Pronghorn speed and relict behavior

Why does the pronghorn need to be that fast? It can run 90 kilometers per hour and keep going for several kilometers. That's speed and endurance, not just a brief burst. Yet no present-day North American predator is nearly fast enough to chase it down. Being faster than your predators makes sense; being so much faster creates the puzzle.

My first thought is that the predators might have changed. Maybe the animal that made this speed necessary is no longer around. The biologist proposes just that: the pronghorn adapted to extinct predators, including American cheetahs and long-legged hyenas. Fossils establish that those animals existed, and they are believed to have been fast enough to catch pronghorns. The predators disappeared, but the speed remained.

That is the idea behind a relict behavior: a behavior continues after the environmental conditions that originally favored it have disappeared. The explanation isn't that pronghorns need this much speed today. It's that a need in the past can help explain a trait we see now.

The second paragraph raises a fair objection. How do we test an explanation when an essential part of the situation is extinct? We can't simply watch a pronghorn encounter an American cheetah. That makes these historical explanations difficult to test, so many biologists are skeptical. But difficulty testing an idea doesn't make the idea false. The writer turns to evidence we can still observe.

Consider herding. More animals mean more eyes watching for predators, and a smaller chance that any particular animal will be attacked. But a herd also brings crowding and competition for food. We should ask whether the protective benefit is still needed. The account says pronghorns have nothing to fear from present-day carnivores, so their continued herding seems to fit a threat from the past. That claim isn't that carnivores have vanished from the environment. It's that those carnivores don't pose the threat that would make herding useful to pronghorns.

Mate choice supplies another piece of evidence. If speed and endurance once made the difference between escaping and being caught, it would make sense for those abilities to matter in choosing a mate. And female pronghorns do choose the winners of sprints and chases between males. That matches what researchers would expect if athletic ability had been important to survival.

The final paragraph asks whether this kind of persistence happens elsewhere. Stickleback fish from populations long free of sculpin still respond to that predator by avoiding its mouth and biting it from behind. Some ground-squirrel populations still recognize rattlesnakes after 70,000 to 300,000 years without snakes. These are concrete examples of defensive behavior continuing long after the original danger has disappeared. They make the pronghorn explanation less of an isolated guess.

Then we get a limit. Arctic ground squirrels separated from snakes for about three million years don't show the same recognition; they display only disorganized caution, even after repeated bites. So the behavior need not last forever. That doesn't erase the evidence that it can persist for a very long time.

Overall, the writer supports an explanation of the pronghorn's speed as a response to vanished predators. The fossil evidence, the pronghorn's other behaviors, and the examples from other animals work together to make that explanation plausible. The final qualification limits how long such behavior may persist; it doesn't reverse the direction of the argument.