Ancient global warming may have caused horses to shrinkS

Most mammals tend to be smaller in hot climates and larger in cold ones. This fact is known as Bergmann's rule (or as I like to call it, Chihuahuafication). Now, with climate change on the horizon, just how much will temperature put pressure on animals to evolve to a different size? Here's what history teaches us.

55 million years ago, the Earth went through a major hot period called the Paleocene-Eocene Thermal Maximum (PETM), 175,000 years where the average temperature was up by about 10°F. During this period, the tiny ancestors of modern horses went through a dramatic size change, shrinking substantially, and then growing once the weather cooled.

Before the PETM, Sifrhippus sandrae weighed around 12-pounds, about the size of a small dog. During this period of increased temperature, their mass shrank by 30% to 8.5-pounds, around what a small cat weighs. Over the 45,000 years after that, they gained that size back, bulking up by 75% to 15-pounds.

Some models of future climate change are suggesting we'll soon be undergoing the same temperature change as the PETM, just on a massively accelerated time-frame. There have even been some suggestions that mammal shrinking is already happening. Maybe we will end up with house hippos and miniature giraffes after all.

[Illustration by Danielle Byerley, Florida Museum of Natural History]