As far back as 50 million years ago, horses figured out that bigger isn’t always better. Researchers have learned that many mammals—horses included—adapted to climate change by getting smaller.

By requiring less nutrition and water to survive and by having more surface area compared to their general size to better “air themselves out,” smaller horses and other mammals can adapt to hotter climates, said Abigail (D’Ambrosia) Carroll, a PhD candidate in the Earth and Environmental Sciences Department at the University of New Hampshire, in Durham.

During recent periods of global warming (56 and 53 million years ago) horses either migrated or evolved to exist as smaller species, Carroll said. In fact, the greater the temperature increase, the smaller the horses got. They might have adapted over many generations to become smaller than their predecessors. Or, already existing smaller horses might have migrated into areas once inhabited by larger horses.

“Maybe smaller-bodied horses of the same species simply migrated northward as Earth temperatures rose, in some attempt to stay cooler in the higher latitudes,” she said.

Smaller animals fare better in warm weather because they can thrive on less and lower-quality food, Carroll said, noting that, in fact, animals might have evolved smaller due to their food’s lesser nutritional value. Increasing climate temperatures would have caused droughts and floods, negatively affecting vegetation growth and nutritional content.

Meanwhile, smaller animals benefit in hotter temperatures from what scientists call “surface-area-to-volume ratio” (SAV). Essentially, by having more square inches of skin per cubic inch of body mass, smaller animals have a more efficient cooling system. “There’s more surface area to lose heat through, which is helpful if you are already living some place pretty warm and need to cool off more easily,” Carroll said.

Global warming can also cause the extinction of horses that aren’t adapted to their new climate, which was the case during a global warming episode about 50,000 years ago, other researchers have reported.

In their study, Carroll and her fellow researchers examined tooth fossils from equine ancestors that lived over many millennia, all from a region now known as the northern Bighorn Basin in northwestern Wyoming. Mammalian teeth provide an excellent view of body size, the researchers said, as science has shown repeatedly that tooth size (especially of the first molar) is directly correlated with body size.

They looked at how tooth size in horses and horselike species (Arenahippus, Diacodexis, Hyopsodus, and Cantius) compared across these prehistoric times:

  • The Paleocene-Eocene Thermal Maximum (PETM), about 56 million years ago;
  • The Eocene Thermal Maximum 2 (ETM2), about 53 million years ago; and
  • The periods before, between, and after these ranges.

During the PETM, annual temperatures in the Bighorn Basin reached between 19° and 26°C (66° to 78°F), up 5° to 8°, the researchers said. They then decreased about 2° to 5° after the PETM. But then they shot right back up to the PETM levels of 19-26°C during the ETM2.

They found that during the ETM2, the equids decreased in size by about 14%. And during the more extreme PETM period of global warming, they dropped by approximately 30%, Carroll said. In fact, in some cases they appear to have reduced their size by as much as 44%. In the years following the global warming, however, they “rebounded,” increasing their size back to about 76% of what it had been before the global warming episode.

So should we expect our own horses to get smaller as we enter this new phase of global warming? Probably not, Carroll said. At least, not domestic horses.

“I think it would take serious dramatic temperature change and changes to the food supply for us to see any difference in domestic horses,” she said. “We humans are pretty good at controlling the environments we live in,and the environments of the animals and livestock we depend on and care about.”

That might not be the case with wild and feral horses, however. “It wouldn’t shock me if free-roaming and wild populations began to show signs of body size decrease,” Carroll said. “Scientists around the world have already been documenting climate-induced body size change in some wild mammal populations, from the red deer in Europe to the California squirrel.”

The horses in Carroll’s study were already significantly smaller than today’s horses, however. “Arenahippus was about the size of a dog when temperatures were ‘normal,’ and when they dwarfed during those extreme warming events, they reached the size of a cat,” she said. “And grasses actually didn’t exist yet when these little guys were running around (and didn’t evolve for another 15-20 million years), so instead they were eating twigs, berries, nuts, and foliage from within the forests that they dwelled.”

And unlike the current global warming episode, those of the PETM and ETM2 occurred over thousands of years. “These early Eocene warming events saw their maximum temperatures reached within 10,000 years,” she said. “That seems to have been enough time for animals to adapt in one way or the other.”

Carroll said today’s climate change is happening at about 10 times the rate of the PETM. “Considering that, I often wonder if our mammals (and other animals) will have enough time to adapt this time around,” she said. “I guess time will tell.”

The study, “Repetitive mammalian dwarfing during ancient greenhouse warming events,” was published in Scientific Advances.