Rising seas, killer storms, droughts, extinctions and money wasted on snowblowers are not the only things to worry about on a warming planet. There is also the shrinking issue.

It happened to Sifrhippus, the first horse, 56 million years ago. Sifrhippus shrank from about 12 pounds average weight to about 8 ½ pounds as the climate warmed over thousands of years, a team of researchers reported in the journal Science on Thursday.

The horse (siff-RIP-us) lived in the Bighorn Basin of Wyoming.

Sifrhippus was not much like modern horses. It was the size of a cat, ate leaves rather than grass and counts as a horse only in scientific classification. It 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 preserved fossils, abundant in the Bighorn Basin, 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 said to have risen by 9 to 18 degrees Fahrenheit at the start, and dropped again at the end.

Scientists have known that many mammals appear to have shrunk during the warming period, and the phenomenon fits well with what is known as Bergmann's rule, which says, roughly, that mammals of a given genus or species are smaller in hotter climates.

Although the rule refers to differences in location, it seemed also to apply to changes over time. But detail was lacking until now.

In Science, 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 other researchers report on the collection and analysis of Sifrhippus fossils from the Bighorn Basin.

They report that the little horse got 30 percent smaller over the first 130,000 years, and shot back up and got 75 percent bigger during the next 45,000 years.

The fossils indicate that at its smallest, Sifrhippus weighed about 8 ½ pounds and at its largest about 15 pounds.

Using fine-grained detail on climate and body size, the researchers concluded that the change in size was, as suspected, driven primarily by the warming trend.

"It seems to be natural selection," Secord said.

He said animals evolved to be smaller during warming because smaller animals did better in that environment, perhaps because the smaller an animal is, the easier it is to shed excess heat.

Paul Koch, of the University of California, Santa Cruz, a specialist in reconstructing ecosystems and climates from many millions of years ago, thinks the question of whether natural selection was the cause of the changes was still open.

Koch, who was not involved in the study, said the disruption of ecosystems during the warming period might have led smaller animals to migrate to new locations.