore, sloths harbor a highly specialized community of commensal beetles, mites, and moths. Documented sloth species known to host these arthropods include the pale-throated three-toed sloth, the brown-throated three-toed sloth, and Linnaeus's two-toed sloth. The mutualistic relationship with moths benefits sloths, as these insects facilitate the fertilization of the algae on the sloth's fur, thereby supplying the sloth with essential nutrients.

Activity

Sloth limbs are specifically adapted for arboreal suspension and prehension, rather than for terrestrial weight bearing. Their muscle mass constitutes only 25 to 30 percent of their total body weight. In contrast, the majority of other mammalian species possess a muscle mass ranging from 40 to 45 percent of their total body weight. Specialized hands and feet, equipped with long, curved claws, enable effortless suspension from branches and facilitate dragging movements across the ground, as bipedal locomotion is not possible for them. In three-toed sloths, the forelimbs are notably 50 percent longer than the hindlimbs.

Sloths exhibit movement only when essential, and even then, at an exceptionally slow pace. Typically, their average speed is 4 meters (13 feet) per minute; however, they can attain a slightly increased speed of 4.5 meters (15 feet) per minute when confronted with immediate predatory threats. Although occasionally observed resting atop branches, sloths predominantly engage in feeding, sleeping, and even parturition while suspended from branches. Instances have been recorded where sloths remain suspended from branches post-mortem. Terrestrially, sloths achieve a maximum speed of 3 meters (9.8 feet) per minute. Two-toed sloths generally demonstrate a greater capacity for terrestrial dispersal between arboreal clusters compared to three-toed sloths.

Remarkably, sloths are proficient swimmers, capable of achieving speeds up to 13.5 meters (44 feet) per minute. Utilizing their elongated forelimbs for propulsion, they are able to traverse rivers and navigate between islands. Sloths possess the physiological capacity to further depress their already low metabolic rate and reduce their heart rate to less than one-third of its typical rhythm, enabling them to sustain breath-holding underwater for durations up to 40 minutes.

Wild brown-throated three-toed sloths exhibit an average daily sleep duration of 9.6 hours. Two-toed sloths are characterized by nocturnal activity patterns. While predominantly nocturnal, three-toed sloths may also exhibit diurnal activity. A significant 90 percent of their time is spent in a state of immobility.

Behavior

Sloths are primarily solitary creatures, engaging in inter-individual interactions infrequently, typically only during the breeding season. However, female sloths occasionally form congregations, a behavior observed more often than among males.

Approximately once every eight days, sloths descend to the ground to defecate. The underlying reasons and mechanisms for this behavior have been subjects of extensive scientific debate. At least five hypotheses have been proposed:

1. One hypothesis suggests that this behavior serves to fertilize trees when feces are deposited at the tree's base.
2. Another hypothesis posits that it functions to conceal feces, thereby mitigating predation risk.
3. A third hypothesis proposes that it facilitates chemical communication among individuals.
4. A fourth hypothesis suggests it allows for the collection of trace nutrients in their claws, which are subsequently ingested.
5. The fifth hypothesis proposes that it promotes a mutualistic relationship with populations of fur moths.

A more recent hypothesis has emerged, offering counter-evidence to the preceding theories. This new perspective suggests that all extant sloth species are descended from ancestors that practiced terrestrial defecation, and that insufficient selective pressure has existed to prompt the abandonment of this behavior, given the infrequent occurrence of predation during defecation events.

Diet

Infant sloths acquire knowledge of their diet through the practice of licking their mother's lips. The leaves of Cecropia constitute a dietary component for all sloth species.

Two-toed sloths exhibit an omnivorous diet, consuming a varied array of insects, carrion, fruits, leaves, and small lizards, and typically range across territories up to 140 hectares (350 acres). Conversely, three-toed sloths are predominantly herbivorous, subsisting on a restricted diet primarily composed of leaves from a limited selection of tree species. Notably, no other mammal exhibits a slower digestive process.

Sloths have developed specific adaptations for an arboreal browsing diet. Their primary food source, leaves, offers minimal energy and nutrients and is challenging to digest. Consequently, sloths possess large, multi-chambered stomachs with a slow digestive process, where symbiotic bacteria facilitate the breakdown of fibrous plant material. The stomach contents can account for up to two-thirds of a well-nourished sloth's body weight, and digestion may extend for a month or longer.

Three-toed sloths descend to the ground approximately once a week to urinate and defecate, meticulously digging and covering their waste. This consistent use of the same location renders them highly susceptible to predation during these excursions. Given the significant energy expenditure and inherent risks associated with descending from the trees, this behavior has long been considered enigmatic. However, recent investigations indicate a complex interaction involving moths that inhabit the sloth's fur. These moths deposit their eggs in the sloth's feces; upon hatching, the larvae consume the waste, and mature moths subsequently ascend to the sloth. This relationship may be symbiotic, as the moths residing in the fur are thought to encourage the growth of algae, a dietary component for sloths. Furthermore, individual sloths typically concentrate their feeding on a specific "modal" tree, and by burying their excreta near its trunk, they might contribute to its nourishment.

Reproduction

Reproductive patterns vary among sloth species: pale-throated and brown-throated three-toed sloths exhibit seasonal mating, whereas the maned three-toed sloth reproduces year-round. The reproductive biology of pygmy three-toed sloths remains undocumented. Gestation periods result in a single offspring, lasting approximately six months for three-toed species and twelve months for two-toed species. Newborns typically remain with their mothers for about five months. Instances of juvenile mortality due to falls have been observed, sometimes attributed to mothers' reluctance to descend from arboreal safety to retrieve their young. While females generally produce one offspring annually, their characteristically low mobility can occasionally extend the interval between successful matings beyond a year. Sloths exhibit minimal sexual dimorphism, a factor that has led to misidentification of sex in several zoological collections.

The precise average lifespan of two-toed sloths in their natural habitat is presently undetermined, owing to the absence of comprehensive longitudinal studies in wild populations. In managed care, the median life expectancy is approximately 16 years, with one notable individual at the Smithsonian Institution's National Zoo living to 49 years.

Distribution

Despite their habitat being restricted to the tropical rainforests of Central and South America, sloths thrive within this specific ecological niche. For instance, on Panama's Barro Colorado Island, sloths are estimated to comprise 70 percent of the total biomass of arboreal mammals. Of the six extant species, four are presently categorized as "least concern." However, the maned three-toed sloth (Bradypus torquatus), endemic to Brazil's diminishing Atlantic Forest, is classified as "vulnerable," and the insular pygmy three-toed sloth (B. pygmaeus) faces critical endangerment. The inherently low metabolic rate of sloths restricts their geographical range to tropical regions, compelling them to employ poikilothermic thermoregulatory strategies, such as basking in the sun.

Human relations

In Costa Rica, the predominant causes of documented sloth fatalities include electrocution from power lines and poaching. Furthermore, their specialized claws offer an unanticipated defense against human hunters; when suspended inverted from a tree, sloths are securely anchored by their claws and frequently remain attached even after being shot.

Sloths are frequently subjected to illegal wildlife trafficking, being sold as exotic pets. Nevertheless, their highly specialized ecological requirements render them unsuitable as companion animals.

References

Rauch, Alan. Sloth (Reaktion Books, 2023). Online review of this publication.

• Rauch, Alan. Sloth (Reaktion Books, 2023) Online review of this book.

The dictionary definition of sloth.

• The dictionary definition of sloth at Wiktionary