The pygmy three-toed sloth (''Bradypus pygmaeus''), also known as the monk sloth or dwarf sloth, is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

sloth Sloths are a Neotropical realm, Neotropical group of xenarthran mammals constituting the suborder Folivora, including the extant Arboreal locomotion, arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of move ...

in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Bradypodidae. The species is

endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...

Isla Escudo de Veraguas Isla Escudo de Veraguas is a small (4.3 km2) 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 Province, Bocas del Toro. The island is located ab ...

, a small island off the

Caribbean The Caribbean ( , ; ; ; ) is a region in the middle of the Americas centered around the Caribbean Sea in the Atlantic Ocean, North Atlantic Ocean, mostly overlapping with the West Indies. Bordered by North America to the north, Central America ...

coast of

Panama Panama, officially the Republic of Panama, is a country in Latin America at the southern end of Central America, bordering South America. It is bordered by Costa Rica to the west, Colombia to the southeast, the Caribbean Sea to the north, and ...

. The species was first described by Robert P. Anderson of the

University of Kansas The University of Kansas (KU) is a public research university with its main campus in Lawrence, Kansas, United States. Two branch campuses are in the Kansas City metropolitan area on the Kansas side: the university's medical school and hospital ...

and Charles O. Handley Jr., of the

Smithsonian Institution The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums, Education center, education and Research institute, research centers, created by the Federal government of the United States, U.S. government "for the increase a ...

in 2001. The pygmy three-toed sloth is significantly smaller than the other three members of its

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

, but otherwise resembles the

brown-throated three-toed sloth The brown-throated sloth (''Bradypus variegatus'') is a species of three-toed sloth found in the Neotropical realm of Central America, Central and South America. It is the most common of the four species of three-toed sloth, and is found in the ...

. According to Anderson and Handley Jr., the head-and-body length is between , and the body mass ranges from . This sloth, like other extant sloths, is

arboreal Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally (scansorial), but others are exclusively arboreal. The hab ...

(tree-living) and feeds on leaves. It has a body adapted to hang by its limbs; the large curved claws help the sloth to keep a strong grip on tree branches. It lives high in the canopy but descends once a week to defecate on the forest floor. It is symbiotically associated with

green algae The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ...

, that can provide it with a

camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the b ...

. Details of mating behavior and reproduction have not been documented. The pygmy three-toed sloth was once thought to be found exclusively in the

red mangrove Red mangrove may refer to at least three plant species: * ''Rhizophora mangle'' * ''Rhizophora mucronata ''Rhizophora mucronata'' (loop-root mangrove, red mangrove or Asiatic mangrove) is a species of mangrove found on coasts and river banks in E ...

s of Isla Escudo de Veraguas, restricted to an area of . A 2012 census restricted to the coastal mangroves of the island initially found a population of around 79 individuals, nonetheless a 2015 study suggests this estimate fell considerably short. The actual population is estimated to be between 500 and 1500 with a high-end estimate of 3200 individuals, many of whom were found further inland. The

IUCN The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the status ...

lists the pygmy three-toed sloth as

critically endangered An IUCN Red List critically endangered (CR or sometimes CE) species is one that has been categorized by the International Union for Conservation of Nature as facing an extremely high risk of extinction in the wild. As of December 2023, of t ...

and they are listed on the world's 100 most threatened species.

Discovery and taxonomy

The pygmy three-toed sloth was first described by Robert P. Anderson of the

and Charles O. Handley Jr., of the

in 2001. The researchers noted that the

three-toed sloth The three-toed or three-fingered sloths are arboreal neotropical mammals. They are the only members of the genus ''Bradypus'' (meaning "slow-footed") and the family Bradypodidae. The five living species of three-toed sloths are the brown-throa ...

s found on Isla Escudo de Veraguas were significantly smaller than those that occur on the nearby outer islands of

Bocas del Toro Province Bocas del Toro (; meaning "Mouths of the Bull") is a Provinces of Panama, province of Panama. Its area is 4,643.9 square kilometers, comprising the mainland and nine main islands. The province consists of the Bocas del Toro Archipelago, Bahía Al ...

. Moreover, they differ from other populations in terms of

pelage A fur is a soft, thick growth of hair that covers the skin of almost all mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture from reaching the skin; the underfur acts as an ...

and

cranial Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Thi ...

characteristics. Hence, they considered the three-toed sloths in Isla Escudo de Veraguas to be an independent species and formally described it from the skin and skull of an adult female. The researchers further pointed out that Isla Escudos de Veraguas is the oldest island and located farthest from the mainland, which began breaking up into small islands due to rises in sea levels 10,000 years ago. They proposed that this species evolved from an isolated population that had originated from the mainland population of

s; it gradually differentiated enough to become an independent species through

insular dwarfism Insular dwarfism, a form of phyletic dwarfism, is the process and condition of large animals evolving or having a reduced body size when their population's range is limited to a small environment, primarily islands. This natural process is disti ...

. In another study the following year, the researchers observed that the mean body size of three-toed sloths on an island decreases linearly as the age of the island increases; the area of the island and the distance from the mainland, however, do not appear to significantly affect dwarfing. The pygmy three-toed sloth is one of the four extant members of the genus '' Bradypus'', and is classified under the family Bradypodidae. According to molecular phylogenetic analysis, Bradypodidae nests with

Megalonychidae Megalonychidae is an extinct family of sloths including the extinct ''Megalonyx''. Megalonychids first appeared in the early Oligocene, about 35 million years (Ma) ago, in southern Argentina (Patagonia). There is, however, one possible find datin ...

Megatheriidae Megatheriidae is a family of Extinction, extinct ground sloths that lived from approximately 23 Annum, mya—11,000 years ago. Megatheriids appeared during the Oligocene, Late Oligocene (Deseadan in the South American land mammal age, SALMA cl ...

and

Nothrotheriidae Nothrotheriidae is a family of extinct ground sloths that lived from approximately 17.5 mya—10,000 years ago, existing for approximately . Previously placed within the tribe Nothrotheriini or subfamily Nothrotheriinae within Megatheriidae, they ...

in the sloth superfamily Megatherioidea. The generic name ''Bradypus'' is the combination of two

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

words: ''brady'' ("slow") and ''pous'' ("foot"). The

specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...

''pygmaeus'' comes from the Greek ''pugmaios'' ("as small as a fist"). 'Monk sloth' and 'dwarf sloth' are two other names for this sloth.

Characteristics

The pygmy three-toed sloth is significantly smaller than the other members of its genus, but otherwise resembles the brown-throated sloth. According to Anderson and Handley Jr., the head-and-body length is between , and the body mass ranges from . The brown-throated sloth is nearly 40% heavier and 15% smaller in head-and-body length than the pygmy three-toed sloth. Moreover, the brown-throated sloth is lighter on the crown. The face is buff to tan; a dark band runs across the brow, surrounded by an orange patch. The throat is gray to brown, lighter than the underbelly; the dark brown back is spotted and has a dark stripe along the midline. Facial hair is short, while the long, rough hair on the

crown A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, parti ...

and shoulders forms a hood. The grayish limbs have three claws each. The tail is long. They have a relatively small and slender skull, with a large

external auditory meatus The ear canal (external acoustic meatus, external auditory meatus, EAM) is a pathway running from the outer ear to the middle ear. The adult human ear canal extends from the auricle to the eardrum and is about in length and in diameter. Str ...

, narrow

squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...

and mandibular processes, a minuscule

stylomastoid foramen The stylomastoid foramen is a foramen between the styloid and mastoid processes of the temporal bone of the skull. It is the termination of the facial canal, and transmits the facial nerve, and stylomastoid artery. Facial nerve inflammation in th ...

, and usually lack foramina for the

external carotid artery The external carotid artery is the major artery of the head and upper neck. It arises from the common carotid artery. It terminates by splitting into the superficial temporal and maxillary artery within the parotid gland. Structure Origin T ...

and anterodorsal (meaning in front and toward the back)

nasopharynx The pharynx (: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates, though its ...

. The

dental formula Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiology ...

of three-toed sloths is: Two of the teeth in each jaw are

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

-like, although those in the upper jaw are small or may be absent. 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. They host algae inside their fur which causes them to have a greenish color. The algae is generally more noticeable on the top of their head and neck, dorsal portions of the forelimbs, and upper backside.

Distribution and status

The pygmy three-toed sloth is unique in that it is found exclusively in the

s of

; the island has a small area of approximately . A 2012 census of pygmy three-toed sloths estimated the total population at 79 – of which 70 occurred on mangroves and 9 in the surroundings. The population density was calculated as . The total area occupied by mangroves on the island was estimated to be around . While their population has presumably always been low due to their restricted range, the 2012 census found far lower numbers than had been estimated (less than 500) by the

in 2010. The IUCN lists the pygmy three-toed sloth as

; it is also listed in

CITES Appendix II CITES (shorter acronym for the Convention on International Trade in Endangered Species of Wild Fauna and Flora, also known as the Washington Convention) is a multilateral treaty to protect endangered plants and animals from the threats of inte ...

. According to the IUCN, conservation efforts are being hampered by conflict between local peoples and the government. Threats to the sloth's survival include timber harvesting and human settlement, that might lead to habitat degradation. After several observations, the corpses of the pygmy three-toed sloth were found to be unharmed physically, suggesting that predation is not a major threat. Instead, disease, habitat loss, or natural causes were larger factors in the species’ death. A study in 2011 showed that there were 79 pygmy sloths in the wild. Studies in 2010 and 2013 suggested a recent

population bottleneck A population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events such as famines, earthquakes, floods, fires, disease, and droughts; or human activities such as genocide, speciocide, wid ...

and decline in

genetic variability Genetic variability is either the presence of, or the generation of, genetic differences. It is defined as "the formation of individuals differing in genotype, or the presence of genotypically different individuals, in contrast to environmentally ...

Ecology and behavior

The pygmy three-toed sloth, like others in its genus, is an

(tree-living) animal. This sloth can spend as many as 15 to 20 hours per day on trees. It moves at an extremely slow speed of , making it one of the slowest animals. The pygmy three-toed sloth is symbiotically related to

; a 2010 study investigated this in detail. Different sloths harbour different types of algae – only '' Tricophilus'' species were found on the brown-throated and pygmy three-toed sloths. These algae discolor the fur of the sloth, giving it a greenish hue – this serves as an efficient

. Some of these algae might be transferred to offspring through the mother, others may be picked up from the surroundings over time. The smaller size of pygmy sloths reduces their energy requirements for survival and reproduction, making them an apparent example of

. A BBC documentary, in which English naturalist

Chris Packham Christopher Gary Packham CBE (born 4 May 1961) is an English naturalist, nature photographer, television presenter and author, best known for his television work including the CBBC children's nature series '' The Really Wild Show'' from 1986 t ...

recognizes the pygmy three-toed sloth as the first in his list of the top ten discoveries in the 2000s, shows a rare clip of a swimming pygmy three-toed sloth. Like other sloths, the pygmy three-toed sloth feeds on leaves. It feeds on red mangrove leaves, which are relatively poor in nutrients and coarser than the tender leaves of ''

Cecropia ''Cecropia'' is a Neotropical genus consisting of 61 recognized species with a highly distinctive lineage of dioecious trees. The genus consists of pioneer trees in the more or less humid parts of the Neotropics, with the majority of the speci ...

'' species eaten by brown-throated sloths on the mainland. Details of mating behavior and reproduction have not been documented.

References

External links