Modern sloths might be slow, but new research reveals their ancestors evolved at an alarming rate.
The rapid rate at which the animals acquired their large body sizes suggests factors such as environmental conditions and competition with other species heavily favored larger sloths before they died out, BioMed Central reported.
Scientists from UCL (University College London) and University College Dublin examined existing models to see how sloths diversified (some were as large as elephants) and shrank down to their current size from the giant ancestor. The study suggests the sloths ancestors shrunk down by about 220 pounds every million years, which is one of the fastest rates of body change ever seen.
"Today's sloths are really the black sheep of the sloth family. 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," Anjali Goswami , of UCL Earth Sciences, reported.
The two groups of sloths alive today share very little resemblance with their ancestors. The species Megatherium americanum was an elephant-sized ground sloth; fossilized track marks suggest the animals could walk on their hind legs. Today's sloths only reach a maximum of about 13 pounds.
To make their findings the researchers looked at information about sloths species in the past and today and tested how they stacked up with evolutionary models. They showed the models that were based only on living species failed to explain how the animals shrunk so quickly. The models that incorporated historical species showed the sloths developed at a very fast rate.
"There are many other groups, such as hyaenas, elephants and rhinos, that, like sloths, have only a few living species. But if we look into the distant past, these groups were much more diverse, and in many cases very different to their current forms," said co-author John Finarelli of the University College Dublin Earth Institute).
The findings were published Sept. 10 in the journal BMC Evolutionary Biology.