Global warming shrank certain animals in the ancient past, and scientists worry it could happen again.

Warm-blooded animals got smaller at least twice in Earth's history when carbon dioxide levels soared and temperatures spiked as part of a natural warming, a new study says.

University of New Hampshire researcher Abigail D'Ambrosia warned that mammals — but not people — could shrivel in the future under even faster man-made warming.

"It's something we need to keep an eye out for," said D'Ambrosia, who led the new work. "The question is how fast are we going to see these changes."

Three different species shrank noticeably about 54 million years ago when the planet suddenly heated up. One of them — an early, compact horse — got 14 percent smaller, going from about 17 pounds to 14.6 pounds, according to an analysis of fossil teeth in the journal Science Advances.

"These guys were probably about the size of maybe a dog, then they dwarfed," said D'Ambrosia. "They may have gone down to the size of a cat."

In hotter climates, mammals and other warm-blooded animals need to shed heat, so they shrink. Smaller animals have more skin — or fur — per pound than bigger animals so more heat can escape, making them better adapted for a warmer climate. Larger animals do better in the cold because they have less skin per pound and keep their heat.

Associated Press