Superslow sloths may have once evolved superfast

Two-toed sloth

Two-toed sloths like this one weigh about 6 kilograms. Their extinct cousins were once thousands of kilograms, suggesting that these slow creatures once evolved quickly.

Jsfouche/Wikimedia Commons (CC BY-SA 3.0)

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.

Ashley Yeager is the associate news editor at Science News. She has worked at The Scientist, the Simons Foundation, Duke University and the W.M. Keck Observatory, and was the web producer for Science News from 2013 to 2015. She has a bachelor’s degree in journalism from the University of Tennessee, Knoxville, and a master’s degree in science writing from MIT.

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