Stay-at-home equine ancestors

Unlike today’s horses, some of their prehistoric ancestors did not travel great distances to find food or water, according to a new study by the University of Cincinnati. In fact, it seems that, five million years ago, Florida was something of a horse paradise, providing everything the animals could want in a relatively small area.

Geologists in University of Cincinnati's McMicken College of Arts and Sciences found that prehistoric horses in coastal Florida lived and died within a comparatively small area.

In contrast, Mongolian (Przewalski's) wild horses travel up to 13 miles a day. In southern Africa, Burchell's zebras are known for their seasonal migrations that take them as far as 300 miles and back as they follow the rains to green grass.

"It seems that these horses in Florida were relatively sedentary. They didn't travel far distances," said Jenelle Wallace, a UC graduate and lead author of the study.

 UC researchers found that prehistoric horses in Florida were sedentary, much like wild horses today in Assateague Island National Seashore. Photo: Michael Miller/UC

The small three-toed animals lived like antelope, browsing leaves in deep forests. But during the Miocene Period (between 23 million and 8 million years ago), horse evolution exploded into 15 different families. Horses developed bigger bodies, longer legs and hard hooves in place of toes to help them cover more ground.

Their teeth also changed, becoming bigger and longer for cropping coarse grass covered in abrasive silica dust instead of plucking soft leaves. It's these teeth that helped the researchers study how extinct horses lived.

They compared the ratio of strontium isotopes (87Sr/86Sr) found in fossilized horse teeth to the strontium in bedrock in different parts of the American Southeast to track the horses' wanderings. Plants such as grass absorb strontium from the earth and the horses, in turn, absorb that strontium while grazing. In this way, strontium serves as a geographic marker.

University of Cincinnati geology professors and study co-authors Brooke Crowley and Joshua Miller have used this technique to track the movements of other animals, both living and prehistoric. Crowley used bones collected from the nests of secretive goshawks to map the birds' travels in Madagascar. She and Miller also are studying the movement of Ice Age mastodons in North America.

"There is a lot of opportunity for expanding the use of strontium to look at a variety of animal groups, time periods and locations," Crowley said.

The study examined seven species of pre-historic horse along with two known leaf-eaters: a prehistoric tapir and a distant relative of elephants called a gomphothere.

The research team found the results surprising. Of all the animals studied, the tapir seemed to have the widest geographic range based on the high variability of strontium found in its teeth. But given that modern tapirs have relatively modest home ranges, researchers thought it more likely that prehistoric tapirs consumed river plants that absorbed nutrients carried far downstream.

Among the horses, there was little variation in the size of their ranges. But the strontium showed a connection between horses and the sea. Like modern horses in places such as Assateague Island National Seashore, prehistoric horses might have fed along the coast. Researchers suggested the vegetation horses consumed was influenced by marine-derived strontium from seaspray, precipitation or saltwater intrusion into groundwater.

Migrating is dangerous business, Animals face injury, illness and starvation when they travel great distances. And in the Miocene Period, horses had to outwit plenty of big predators such as saber-toothed cats.

"The study suggests we're not the only couch potatoes. If animals don't have to move, they won't," Miller said. "The energetic costs of moving are high."

Crowley, who also teaches in UC's Department of Anthropology, said studies like this shed light on the habitat needs of animals long before they were influenced by human activities.

"Having a deep perspective is really important for understanding a species' needs in conservation and management," Crowley said. "If we just look at a narrow window of time -- like 50 or 100 years -- we don't get a good picture of a species when it's not in crisis."

Using the geologic record, researchers can piece together how animals interacted, what allowed them to thrive and what ultimately caused them to perish, she said.

"By using this technique, we can answer questions in a way we couldn't otherwise," Crowley said. "That's the cool thing about geochemistry. You can unlock secrets in teeth and bones."

The study was funded by grants from the UC Geology Department, Sigma Xi, the Geological Society of America and the American Society of Mammalogists along with the Association for Women Geoscientists Winifred Goldring Award.

For more details, see:

Investigating equid mobility in Miocene Florida, USA using strontium isotope ratios
Jenelle P. Wallace, Brooke E. Crowley, Joshua H. Miller
Palaeogeography, Palaeoclimatology, Palaeoecology, (2019) 516, pp232-243
https://doi.org/10.1016/j.palaeo.2018.11.036