An analysis of the sloth family tree suggests three different groups of the animals evolved to gigantic sizes in response to cold and dry conditions
Ancient sloths came in a variety of sizes Diego Barletta
A cooling, drying climate turned sloths into giants – before humans potentially drove the huge animals to extinction.
Today's sloths are small, famously sluggish herbivores that move through the tropical canopies of rainforests. But for tens of millions of years, South America was home to a dizzying diversity of sloths. Many were ground-dwelling giants, with some behemoths approaching 5 tonnes in weight.
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That staggering size range is of particular interest to Alberto Boscaini at the University of Buenos Aires in Argentina and his colleagues.
"Body size correlates with everything in the biological traits of an animal," says Boscaini. "This was a promising way of studying [sloth] evolution."
Boscaini and his colleagues compiled data on the physical features, DNA and proteins of 67 extinct and living sloth genera – groups of closely related species – to develop a family tree showing their evolutionary relationships.