Sloths Speed in the Evolution Race: How These Mammals Evolved Massive Bodies

Sloths may be slow but in the race of evolution, they're some of the fastest mammals out there. Scientists have taken a closer look at the evolutionary history of sloths, examining how and why these large mammals of the past evolved into the small, slow animals that they are today.

The scientists examined existing models for reconstructing how sloths diversify. More specifically, they took information about all known sloth species, both living and in the fossil record, and tested how existing evolutionary models explained the range in body sizes.

Some species of the past were as large as elephants; in fact, the researchers found that some sloth lineages increased in size by over 100 kilos every million years, which is some of the fastest rates of body size evolution known to mammals. It's likely that environmental conditions in the past largely favored larger body sizes.

"Today's sloths are really the black sheep of the sloth family," said Anjali Goswami, one of the authors of the new paper detailing the findings, in a news release. "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."

In fact, only two existing groups of sloth live today. They bear little resemblance to the sloths of the past; one species, Megatherium americanum, was an elephant-sized ground sloth that could walk upright on its hind legs. It, along with all but the two sloth groups, died out about 11,000 years ago.

The findings reveal a bit more about the evolutionary history of these animals. In addition, it shows that this same method could be used to examine the evolutionary past of other species.

"There are many other groups, such as hyenas, elephants and rhinos that, like sloths, have only a few living species," said John Finarelli, co-author of the study. "But if we look into the distant past, these groups were much more diverse, and in many cases very different to their current forms."

The findings are published in the journal BMC Evolutionary Biology.