Ancient prairie predators leave ghostly legacy

By Gary Raham
Nature Writer and Illustrator

A friend of mine named Mike hunted pronghorn. It really was more of an excuse to pass some time on the prairie admiring the big Wyoming sky because "hunting" pronghorn involves creating a blind behind a decoy and waiting for a nosey animal to amble by.

In reality, Mike ambushed pronghorn, because they aren't easily caught any other way. Pronghorn canter at 25 miles per hour, gallop without straining at 45 mph and can run 56 to 62 mph at top speed. They are the fastest animals in North America. No predator in their habitat today can catch an adult without ambushing them, unless the pronghorn is sick or injured. Why, we wonder, do pronghorn expend the energy for Ferrari-speed when Honda-speed economy would suffice?

John A. Byers, a pronghorn researcher, began asking himself this question in the 1980s when he studied the behavior of these superficially antelope-like grass-burners in Montana's National Bison Range wildlife refuge. The answer he deduced during 14 years of fieldwork involved ghosts--or at least ghostly survival imperatives--etched into pronghorn genes of by-now extinct predators that could match their prey's speed.

Our prairies were once quite dangerous places--not a mere 200 years ago, when wolves and bears called the Front Range home--but 10,000 years ago when post-Ice Age hunters left spear tips in giant bison and mammoth vertebrae. Here's a list of the animal dangers that pronghorn had to deal with just 5,000 generations in their past. (Pronghorn females usually produce their first fawns at age 2.)

Dog-like predators included modern-style wolves, giant dire wolves (like those whose fossils have been recovered from California's LaBrea Tar Pits), wolf-sized and hyena-like plundering dogs (genus Borophagus), and two species of pack-hunting carnivores (Cuon alpinus and Protocyon). Hyenas, whose long limbs imply that they might have been fast runners, also prowled the prairies.

In addition to fairly modern types of black bears and grizzlies, the giant short-faced bear (Arctodus simus), 67 percent larger than a grizzly, possessed relatively long legs and feet oriented from front to back in a way implying that they were fast runners, too.

Most people have seen pictures of the saber-toothed cat, Smilodon. This heavy-shouldered pussycat probably attacked big, slow prey like mammoth, cutting jugular veins with their impressive teeth. But fewer people know that our prairies were also home to a giant lion (Panthera leo atrox), a jaguar and a cheetah, not to mention the familiar cougar (Felis concolor).

All these predators most likely gave mother pronghorns lots to worry about. Even though fawns can be up and running faster than a human in four days, they would certainly have been vulnerable to many of the predators listed above. Few of these predators, however, could have outrun a healthy adult. Wolves today, for example, rarely have been observed to take adult pronghorn, preferring instead to pick on bigger game, like elk. Based on the behavior of modern animals, it's also quite likely that the lions, jaguars and cougars would have ambushed pronghorn only when the right opportunity presented itself. The behaviors of extinct predators, with no definitive modern counterparts with which to compare, are always somewhat problematic. We can speculate that the Ice Age hyena and short-faced bear may have beset pronghorn, but they are different enough from modern species to raise questions about their dining tastes. Fossil cheetahs look enough like modern varieties to make more confident predictions. Modern cheetahs love to dine on African ungulates with pronghorn habits and size, and they can shift into running-speed gears that can propel them at 60 to 70 miles per hour. Moreover, they coexisted with pronghorn for at least 2.5 million years--long enough, it would seem, to make speed a high priority for pronghorn survival.

Cheetahs chased pronghorns for 1.25 million generations. It's not surprising that pronghorn still run for their lives after just 5,000 generations of predator relief. More than 99 percent of their experience tells them to run and run fast. Various scientific studies have shown that behaviors in animals change slowly after selection pressure is relaxed--not surprising when such basic animal behavior gets hard-wired in the genes.

Other animals seem haunted by their pasts as well. Hawks flying over large Madagascan lemurs cause fear reactions, even though none of these birds is large enough to harm them. Fossils show that large raptors that could have been a serious threat once existed on the island. Ground squirrels in California all show fear responses to snakes, although only some have snake predators in their modern habitats. Spanish horses that went wild in North America quickly grouped together in herds, following the instincts of forebearers once threatened by many, if not all, of the same predators as pronghorn.

The Ice Age still lives in all of us, which is why I can imagine Mike behind his blind, watching clouds build on the horizon, anticipating the nervous approach of a handsome buck, and find myself wishing on some deep and visceral level that I were there, too.

Reference: Byers, John A. "American Pronghorn: Social Adaptations and the Ghosts of Predators Past." Chicago and London: The University of Chicago Press, 1997.