Pygmy Three-toed Sloth
Encyclopedia
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. 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.
, narrow squamosal
and mandibular process, no foramina in the anterodorsal nasopharynx
, a minuscule stylomastoid foramen
, and usually lacks foramina for the external carotid artery
. 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.
tree eaten by B. variegatus on the mainland.
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.
Predation on B. pygmaeus has not been documented.
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.
. 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.
Three-toed sloth
The three-toed sloths are tree-living mammals from South and Central America. They are the only members of the genus Bradypus and the family Bradypodidae. There are four living species of three-toed sloths...
. It is endemic to Isla Escudo de Veraguas
Isla Escudo de Veraguas
Isla Escudo de Veraguas is a small isolated Caribbean island of the Republic of Panama. Despite its name, it is not part of the province of Veraguas, but rather Bocas del Toro...
, an island off the coast of Panama
Panama
Panama , officially the Republic of Panama , is the southernmost country of Central America. Situated on the isthmus connecting North and South America, it is bordered by Costa Rica to the northwest, Colombia to the southeast, the Caribbean Sea to the north and the Pacific Ocean to the south. The...
, which separated from the mainland nearly 8900 years ago. 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
Insular dwarfism
Insular dwarfism, a form of phyletic dwarfism, is the process and condition of the reduction in size of large animals – typically mammals – when their population's range is limited to a small environment, primarily islands. This natural process is distinct from the intentional creation of dwarf...
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.
Morphology
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. 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). The relatively small skull has a large external auditory meatusAuditory meatus
Auditory meatus can refer to:* external auditory meatus* internal auditory meatus...
, narrow squamosal
Squamosal
The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...
and mandibular process, no foramina in the anterodorsal nasopharynx
Nasopharynx
The nasopharynx is the uppermost part of the pharynx. It extends from the base of the skull to the upper surface of the soft palate; it differs from the oral and laryngeal parts of the pharynx in that its cavity always remains patent .-Lateral:On its lateral wall is the pharyngeal ostium of the...
, a minuscule stylomastoid foramen
Stylomastoid foramen
Between the styloid and mastoid processes of the temporal bone is the stylomastoid foramenIt is the termination of the facial canal, and transmits the facial nerve and stylomastoid artery.- Clinical Relevance :...
, and usually lacks foramina for the external carotid artery
External carotid artery
In human anatomy, the external carotid artery is a major artery of the head and neck. It arises from the common carotid artery when it bifurcates into the external and internal carotid artery.-Course:...
. 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.
Ecology
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 CecropiaCecropia
Cecropia is a Neotropical genus presently consisting of sixty-one recognized species with a highly distinctive lineage of dioecious trees....
tree eaten by B. variegatus on the mainland.
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
Dwarfism
Dwarfism is short stature resulting from a medical condition. It is sometimes defined as an adult height of less than 4 feet 10 inches , although this definition is problematic because short stature in itself is not a disorder....
in large land mammals and enlargement
Island gigantism
Island gigantism or insular giantism is a biological phenomenon in which the size of animals isolated on an island increases dramatically in comparison to their mainland relatives....
of small land mammals are still heavily debated.
Predation on B. pygmaeus has not been documented.
Reproduction
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, although captive bred sloths can give birth as early as 6 months after mating. Mammary glands are found near the armpits of the mother and infants cling to the mothers’ ventral side. Captive-bred young are independent of their mothers around 6 months of age. 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. Infant B. variegatus learn from their mother by focusing on her activities and mimicking behaviors. 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. B. variegatus males are called to mate by females’ emitting high frequency sounds around the end of the rainy season.Behavior
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. 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.
Population and threats
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. 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. 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 ListIUCN Red List
The IUCN Red List of Threatened Species , founded in 1963, is the world's most comprehensive inventory of the global conservation status of biological species. The International Union for Conservation of Nature is the world's main authority on the conservation status of species...
. 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.