Why Did Sloths Get Smaller

These prehistoric sloths roamed the land, feeding on leaves they could reach with long tongues, much like a giraffe. Others, like the Shasta ground sloth, were much smaller and occupied very different environments. They lived in the deserts of North America and were known for their fondness for cacti.

The sloth family tree once sported a dizzying array of branches, body sizes and lifestyles, from small and limber tree climbers to lumbering bear. Giant Sloths the Size of Elephants Once Walked Along the Ground. Here's How the Massive Animals Evolved and Declined Researchers analyzed fossils and DNA to get a big.

Scientists combined fossil measurements, DNA, and ecological data to uncover why some sloths evolved to become massive, while others remained small and nimble. "They looked like grizzly bears but five times larger," said Rachel Narducci, a co. In response, sloths shrank in size, possibly because warmer temperatures expanded forest habitats that favored smaller, more agile sloths and because smaller bodies help manage heat stress.

The results were striking. Whether sloths lived on the ground or in trees turned out to be the biggest factor in how large they became. Early sloths were mostly large, land-based grazers.

But as forests spread and open spaces gave way to tree cover, sloths evolved smaller bodies and took to the canopy, often independently in separate lineages. Though the ancient sloth lineages quickly grew into giants, today's sloths are having a tough time keeping up with the pace of their habitat destruction. And the sloths currently hanging out in Central and South America can't rely on size.

Instead, many sloth species. Existing sloth species appear to be the "black sheep" of the sloth family, obscuring what is otherwise a strong signal in the fossil record of ever increasing body size. As the researchers say, it is hard to infer from a group of small.

The earliest sloths, emerging in South America around 37 million years ago, were small terrestrial creatures. As global temperatures fluctuated, so did the availability of forests and grasslands, shaping sloth lifestyles accordingly. Today's sloths might be known as slow, small animals, but their ancestors developed large body sizes at an amazing rate, according to an evolutionary reconstruction.

The fast rate of change.