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Oct. 04, 2011

The Pronghorn of North America: Running from the Past

by Kara Rogers

Click to enlarge images

Antilocapra americana

The pronghorn of the plains, deserts, and grasslands of North America is the fastest mammal found outside of Africa. In a sprint, Antilocapra americana can reach speeds in excess of 80 or 90 km/hr -- which would keep pace with a slow cheetah. Over distance the pronghorn can sustain speeds of about 60-65 km/hr. But none of the pronghorn's predators come close to matching its speed, which makes one wonder why the species evolved to be so fast. The reason, according to some biologists, could be that the pronghorn is running from the past -- from the ghosts of extinct predators.

The relationship in speed between pronghorn and their living predators, which include golden eagles, wolves, coyotes, mountain lions, and bobcats, is akin to a formula one car racing against a passenger car. With its long, slender legs; lung and heart volumes unusually large for its size; and a remarkably high capacity for oxygen uptake, the pronghorn is specially adapted for running at high speeds and for sustaining speed over long distances. But while pronghorn can easily outrun their modern-day predators, twenty thousand years ago, when the now extinct American cheetah (Miracinonyx) roamed the plains of North America, the situation was probably very different. Indeed, excessive speed and endurance likely were required for pronghorn survival.

Speed, however, is not the only unusual trait of pronghorn. For example, although it is sometimes referred to as the American antelope, the pronghorn actually is only distantly related to antelope. In fact, the pronghorn is classified in an entirely separate family -- Antilocapridae -- and is the only living member of this group.

Antilocapra americana

One reason for its distinct classification is that the pronghorn has atypical horns. Flattened and hollow, the horns split into two branches, one of which forms a forward-angled point known as a prong (giving rise to the animal's common name) and the other of which is a larger extension stretching upward behind the prong and curving slightly inward. Pronghorn shed their hollow horns each year, which makes them different from animals such as antelope, bison, goats, and sheep, which have hollow, unbranching horns that are never shed, as well as from animals such as deer and elk, which have solid, branching antlers that are shed annually. Similar to reindeer, however, both male and female pronghorns bear horns, although females' horns often are very small or even absent.

Another curious feature of the pronghorn is that females, after their first year of giving birth, almost always bear twins in subsequent births. Twinning is uncommon among wild ruminants, with the exception of goats and some types of sheep. Fawn mortality among pronghorn, however, is naturally high, so twinning may serve as a mechanism to ensure the survival of at least one fawn.

About 200 years ago, an estimated 35 million pronghorn roamed the plains of North America. By the early 1920s, however, hunting had reduced their numbers to about 20,000. Conservation measures implemented shortly thereafter allowed populations to rebound, such that about 700,000 pronghorn now exist. The Sonoran pronghorn (A. a. sonoriensis), however, which is one of five recognized subspecies of pronghorn and which inhabits the Sonoran Desert of the southwestern United States and northern Mexico, is endangered, with between 200 and 500 individuals left in the wild.
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Kara Rogers is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is a member of the National Association of Science Writers and author of Science Up Front on the Britannica Blog. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.

This post also appears on the Britannica Blog.

About Kara Rogers

Kara is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is the author of Out of Nature: Why Drugs From Plants Matter to the Future of Humanity (University of Arizona Press, 2012).

The views expressed are those of the author and are not necessarily those of Science Friday.

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