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.
What this question is testing
Topic
The author is investigating a small mystery: why is the pronghorn (an antelope-like animal) so weirdly fast when nothing currently chases it?
Framework
Problem-Solution. The author presents the puzzle and walks through a proposed answer with its supporting evidence.
Main Point
The simpler version: pronghorns can run 90 km/h, but no current predator is anywhere close to that fast. Why? One theory says they're still running from predators that went extinct 10,000 years ago — basically, evolution hasn't caught up. Biologists are usually skeptical of "they're running from ghosts" explanations because they're hard to test, but in the pronghorn case the rest of the animal's behavior (herd grouping, mate preferences) lines up. Other animals show similar leftover behaviors too — fish that fear extinct fish predators, squirrels that recognize rattlesnakes despite no contact for thousands of years.
P1: The mystery
Pronghorns run way faster than anything currently chasing them. One biologist's theory: they're adapted to predators that went extinct 10,000 years ago — American cheetahs and long-legged hyenas.
P2: Why this kind of theory is hard to swallow
"Relict behavior" — a behavior left over from extinct conditions — is hard to test because the predator is gone. Biologists usually only accept it when nothing else works.
P3: The evidence stacks up
Pronghorns herd — useful for spotting and avoiding predators, but costly because of food competition. Why bother if no predator threatens? Looks like a leftover. Similarly, female pronghorns choose mates by speed, which would have mattered when fast predators existed.
Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.