New Study Sheds Light on the Emergence and Demise of Giant Sloths

There are only two species of sloths today, but historically there were dozens, including one with a "bottle nose" that fed on ant ants, and another that probably resembled the ancestors of modern armadillos. Most extinct sloths also didn't live in trees because they were too big. The largest sloths, members of the genus Megatherium (megatherium), were the size of Asian elephants and weighed about 3.5 tons.

"They looked like grizzly bears, only five times bigger," said Dr. Rachel Narducci, head of the vertebrate paleontology collection at the Florida Museum of Natural History.

In the new study, Dr. Narducci and her colleagues analyzed ancient DNA and compared more than 400 fossils from 17 natural history museums to find out how and why extinct sloths got so big.

Terrestrial sloths varied greatly in size, from the truly massive Megatherium to the humble Shasta sloth that terrorized cacti in the desert southwest of North America.

The same cannot be said for sloths who were fond of climbing trees. Sloths that lived exclusively in trees were and remain equally small, with an average weight of 6 kg, while those that spent part of their time on the ground weighed on average about 79 kg.

Ground sloths especially love caves, and their size undoubtedly played a role in their ability to find and build shelters. The medium-sized Shasta sloth favored small, natural caves created by the power of wind and water in the Grand Canyon's cliffs, like the alveoli of a giant geological lung.

Larger sloths were not limited to pre-existing caves. Using claws, which are among the largest of any known mammal living or extinct, they could carve their own out of bare earth and rock. Many of the caves they left behind still have claw marks on the interior walls, evidence of their ancient nesting digs.

Other factors that may have influenced size differences include climate, degree of relatedness between sloth species, and metabolic rate. A significant amount of data of different types was required to accurately distinguish between these possibilities.

The authors combined information about the shape of the fossils with DNA from living and extinct species to create a tree of life for sloths that traces their lineage back to an origin more than 35 million years ago.

To this tree were added the results of decades of research on where sloths lived, what they ate, and whether they were climbing or walking. Because the paleontologists were particularly interested in the evolution of size, they gathered data for the final analytical ingredient by measuring hundreds of museum fossils, which they used to estimate the sloth's weight.

According to the team of scientists, differences in sloth size were primarily influenced by the types of habitats in which they lived and, as a result, climate change. "Taking all these factors and running them through evolutionary models with many different scenarios was a major challenge that hadn't been done before," says Dr. Narducci. The sloth dynasty coincided with significant, life-changing changes in the climate of Earth.

Analysis by the research team shows that the earliest sloths were likely small land dwellers, about the size of a dog breed dog.

At various points in their evolutionary history, sloths have shifted to a semi-arboreal lifestyle (spending most of their time in trees). However, not all of them stayed in trees. The largest sloths, including Megatherium and Mylodon (mylodon), probably evolved from a tree-adapted sloth that eventually decided to settle firmly on the ground.

Amid the hesitancy of climbing and walking sloths, their size remained virtually unchanged for about 20 million years, regardless of their preferred mode of locomotion. Then something earth-shattering happened.

Between the modern states of Washington and Idaho, through parts of Oregon and Nevada, a giant wound opened up and magma rushed out. The result was a "frozen scar" of nearly 2.5 million cubic kilometers in the Pacific Northwest. It can still be seen in some places along the Columbia River, where millions of years of flowing water cut through and polished a colonnade of basalt.

These stone pillars have a distinct hexagonal shape resulting from the way magma solidified and cracked as it cooled. The volcanic event that formed them was a slow burning event that lasted about 750,000 years and coincided with a period of global warming called the mid-Miocene climatic optimum.

Greenhouse gases from volcanic eruptions are now considered the most likely cause of warming. In response, sloths have gotten smaller.

This may be because warmer temperatures have led to increased precipitation, allowing forests to expand and thus creating more habitat for smaller sloths. Size reduction is also a common way for animals to cope with heat stress and has been recorded in the fossil record on several different occasions.

After the volcano subsided, the Earth remained warm for about a million years. Then the planet resumed a long-term cooling that continues intermittently to this day. The sloths changed course, too. The more the temperature dropped, the bigger they got.

Sloths that lived primarily in trees and those that partially lived this lifestyle had the obvious limitation of having to live near trees, but ground sloths lived almost anywhere their feet could take them.

They have climbed the Andes Mountains, spread across open savannas, migrated into the deserts and deciduous forests of North America, and found a home in the boreal forests of Canada and Alaska.

There were even sloths adapted to the marine environment. Thalassocnus (thalassocnus means "sea sloth" in ancient Greek) lived in the arid strip of land between the Andes and the Pacific Ocean. They survived in this harsh region by foraging for food in the ocean.

"They developed adaptations similar to those of manatees," says Dr. Narducci. "They had dense ribs to help maintain buoyancy and longer snouts to feed on seagrass."

The diverse habitat posed unique challenges that ground sloths solved, including increased feeding.

"This would allow them to conserve energy and water and move more efficiently through habitats with limited resources," Dr. Narducci says. "And if you're in open grasslands, you need defense, and increased size gives some of it. Some ground sloths have had small osteoderms embedded in their skin that look like pebbles."

Just as importantly, larger bodies helped sloths cope with the cold weather. They reached their greatest growth during the Pleistocene Ice Age, shortly before their extinction. "About 15,000 years ago there is really a decline," Dr. Narducci says.

There is still debate about what happened to the sloths, but given that humans appeared in North America around the same time that sloths went extinct en masse, it's not hard to assume.

Paradoxically, the large size that allowed them to defend themselves against most predators and insulated them from the cold became a hindrance. With neither speed nor good defense, ground and semi-ground sloths were easy prey for the first humans.

Tree sloths watched the carnage unfold beneath them from the safety of the treetops, but even there they did not escape unscathed. After their terrestrial relatives became extinct everywhere, two species of tree sloths in the Caribbean survived until a period of 4,500 years ago. Humans appeared in the Caribbean around the same time that the Egyptians were building pyramids. Shortly afterward, Caribbean tree sloths became extinct.

"Paleoclimatic changes do not explain the rapid extinction of ground sloths that began about 15,000 years ago," the researchers said. "Their abrupt demise suggests that humans were involved in the decline and extinction of ground sloths."