That according to a team of researchers from Sorbonne University, who tested the bone density of ancient sloth specimens.
In their paper published in Proceedings of the Royal Society B: Biological Sciences, the researchers showed that over time, the bone density of sloth fossils increased, evidence of the mammals' more frequent forays into the ocean -- a slow but steady return to their primordial origins.
The researchers fetched the fossils from museums in Lima, Peru and Paris, France, and used a CT scanner to determine density. The bones were already dated, so it was easy to track the shift in density over time.
The study looked at the bones of five different species of sloths, all dated between five and eight million years old. Ground sloths, which roamed much of North and South America, have been extinct for at least 10,000 years.
Land mammals are blessed with bones low in density, which makes movement less taxing on muscles and energy levels. But in water, low-density means buoyancy, making it harder to dive beneath the ocean's surface for food.
"Think about a scuba diver who has a weight belt," Eli Amson, lead author and paleontology grad student at Sorbonne, told National Geographic. “It allows them to sink.”
The earliest aquatic sloths likely just came to nibble on shallow sea grasses. But as food became scarcer on land, the animals may have grown bolder, wading farther in, diving deeper.
"Over time, [the sloths] become better adapted to an aquatic habitat where they go out and swim, and dive down in order to feed more often and not just with the tides," explained Greg McDonald, a senior curator of natural history for the U.S. National Park Service.
Though the sloths never made a full transition into the ocean -- like their mammal relatives, the seal and manatee -- the research is an exciting glimpse into just how quickly a species can make evolutionary adjustments for the demands of survival.