LINCOLN, Neb., Feb. 24 (UPI) -- A prehistoric horse no bigger than a house cat got even smaller during an ancient episode of global warming, U.S. paleontologists say.
Some 56 million years ago Sifrhippus, the first horse, shrank from an average weight of about 12 pounds down to about 8-1/2 pounds as the climate warmed over thousands of years, researchers reported Thursday in the journal Science.
Sifrhippus counts as a horse only in scientific classification and was not much like modern horses, researchers said, eating leaves instead of grass.
Preserved fossils in the Bighorn Basin of Wyoming show a size change over a warm period in the Earth's history known as the Paleocene-Eocene thermal maximum, in line with a theory known as Bergmann's rule that suggests mammals of a given genus or species tend to be smaller in hotter climates.
Researchers say the little horse got 30 percent smaller over a period of 130,000 years.
"It seems to be natural selection," researcher Ross Secord of the University of Nebraska-Lincoln told The New York Times.
Animals evolved to be smaller during warming because smaller animals did better in that environment, he said, perhaps because the smaller an animal is the easier it is to shed excess heat.
Current global warming is occurring on a scale of hundreds of years, not thousands, and scientists say they can only speculate if modern mammals will shrink.
"It's difficult to say that mammals are going to respond in the same way now," Secord said, adding he thinks some will get smaller.