Study Shows Big Game Hunters, Not Climate Change, Killed Off Sloths

Determining whether the first arrival of humans orthe warm-up of the American continent at the end of the last Ice Agewas responsible for the demise of prehistoric sloths has puzzledscientists because both events occurred at the same time, about 11,000years ago. But by using radiocarbon to date fossils from Cuba andHispaniola, where humans appeared later than on the North Americancontinent, long after the last Ice Age occurred, UF ornithologist DavidSteadman was able to separate the two events.

He and hiscolleagues found the last record of West Indian ground sloths coincidedwith the arrival of humans 4,400 years ago. The results are publishedin a Proceedings of the National Academy of Sciences paper this week.

"Ifclimate were the major factor driving the extinction of ground sloths,you would expect the extinctions to occur at about the same time onboth the islands and the continent since climate change is a globalevent," Steadman said.

Gary Haynes, anthropology professor at theUniversity of Nevada, Reno, said Steadman's study "clearly shows thatground sloth extinctions in the New World didn't happen after seriouschanges in climate or vegetation -- and that the first appearance ofhumans must have been the decisive factor."

The fossil recordshows the people who arrived in North America were making sophisticatedtools out of stone, bone and ivory, Steadman said. These "big-gamehunters" had a traumatic effect on the animals living there, he said.

Morethan three-fourths of the large species of mammals that roamed theNorth American landscape became extinct within a few thousand years,which, besides ground sloths, included mammoths, mastodons,saber-toothed tigers and giant bears, Steadman said.

"It was as dramatic as the extinction of the dinosaurs 65 million years ago," he said.

Byunderstanding when, and to some extent how, ground sloths becameextinct, scientists may be able to determine the biological potentialof an area for restoration if human contact could be eliminated, suchas in a national forest, a national park or an island, Steadman said.

"I'mnot a Steven Spielberg type in that I don't believe that DNA wouldbring these things back," he said. "But in lieu of Jurassic Park, Ithink we can come up with sound ideas using the nearest livingrelatives. For example, we might want to consider taking living treesloths and introducing them to protected forested areas on Cuba orHispaniola."

While the largest of the prehistoric ground slothsgrew to the size of a modern elephant and fed on bushes and the leavesof lower branches of trees, today's only surviving descendants areseveral small tree sloths whose range extends from southern Mexico tosouthern Brazil, he said.

Such an experiment might be similar tothe one that involved restoring bison, once native to Florida, toPaynes Prairie Preserve State Park near Gainesville, Steadman said."With the work we're doing, especially on islands, to reconstruct whichkinds of plants and animals -- particularly birds and mammals -- usedtolive there, we can open up possibilities for restoring parts of theseislands to something near their original condition," he said.

Theonly reason the living species of sloths survive is that they live highup in trees, where their green-algae-colored fur camouflages them,Steadman said. "God save the sloth that comes down to the groundbecause usually somebody is there to kill it," he said.

For thestudy, Steadman sent samples from the large collection of ground slothskeletons at the Florida Museum of Natural History on the UF campus, toPaul Martin, a professor emeritus of geosciences at the University ofArizona, for radiocarbon dating.

Steadman said he was notsurprised to find that humans were more significant than changes inclimate because most species of plants and animals can adjust tochanges in temperature. However, the transition between the glacial andinter-glacial period, which resulted in shifts in habitat and theranges of plants, may have made animal species more vulnerable thanthey otherwise would have been, he said.

"This is the first timeit's been demonstrated for West Indian ground sloths, and West Indianground sloths are sort of the poster child of big extinct West Indianmammals," he said. "I think this will go a long way to finally put torest this whole idea that large extinct animals from the West Indiesdied out in the Ice Age during the Pleistocene Epoch."