Back in the day of the first mammals, horses started out the size of house cats, weighing about 8 pounds and standing only a few inches tall. Then they got even littler, a direct result of the warm temperatures that characterized the Eocene era. Only when Earth cooled down a bit did the beasts get big, according to a new study — the first evidence that temperature directly affects body size. Interesting results when you think about rising global temperatures. Are Earth’s animals about to undergo a New Shrinkening?