There are two distinct groups of Sloths – three-toed (Bradypus) and two-toed (Choloepus), although both families have three toes on both their rear limbs. The ancient ancestors of two toed sloths were significantly larger, with the largest growing to the size of modern elephants. These huge creatures were ground animals that are believed to have lived in multi-generational burrows. Most of the suborder Folivera died out in the Quaternary extinction event around 12,000 years ago. Their closest relatives (all in the super-order Xenarthra) are anteaters followed by the armadillo.

The three and two-toes sloths are believed to have individually branched off somewhere around 32 million years ago, and each evolved their upside-down, arboreal lifestyle independently (convergent evolution). As such, there are a surprising number of differences between them, despite the very obvious similarities in appearance, diet, lifestyle and speed of motion.

• Two-toed sloths are much larger than their three toed cousins, reaching 80 cm and up to 11kg.
• They also have much longer gestation periods (11.5 months vs ~6 months).
• Three toed sloths are even slower than two-toed sloths, both in terms of general activity and rate of movement.
• The three toed sloth has longer arms than legs, while both sets are of equal length in two-toed sloths.
• Three toed sloths have a small stubby tail, while the two toed does not.
• Two toed sloths have 6-7 vertebrae, while three toed have nine.
• 2 toed sloths spend more time hanging from horizontal branches, while 3 toed often sit in trees. This is generally attributed to the disparity in arm/leg length in 3 toed sloths
• The two families cannot interbreed, so there are no hybrid 2/3 toed sloths
• Two toed sloths have more ribs than any other mammal – 46. That’s almost twice as many as humans, who have 24. Three toed sloths only have 28, a significant difference between the two. These extra ribs help support their stomach when upside down, and are very flexible, making them hard to break. These also protect the sloth on the rare fall from a tree.  Two toed sloths can survive a ten-story fall to the forest floor (100’)

For a well-known creature that captures the imagination of children and adults alike, there is relatively little known about them.  Look on multiple “authoritative” websites, and you will hear facts which vary: sloths are colorblind – sloths see color.  The gestation period of the three toed sloth is not well documented yet (with most information coming from sloths in captivity, which is notoriously unreliable). Even the number of ribs and diet are cited differently across multiple sites. Studies about the symbiotic relationship between sloths and the algae, and fungus that grow on their fur and the moths that accompany them are very recent  (see research above) and there is much yet to learn.

Sloths range from There are two distinct populations of Hoffman’s sloths on either side of the Andes mountain range. These two populations are thought to have diverged up to 7 million years ago. There are 5 suggested subspecies.

Reproduction

Sloths are mammals and reproduce sexually in which a female egg is fertilized by a male sperm cell. Like all other sloth activities, mating occurs whikle hanging in trees. Sloths mate between the months of September and November, which is the end of the wet season. This ensures that offspring, born 11½ months later spend their first months in the dry season (there are observations that suggest this oft-cited figure is erroneously specific and that gestation actually varies significantly). This may be important, because sloths cannot regulate their body temperatures, and very young babies may struggle by being cooled by the heavy rains of the wet season.  Sloths give birth to single offspring. In very rare cases where two are born together, the mother will abandon one, as she does not have the physical resources to support two.

The newborn are able to grip the belly of the mother immediately after birth, where they will remain for months. Unlike most female mammals, sloths do not produce large stores of milk for the baby, because that requires significant energy stores that sloths do not have.  The model of efficiency, the mother produces milk as needed by the infant, which feeds very frequently.

The very slow movement rate of sloths adds to reproductive challenges.  Sloths are solitary creatures, and they have to find and travel to each other to mate. When females are in heat they will often scream to attract nearby males. It is also speculated that they leave pheromone markers on trees to alert others of their presence .

Genetic Diversity

Isolation and Shrinking Habitat

Diet & Digestion