Coyotes（土狼） today are pint-sized compared to their Ice Age counterparts, finds a new fossil study. Between 11,500 and 10,000 years ago -- a mere blink of an eye in geologic（地质的） terms -- coyotes shrunk to their present size. The sudden shrinkage was most likely a response to dwindling food supply and changing interactions with competitors, rather than warming climate, researchers say. In a paper appearing this week in Proceedings of the National Academy of Sciences, researchers studied museum collections of coyote skeletons dating from 38,000 years ago to the present day. It turns out that between 11,500 and 10,000 years ago, at the end of a period called the Pleistocene, coyotes in North America suddenly got smaller.

"Pleistocene coyotes probably weighed between 15-25 kilograms, and overlapped in size with wolves. But today the upper limit of a coyote is only around 10-18 kilograms," said co-author Julie Meachen of the National Evolutionary Synthesis Center in Durham, North Carolina.

"Within just over a thousand years, they evolved into the smaller coyotes that we have today," she added.

What caused coyotes to shrink? Several factors could explain the shift. One possibility is warming climate, the researchers say. Between 15,000 and 10,000 years ago, global average annual temperatures quickly rose by an average of six degrees. "Things got a long warmer, real fast," Meachen said.

Large animals are predicted to fare worse than small animals when temperatures warm up. To find out if climate played a role in coyotes' sudden shrinkage, Meachen and co-author Joshua Samuels of John Day Fossil Beds National Monument in Oregon measured the relationship between body size and temperature for dozens of Ice Age coyotes, and for coyotes living today, using thigh bone circumference to estimate body size for each individual.

But when they plotted body size against coldest average annual temperature for each animal's location, they found no relationship, suggesting that climate change was unlikely to be the main factor.

If the climate hypothesis is true, then we should see similar changes in other Ice Age carnivores（食肉动物） too, Meachen added. The researchers also studied body size over time in the coyote's larger relative, the wolf, but they found that wolf body sizes didn't budge（挪动，微微移动） . "We're skeptical that climate change at the end of the Pleistocene was the direct cause of the size shift in coyotes," Meachen said.