Big mammals might be unexpectedly resilient in the face of global warming, suggests a new study that looked to the past for insights into the future.
The study found that llamas, tapirs, deer and other large mammals changed their diets when glaciers retreated from North America more than a million years ago.
Scientists have long assumed that animals would be rigid about what they ate and what niches they occupied during periods of climate change — making them especially vulnerable to those shifts.
"This really questions that assumption," said Larissa DeSantis, a paleoecologist at the University of Florida, Gainesville. "We were able to show these animals do change their diets quite dramatically."
DeSantis and colleagues studied mammal teeth from two sites on Florida's Gulf Coast. One site dated back 1.9 million years to a glacial period when North America was relatively cold. The other site dated back 1.3 million years to an interglacial period that was much warmer and drier. Both sites contained lots of fossils from many of the same large mammals, including peccaries, pronghorn, horses and elephant-like creatures called gomphotheres.
The researchers drilled into 115 fossil teeth and analyzed the enamel powder for chemical signatures that reflected what the animals had been eating when they were alive. Grasses, for example, contain different forms of carbon and oxygen than leaves and shrubs do. Those signatures are preserved in an animal's tissues.
"You are what you eat," DeSantis said. "And you are what you drink."
During the glacial period, the researchers reported this week in the journal PLoS One, most of the mammals were browsing on leaves and shrubs. When conditions grew warmer, the majority of animals added grasses to their diets. Llamas and peccaries made especially dramatic adjustments to their diets.
"You'd expect to have very distinct groups of animals in each of these environments, but you have the same cast of characters," said Mark Clementz, a paleobiologist at the University of Wyoming in Laramie. "Those guys seem to be doing just fine."
When scientists try to predict how species will respond to climate warming, their models often assume that animals will continue to eat only what they're eating now. Those models might have to change, at least for large mammals, Clementz said.
"The big message is that many species may be more flexible than what we believe at the moment," Clementz said. By adding new foods to their diets, he said, the animals seemed to weather the climate shift. "That could have a big effect on how they react to climate change in the future."
Exactly how any given animal will react to future changes remains unclear. Warming is happening more quickly now than it did in the past, among other differences. Future studies will also need to consider birds, reptiles, small mammals or other groups.
Still, the message of the new study is at least somewhat hopeful.
"There's a little bit of optimism," Clementz said, "That species are a bit more resilient than we may be giving them credit for."
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