Giant ground sloths evolved three different times for the same reason

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.

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.

The researchers then took this evolutionary history, which covered a span of 35 million years, and added information about each sloth’s habitat, diet and lifestyle. They also studied trends in body-size evolution, making body mass estimates of 49 of the ancient and modern sloth groups.

The results suggest sloth body-size evolution was heavily influenced by climatic and habitat changes. For instance, some sloth genera began living in trees – similar to today’s sloths – and shrank in body size as they did so.

Meanwhile, three different lineages of sloths independently evolved elephantine proportions – and it seems they did this within the last several million years, as the planet cooled and the growth of the Andes mountains made South America more arid.

“Gigantism is more closely associated with cold and dry climates,” says team member Daniel Casali at the University of São Paulo, Brazil.

Many of these diverse sloths disappeared during two stages: one around 12,000 years ago and the other around 6000 years ago, says Boscaini.

“This matches with the expansion of Homo sapiens, first over the entire American supercontinent, and later in the Caribbean,” he says — which is where some giant sloths lived. Notably, the only surviving sloth species live in trees so are much harder for humans to hunt than massive ground sloths.

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The idea that humans were the death blow for ancient megafauna is well-supported, says Thaís Rabito Pansani at the University of New Mexico, who wasn’t involved in the study.

“However, in science, we need several lines of evidence to reinforce our hypotheses, especially in unresolved and highly debated issues such as the extinction of megafauna,” she says. The new evidence shores up this story.

“Sloths were thriving for most of their history,” says Casali. “[The findings] teach us how a very successful [group] can become so vulnerable very quickly.”

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