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.
Topic
Why pronghorns run faster than any modern North American predator — the "relict behavior" hypothesis.
Framework
Highlight Noteworthy / Problem-Solution. The author presents the puzzle, the relict-behavior hypothesis, the supporting evidence (including pronghorn herding), and similar relict behaviors in other animals.
Main Point
The pronghorn's extraordinary speed and other behaviors are best explained as adaptations to long-extinct predators — relict behaviors. The supporting evidence (pronghorn herding, mate selection, plus parallel cases in fish and ground squirrels) increases the plausibility of relict-behavior explanations.
P1: The puzzle and the proposed solution
Pronghorns can run 90 km/h, far faster than any modern North American predator. One biologist proposes that pronghorns adapted to predators known from the fossil record (American cheetahs, long-legged hyenas) that have been extinct for 10,000 years.
P2: Why biologists are wary of relict behaviors
Most biologists distrust relict-behavior explanations because they're hard to test (the principal component is extinct). They're typically considered only when alternatives fail. But present-day observations sometimes yield supporting evidence.
P3: Evidence for the pronghorn case
Several pronghorn behaviors look like adaptations to extinct enemies. Pronghorns herd, which gives more eyes to watch for predators and reduces the chance any one animal is attacked — but causes overcrowding and food competition. Since pronghorns have nothing to fear from present-day carnivores, herding looks like a relict adaptation. Similarly, female pronghorns choose mates by speed and endurance, which would have been useful when fast predators existed.