A new study suggests that the earliest known horse species shrank significantly in size over a 135,000-year span as a consequence of a warming climate. Using geochemical testing and measurements of fossilized teeth dating back more than 50 million years, U.S. researchers found that a 30-percent decrease in the body size of the species, Sifrihippus sandae, corresponded closely with changes in global temperatures. As the average global temperature rose by about 10 degrees F during the first 135,000 years of that period — known as the Paleocene-Eocene Thermal Maximum (PETM) — the early horse, which lived in the forests of North America, declined in size from about 12 pounds to about 8.5 pounds, presumably because a declining amount of available oxygen. According to the study, published in the journal Science, the species regained much of its size during the final 45,000 years of the PETM, bulking up to about 15 pounds. Researchers say the so-called “dwarfing” phenomenon could provide insights into how animals will respond to a projected future increase in global temperatures.