Researchers have uncovered a direct link between global temperatures and body size, leading them to conclude that future climate change could mean species getting smaller.
A team led by scientists from the University of Florida and the University of Nebraska followed the evolution of the earliest horses about 56 million years ago, and found that as temperatures increased, their body size decreased.
"Horses started out small, about the size of a small dog like a miniature schnauzer," says Jonathan Bloch, associate curator of vertebrate paleontology at the Florida Museum of Natural History.
"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."
The earliest-known horse, Sifrhippus, first appeared in North America during the 175,000-year Paleocene-Eocene Thermal Maximum, when increased concentrations of carbon dioxide in the atmosphere and oceans caused average global temperatures to rise 10 to 20 degrees.
The team's traced its evolution from an estimated 12-pound animal to one the size of a domestic cat 130,000 years later. Over the following 45,000 years, it increased in size once again to reach 15 pounds.
"We could immediately see that the shifts in size of horses and temperature were mirror images of each other," says Ross Secord of the University of Nebraska-Lincoln.
The findings raise the obvious questtion of 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," says Secord.
"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."
And, of course, human beings could shrink too.
"I joke about this all the time - we’re going to be walking around three feet tall if we keep going the way we’re going," says Philip Gingerich, a researcher at the University of Michigan.
"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."