Sloths may be slow at almost everything, but their bodies changed rapidly over millions of years.
Species ballooned from weighing a few kilograms around 35 million years ago to being as big as an elephant at up to 3,800 kilograms. These giant ground sloths, such as Megatherium americanum, lived about 3 million to 11,700 years ago. Modern sloth species, however, have shrunk back down to a more measily size, at about 4 to 5 kilograms.
Such disparity in body size suggests that some sloths would have needed to put on up to 126 kilograms every million years to reach elephant size. That rate for adding body weight seems slow, but it's one of fastest reported in mammals, scientists say. The results appear September 10 in BMC Evolutionary Biology.