The incredible shrinking horse

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The incredible shrinking horse

Updated: February 25, 2012 - 4:29 PM

New research reveals how mammals evolve to become smaller amid a warming planet, which raises questions about the future of mammals.

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Artist's reconstruction of Sifrhippus sandrae (right) touching noses with a modern Morgan horse (left) that stands about 5 feet high at the shoulders and weighs approximately 1000 lbs.

Photo: Florida Museum of Natural Science, Illustration by Danielle Byerley

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Rising seas, killer storms, droughts and extinctions are not the only things to worry about on a warming planet. There is also the shrinking issue. It happened to Sifrhippus 56 million years ago.

Sifrhippus, the first horse, shrank from about 12 to about 8 1/2 pounds as the climate warmed over thousands of years, researchers reported in the journal Science. The horse (siff-RIP-us, if you have to say the name) lived in what is still horse country, in the Bighorn Basin of Wyoming, where wild mustangs roam.

Sifrhippus was not much like modern horses. It was the size of a cat and might have made a nice pet if anyone had been around to domesticate it, but the first hominids were a good 50 million years in the future. Its fossils provide an excellent record of its size change over a 175,000-year warm period in the Earth's history known as the Paleocene-Eocene thermal maximum, when temperatures are estimated to have risen by 9 to 18 degrees at the start, and dropped again at the end. Scientists have known that many mammals appear to have shrunk during the warming period, but detail was lacking until now. Ross Secord, of the University of Nebraska-Lincoln; Jonathan Bloch, of the Florida Museum of Natural History at the University of Florida in Gainesville; and a team of researchers report that the little horse got 30 percent smaller over the first 130,000 years, and then -- as always seems to happen with weight loss -- shot back up and got 75 percent bigger over the next 45,000 years. Secord said animals evolved to be smaller, perhaps because the smaller an animal is, the easier it is to shed excess heat. The question is, Secord said, is that if warming continues at the highest rate projected: "Can mammals keep up?"

NEW YORK TIMES

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