Two-toed Sloths (Choloepus spp.) Fact Sheet: Taxonomy & History

• Large difference in chromosomes (number and characteristics) within Choloepus may mean there are even more than 2 species of two-toed sloths
• Two-toed and three-toed sloths were formerly placed in the same family but the two genera have profound behavioral and anatomical differences and are believed to come from two different fossil lineages. They are now placed in separate families.
  • 2-toed is larger, faster, and nocturnal. Diet is more varied - eats leaves and fruit. 6 or 7 neck vertebrae and vestigial tail
  • 3-toed is smaller, slower and both diurnal and nocturnal. Highly specialized browsers - eat only leaves. 8 or 9 neck vertebrae. Stout tail is 68 mm (2.7 in ) long
• All sloths have 3 claws on their hindlimbs
• Sloths are more closely related to anteaters than armadillos. Members of this group of animals have a reduced number of teeth.
  • Anteaters are toothless
  • Sloths have only 10 upper teeth and 8 lower teeth
  • Anteaters and sloths were once grouped together as "Edentata" meaning 'without teeth'.
• Common names for the 2-toed sloths:
  • Choloepus didactylus: Southern two-toed sloth, Linnaeus's two-toed sloth
  • Choloepus hoffmanni: Hoffman's two-toed sloth
• Local names: Prezoso de dos dedos is used in many Spanish-speaking countries ("perezoso" means "lazy"). Preguica real or unau (in Brazil); perico ligero (in Columbia);cucala (in Honduras)

• Extensive fossil record dates from Eocene - more than 200 genera (Barros et al 2008)
  • Found in South, Central, and North America, Caribbean and Antarctica. Size varied from a medium dog to elephant. (Nowak 2008)
• Phylogenetic relationships are not resolved. Gaudin and McDonald postulate four families: Megalonychidae, Megatheriidae, Nothrotheriidae and Mylodontidae. Bradypus is placed as a sister taxon to all other sloths. Choloepus is incorporated into the Megalonychidae.
• At least three genera of giant ground sloths lived in Southern California during the Pleistocene. A Shasta's ground sloth skeleton was recently excavated in Carlsbad (thought to be about 2.1 m (7 ft) long and weighing about 136 - 181 kg (300 - 400 lbs) — bear sized.
• First sloths arrived in North America about 7 million years ago (presumably by swimming between islands from South America)
• By 13,000 years ago, all ground sloths were extinct except for a few populations on Caribbean islands.

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia (Linnaeus, 1758) – mammals

Order: Pilosa (Flower, 1883) - a group of placental mammals found in the Americas, including anteaters and sloths, and extinct ground sloths

Family: Megalonychidae (Ameghino, 1889) – two-toed sloths

Genus: Choloepus (Illiger, 1811) – two-toed sloths

Species: Choloepus didactylus (Linnaeus, 1758) – southern two-toed sloth, Linnaeus's two-toed sloth, Linné's two-toed sloth
Species: Choloepus hoffmanni (Peters, 1858) – Hoffmann's two-toed sloth

Source: Integrated Taxonomic Information System (2017)

Illustration of a two-toed sloth from a 1900 school textbook.

From Introduction to zoology; a guide to the study of animals, for the use of secondary schools (1900); p.343.

Image credit: Made available on Flickr by Internet Archive Book Images. No known copyright restrictions.

Anderson and Hanley (2001)
Barros et al. (2008)
Gaudin and McDonald (2008)
Naish (2005)
Wilson & Reeder (2005)