Earliest horses show past global warming affected body size of mammals

In a study appearing in Science Feb. 24, researchers from eight institutions led by scientists from the University of Florida and University of Nebraska found a correlation between temperature and body size in mammals by following the evolution of the earliest horses about 56 million years ago: As temperatures increased, their body size decreased.

"Horses started out small, about the size of a small dog like a miniature schnauzer," said co-author Jonathan Bloch, associate curator of vertebrate paleontology at the Florida Museum of Natural History on the UF campus. "What's surprising is that after they first appeared, they then became even smaller and then dramatically increased in size, and that exactly corresponds to the global warming event, followed by cooling. It had been known that mammals were small during that time and that it was warm, but we hadn't understood that temperature specifically was driving the evolution of body size."

Sifrhippus, the earliest-known horse, first appeared in the North American fossil record during the Paleocene-Eocene Thermal Maximum. During this 175,000-year climate event, increased concentrations of carbon dioxide in the atmosphere and oceans caused average global temperatures to rise 10 to 20 degrees. By analyzing the size and isotopes of fossils collected in Wyoming's Bighorn Basin, researchers traced the evolution of Sifrhippus from an estimated 12-pound animal that shrank during a 130,000-year period about 30 percent to 8.5 pounds -- the size of a small house cat -- then increased to about 15 pounds during the next 45,000 years.

"This is the highest-resolution terrestrial record of its kind from anywhere in the world and it shows how climate changed in Wyoming at that time," said lead author Ross Secord, who began geochemical analysis of the horse teeth and other mammals as a postdoctoral researcher with Bloch before joining the University of Nebraska-Lincoln in 2008. "When Jon and I started plotting oxygen data from the mass spectrometer, we could immediately see that the shifts in size of horses and temperature were mirror images of each other."

Bloch said the project began about seven years ago when former UF student and study co-author Stephen Chester, now an anthropology doctoral candidate at Yale University, measured horse teeth that seemed to be too large for their age and became smaller through the geologic section. The findings raise important questions about how animals might respond to future rapid climate change.

"We're seeing about a third of the mammals getting smaller and some of them getting a lot smaller, by as much as half of their original body size," Secord said. "Because warming happened much slower during the Paleocene-Eocene Thermal Maximum, mammals had more time to adjust their body size. So, it's not clear that we're going to see the same thing happening in the near future, but we might."

Philip Gingerich, who first recorded the phenomenon of decreasing body size during the Paleocene-Eocene Thermal Maximum in 1989, said the study documentation clearly demonstrates the relationship between temperature and body size. He agrees this may occur as a result of current warming patterns.

"I joke about this all the time -- we're going to be walking around 3 feet tall if we keep going the way we're going," said Gingerich, a researcher at the University of Michigan and director of its Museum of Paleontology. "Maybe that's not all bad and if that's the worst it gets, it will be fine. You can either adapt, or you go extinct, or you can move, and there's not a lot of place to move anymore, so I think it's a matter of adaptation and becoming smaller."

Researchers also analyzed correlations with aridity and carbon dioxide levels but found temperature to be the most likely driving factor in body size. Although little is known about how animals arrived in North America at that time, the Paleocene-Eocene Thermal Maximum is a significant event in geologic time in terms of mammalian history, Bloch said.

"The PETM is really important because it marks the beginning for the first appearance of several major groups of mammals, including crown-group primates (ancestors of modern primates) and the first even- and odd-toed modern ungulates (mammals with hooves)," Bloch said. "This sets the scene for the entire diversity of animals we see on the planet today."

Study co-authors include Doug Boyer of Brooklyn College, Aaron Wood of South Dakota School of Mines and Technology and the Florida Museum, Scott Wing of the Smithsonian Institution National Museum of Natural History, Mary Kraus of the University of Colorado, Francesca McInerney of Northwestern University and John Krigbaum of UF.