A team of scientists led by researchers from the University of Florida and University of Nebraska has discovered a correlation between environmental temperature and body size in mammals by following the evolution of the earliest known horse Sifrhippus sandrae.

This horse lived in the forests of North America 56 million years ago, including during a 175,000-year interval of time known as the Paleocene-Eocene Thermal Maximum (PETM). When average global temperatures rose by some 10 degrees Fahrenheit during this period, about a third of mammal species responded with significant reduction in size.

By analyzing the size and the chemistry of the Sifrhippus’ fossil teeth found in Wyoming, the team has tried to figure out what the environment was like when the teeth were growing.

The results, published in the international journal Science, show that S. sandrae initially weighed in at around 12 pounds (5.6 kg), but then shrank by approximately 30 percent to the size of a small housecat (about 8.5 pounds or 4 kg) at the start of the PETM, when climate warmed.

Then, the species grew abruptly at the end of the period as climate cooled, reaching about 15 pounds (7 kg).

The scientists have suggested that rising temperatures or high concentrations of carbon dioxide primarily caused the phenomenon in S. sandrae and other mammals during this period.

“For the first time, going back into deep time – going back tens of millions of years – we were able to show that indeed temperature was causing essentially a one-to-one shift in body size within this lineage of horse,” explained Dr. Jonathan Bloch, a co-author on the study and an associate curator of vertebrate paleontology at the Florida Museum of Natural History of the University of Florida in Gainesville.

“Because it’s over a long enough time, you can argue very strongly that what you’re looking at is natural selection and evolution – that it’s actually corresponding to the shift in temperature and driving the evolution of these horses.”

“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 Dr. Ross Secord of the University of Nebraska-Lincoln, lead author on the paper. “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.”

The findings also raise important questions about how animals might respond to rapid global warming in the near future.

“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,” Dr. Secord said. “Because warming happened much slower during the PETM, 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.”

The researchers also analyzed correlations with aridity and carbon dioxide levels but confirmed 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 PETM is a significant event in geologic time in terms of mammalian history,” Dr. 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). This sets the scene for the entire diversity of animals we see on the planet today.”