Pygmy three-toed sloth

Pygmy three-toed sloth
Conservation status
Critically Endangered (IUCN 3.1)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Pilosa
Family:	Bradypodidae
Genus:	Bradypus
Species:	B. pygmaeus
Binomial name
Bradypus pygmaeus Anderson & Handley 2001
Pygmy three-toed sloth range

The pygmy three-toed sloth (Bradypus pygmaeus), also known as a monk sloth or dwarf sloth, is a small three-toed sloth, endemic to Isla Escudo de Veraguas, a small island off the coast of Panama, which separated from the mainland nearly 8900 years ago. Only described as a separate species in 2001, they are thought to have originated from isolation of individuals of the mainland population of brown-throated three-toed sloths. 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.

Characteristics

Pygmy three-toed sloths have a tan face with a dark brown band across the brow and orange eye patches. The back can exhibit either uniform or blotchy color distribution, but is usually dark brown with an obvious dorsal stripe. Pygmy sloths are unique in that they have long hairs on the crown and the sides of the head, giving the distinct impression of a hood. Compared to the related brown-throated three-toed sloth, the pygmy species is, on average 40% smaller in body mass, weighing 2.5 to 3.5 kg (5.5 to 7.7 lb), and 15% smaller in body length. Adults measure 48 to 53 cm (19 to 21 in), with a tail 4.5 to 6.0 cm (1.8 to 2.4 in) long.

They have a relatively small skull, with a large external auditory meatus, narrow squamosal and mandibular processes, no foramina in the anterodorsal nasopharynx, a minuscule stylomastoid foramen, and usually lack foramina for the external carotid artery. They have 18 teeth, 10 in the upper jaw and 8 in the lower. Two of the teeth in each jaw are incisor-like, although those in the upper jaw are small or may be absent. The incisor-like teeth in the lower jaw are compressed anteroposteriorly. Many of the features found in pygmy sloths are thought to be indicative of a relatively rapid evolution of a new species in an isolated, island habitat.

Pygmy sloths are also 12–16% smaller in cranial dimensions than the mainland species (length: 67.5 to 72.2 mm (2.66 to 2.84 in); width: 38.8 to 45.7 mm (1.53 to 1.80 in).

Ecology

All three-toed sloths are arboreal mammals that feed on leaves; the pygmy sloth is unique in that it is found exclusively in the red mangroves, and feeds on coarse leaves. Red mangrove leaves are a relatively poor source of nutrients, in comparison with the tender leaves of the Cecropia tree eaten by brown-throated sloths on the mainland.

The smaller size of pygmy sloths reduces their energy requirements for survival and reproduction, making them an apparent example of insular dwarfism. No predators of pygmy three-toed sloths have been documented.

Reproduction

Mating, gestation, birth and post-birth dynamics have not been observed for pygmy sloths, but these features may be inferred from studies of other species in the genus.

Individuals of other species reach sexual maturity around three years of age and typically give birth after 12 months gestation, although captive-bred sloths can give birth as early as six months after mating. Mammary glands are found near the armpits of the female, and infants cling to the mothers’ underside. Captive-bred young of other species are independent of their mothers around six months of age. Some reports suggest that female sloths give birth to a single offspring, but observations of a female brown-throated sloth in the wild with two infants suggest that they are capable of producing twins.

Like other sloths, the pygmy sloth is a good swimmer.

Population and threats

A 2012 study found only 70 pygmy three-toed sloths in the mangroves Escudo de Veraguas, as they had been previously reported as only occurring in mangrove thickets and being obligate red mangrove specialists and not folivorous generalists like other Bradypus sloths. The 2012 study also found a much smaller mangrove habitat area (10.7 ha), compared with a 2010 estimate by the IUCN of 130 to 150 ha of mangroves with declining area and quality. While their population has presumably always been low due to their restricted range, the 2012 census found far lower numbers than had been estimated (>500) in 2010. The 2010 estimate has been described as "a rough estimate based on anecdotal evidence", something not clearly stated in the 2010 threat assessment, which has served as an important policy document. Although the island has no human population, the World Conservation Union stated in 2006 that visiting fishermen poach the sloth, which is an easy target because it lives in the mangrove forests by the sea. However, this claim has been neither substantiated nor falsified. People living on Escudo itself and nearby communities claim that they never hunt and eat the sloths. Although protected as a wildlife refuge, the enforcement is lax.

Pygmy sloths are listed as critically endangered by the IUCN Red List.

External links

• EDGE of Existence Saving the World's most Evolutionarily Distinct and Globally Endangered (EDGE) species