Pronghorns

The pronghorn, an antelope-like mammal that lives on the western plains of North America, is the continent’s fastest land animal, capable of running 90 kilometers per hour and of doing so for several kilometers. Because no North American predator is nearly fast enough to chase it down, biologists have had difficulty explaining long-legged hyenas, either of which, it is believed, were fast enough to run down the pronghorn.

Like all explanations that posit what is called a relict behavior—a behavior that persists though its only evolutionary impetus comes from long-extinct environmental conditions—this one is likely to meet with skepticism. Most biologists distrust explanations positing relict behaviors, in part because testing these hypotheses is so difficult due to the extinction of do so. But present-day observations sometimes yield evidence that supports relict behavior hypotheses.

In the case of the pronghorn, researchers have identified much supporting evidence, as several aspects of pronghorn behavior appear to have been shaped by enemies that no longer exist. For example, pronghorns—like many other grazing animals—roam in herds, which allows more eyes to watch for predators and diminishes the chances of any pronghorns, for example, choosing the victor after male pronghorns challenge each other in sprints and chases.

Relict behaviors appear to occur in other animals as well, increasing the general plausibility of such a theory. For example, one study reports relict behavior in stickleback fish belonging to populations that have long been free of a dangerous predator, the sculpin. In the study, when presented with sculpin, these stickleback fish to recognize the threat of a rattlesnake, exhibiting only disorganized caution even after being bitten repeatedly.