The ancestor of the modern horse, the Sifrhippus, weighed about 6 kilograms when it appeared in North America during the Paleocene-Eocene Thermal Maximum (PETM). During this 170,000-year climate event, the average global temperatures increased by 10 degrees. Researchers of the University of Florida and the University of Nebraska found that after its arrival the body size of the Sifrhippus went down, as temperature went up during the PETM.
“Horses started out small, about the size of a small dog like a miniature schnauzer,” says co-author Jonathan Bloch. “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.”
By analyzing the size and isotopes of fossils, the researchers traced the evolution of the Sifrhippus that shrank by 30 percent until the animal weighed about 4 kilograms and had the size of a small house cat. In the last 45,000 years of the PETM, when the weather became colder, the horse increased in size again to about 8 kilograms.
The researchers investigated the dryness of environments, atmospheric pressure and even the nutritional content of plants exposed to higher levels of carbon dioxide, and ultimately concluded that temperature was the primary driver of size evolution in Sifrhippus.
So how will animals 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 lead author Ross 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.”
Secord, R., Bloch, J., Chester, S., Boyer, D., Wood, A., Wing, S., Kraus, M., McInerney, F., & Krigbaum, J. (2012). Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum Science, 335 (6071), 959-962 DOI: 10.1126/science.1213859
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