LOS ANGELES, CALIFORNIA—When Earth warms, mammals shrink. That’s what researchers found when they looked back 56 million years, during a time when global temperatures increased about 6° for a period of 20,000 years. Early horses, for example the now-extinct Hyracotherium (shown on the right next to a modern horse on the left), shrunk by about 30%, presumably to increase the ratio of skin area to body volume, and thus lose heat more easily. Now, scientists have found that this wasn’t a one-off event. At last week’s meeting here of the Society of Vertebrate Paleontology, another team of U.S.-based researchers looked at a slightly later but somewhat less severe warming period, which happened about 53 million years ago. Using molar size as a proxy for body size, the researchers looked at mammals in sediments from the fossil-rich Bighorn Basin of Wyoming. They found that the same thing happened: Mammals, including deer and small primates that resembled today’s lemurs, got small again, with horses like Hyracotherium downsizing by about 22% this time. In both cases, the animals rebounded to their previous sizes when the warming episode was over. Both this and the earlier warming episode were preceded by big increases in atmospheric carbon dioxide, similar to what we are seeing today. Time to get small, everyone?