| Pygmy three-toed sloth[1] | |
|---|---|
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Pilosa |
| Family: | Bradypodidae |
| Genus: | Bradypus |
| Species: | B. pygmaeus |
| Binomial name | |
| Bradypus pygmaeus Anderson & Handley 2001 |
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| Pygmy three-toed sloth range | |
The pygmy three-toed sloth (Bradypus pygmaeus) is a three-toed sloth. It is endemic to Isla Escudo de Veraguas, an island off the coast of Panama, which separated from the mainland nearly 8900 years ago.[3] The species is thought to have originated from isolation of individuals from the mainland population of Bradypus variegatus. The population became a distinct species through insular dwarfism on the island. Studies suggest an inverse, linear relationship between mean body sizes and age of the island for island populations of sloths in this region.[4]
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B. pygmaeus has a tan face with orange eye patches. The dorsal side can exhibit either uniform or blotchy color distribution, usually dark brown with an obvious dorsal stripe. The brow is dark brown. B. pygmaeus is unique in that it has long hairs on the crown and the sides of the head, giving it the distinct impression of a hood.[4] Compared to the mainland B. variegatus, B. pygmaeus averages 40% smaller in body mass (2.5–3.5 kg), 15% smaller in total length (excluding tail: 483–530 mm), and 12–16% smaller in cranial dimensions (length: 67.5-72.2 mm; width: 38.8-45.7 mm).[3] The relatively small skull has a large external auditory meatus, narrow squamosal and mandibular process, no foramina in the anterodorsal nasopharynx, a minuscule stylomastoid foramen, and usually lacks foramina for the external carotid artery.[3] The skull and jaw have 18 total teeth, 10 upper and 8 lower. Two upper and two lower teeth are incisor-like. The upper incisor-like teeth are small or may be absent while those in the lower jaw are compressed anteroposteriorly.[3]
Three-toed sloths are arboreal mammals that feed on leaves. Bradypus pygmaeus is unique in that it is found exclusively in the red mangroves and feeds on coarse leaves. Red mangrove leaves are considered a poor nutrient source when compared to the tender leaves of the Cecropia tree eaten by B. variegatus on the mainland.[4]
The smaller size of B. pygmaeus reduces its energy requirements for survival and reproduction. Further studies are needed to better understand how optimal body size relates to metabolism, diet and reproductive success on islands as theories of island dynamics leading to dwarfism in large land mammals and enlargement of small land mammals are still heavily debated.[4]
Predation on B. pygmaeus has not been documented.
Mating, gestation, birth and post-birth dynamics have not been observed for B. pygmaeus, but these features may be inferred from studies of other species in the genus. Individuals reach sexual maturity around 3 years of age and typically give birth after 12 months gestation,[4] although captive bred sloths can give birth as early as 6 months after mating.[5] Mammary glands are found near the armpits of the mother and infants cling to the mothers’ ventral side.[6] Captive-bred young are independent of their mothers around 6 months of age.[5] Some reports suggest that female sloths give birth to a single offspring, but observations of a female B. variegatus in the wild with two infants suggests that they are capable of producing twins.[5] Infant B. variegatus learn from their mother by focusing on her activities and mimicking behaviors.[6] Mother B. variegatus will provide the infant with food and protect them from possible injury by deliberate movements of their body and arms that restrict the infants’ ability to move.[6] B. variegatus males are called to mate by females’ emitting high frequency sounds around the end of the rainy season.[5]
The behavior of B. pygmaeus has not been reported, but can be inferred from behaviors in its close relative, B. variegatus. Two B. variegatus males were observed fighting in the wild by striking one another using their forefeet. This observed dispute probably took place over access to new greenery and fruits in a Cecropia tree. In other cases, disputes between male sloths may be for rights to mate. Captive females will fight for resources.[7] Like other sloths, this sloth is a good swimmer.
Sleep behavior of B. variegatus in the wild contrasts with observed sleeping behavior of captive sloths. Sleep time in the wild is 6 hours less than that of captive sloths, perhaps because wild sloths need to forage and avoid predation although age cannot be ruled out as a possible reason for the discrepancy.[8]
The exact number of pygmy three-toed sloths is not known; its population has presumably always been low due to its restricted range, and it is estimated that there are around 300 left.[9] Individuals of the related B. variegatus are able to sustain themselves in a small area (<2 ha) because they stay in the same tree for ∼ 1.5 days before moving through the canopy to the next tree. However, if the habitat becomes too restrictive due to human encroachment causing gaps in the forest, brown-throated sloths are unable to cross the gaps and face greater extinction risk.[10] This is important to understand considering B. pygmaeus lives on a small island that is potentially threatened by development for tourism. It is listed as critically endangered by the IUCN Red List.[2] The island on which it lives is not inhabited by humans. However, fishermen who visit the island poach the sloth, and it is an easy target, for it only lives in the mangrove forests by the sea. Although protected as a wildlife refuge, the enforcement is lax.[9]
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