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Some Ancient Sloths Ventured Into the Ocean, Study Says

An aquatic sloth feeds on seagrasses in an ancient ocean. Illustration by Emily M. Eng.
An aquatic sloth feeds on seagrasses in an ancient ocean. Illustration by Emily M. Eng.

Modern-day sloths are tree dwellers, only occasionally venturing down to the ground. But about five to eight million years ago, five sloth species ventured into the sea.

Now, new research suggests that these ancient animals went much further into the water than we ever knew. Instead of just living near the ocean and making brief forays in, as scientists had previously thought, it appears that ancient aquatic sloths swam out and dove toward the bottom for food. The study confirms habits scientists had speculated about for years.

In studying the aquatic sloth fossils, a team of scientists found that cavities present in the bones of terrestrial animals were absent in the sloth specimens. They were instead filled with solid bone, which aided in diving.

“Think about a scuba diver who has a weight belt,” says Eli Amson, a paleontologist at the National Museum of Natural History in Paris and lead study author. “It allows them to sink.”

The bones of terrestrial mammals, by contrast—including our own—are filled with small cavities.

Dense bone is one of the key adaptations seen in mammals such as manatees and dugongs that returned to the sea, where life began. Dense bones would have been especially important in helping aquatic sloths dive because they had big bellies, like modern sloths do, which would have acted like flotation devices, says Greg McDonald, a senior curator of natural history for the U.S. National Park Service.

The earliest aquatic sloths probably came down to the beaches to munch on sea grasses exposed to the air during low tides, McDonald says. The animals may have waded into shallow water to graze on vegetation.

“Over time, [the sloths] become better adapted to an aquatic habitat where they go out and swim,” McDonald says, “and dive down in order to feed more often and not just with the tides.”

Beach Bums?

Other adaptations to a watery lifestyle can be seen in the ancient sloths’  limbs and tails. On average, these animals were 6.5 to 8 feet (2 to 2.5 meters) long, with about 3 feet (a meter) of that length being all tail, says Amson.

“The tail is actually reminiscent of a platypus tail or a beaver tail,” he says. But the sloths probably weren’t using their paddle-like tail for locomotion underwater. It was probably working to keep the animals stable as they dove, Amson explains.

Modifications to bones in their upper and lower legs also point to a shift to a more aquatic lifestyle, McDonald says.

Sloths didn’t get to a point where they were as aquatic as modern seals or sea lions, he adds. Aquatic sloths probably did come back to land to bask in the sun and warm up between meals.

And what a sight that would have been, to see six-foot (two-meter) sloths lazing about a beach. “Even by sloth standards, it’s a weird animal,” McDonald says. (Learn more about aquatic sloths.)

The new study was published this week in the Proceedings of the Royal Society B.

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