Sloth Evolution Not So Slothful After All

| by Janet Fang

Photo credit: Hoffmann's two-toed sloth (Choloepus hoffmanni) / Geoff Gallice via Flickr

The fossil record is chock-full of ground-dwelling sloths ranging from medium-sized to mammoth (literally). But these days, there are only a few small sloths, and they all live in trees. Researchers reconstructing sloth evolution found that extinct sloths developed large body sizes at an amazing rate. Existing sloths are the black sheep of the sloth family: not only do they not reflect the overall evolutionary trends of the group, but they also obscure the strong (but long gone) signal in the fossil record of ever increasing body size.

Sloths were incredibly diverse in the past, with more than 50 known species distributed among eight families. Animals in at least half of those families weighed over 1,000 kilograms. Megatherium americanum grew to the size of an elephant, and the claws of Eremotherium eomigrans were about a third of a meter long. They showed up around 50 million years ago, but multiple megafaunal extinction events—from 2.6 million to 11,700 years ago—cut their diversity down by about 90 percent. 

All six sloth species today belong to just two genera, Bradypus and Choloepus, each in their own family: four three-toed sloths in Bradypodidae, and two two-toed sloths in Megalonychidae. The two lineages may seem similar, but they diverged from one another about 30 million years ago. So oddly enough, they must have independently evolved their small size—no more than six kilograms—and arboreality, or tree-living.

Using a data set of all 57 species of known living and fossil sloths, a trio of researchers led by John Finarelli of University College Dublin examined changes in body mass through sloth evolution. The team found a clear trend for the evolution of larger and larger body sizes through time—and they evolved at an extremely fast rate. The work was published in BMC Evolutionary Biology this week. 

Megatherium’s family saw an average mass increase of 129 kilograms per million years—one of the fastest rates of body size evolution known for mammals. Even the family Megalonychidae (which includes today’s two-toed sloths) had an average body mass increase of 2.6 kilograms per million years. 

The findings suggest that environmental conditions at the time, such as climate or species competition, must have really favored larger body sizes. Whatever the cause of their eventual decline, only small, arboreal sloths survived these events, completely reversing the millions-year-old trend towards larger and larger sloths, the BMC blog reports. 

“If we ignore the fossil record and limit our studies to living sloths, as previous studies have done, there's a good chance that we'll miss out on the real story and maybe underestimate the extraordinarily complex evolution that produced the species that inhabit our world," says study co-author Anjali Goswami of Univeristy College London in a news release. 

And sloths may not be the only group where modern-day species are unrepresentative of overall evolutionary trends. The diversity of hyenas, elephants, and rhinos, for example, are only a fraction of what it was in the past.