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The Pronghorn – The American Almost Antelope

Sunday, 16 October 2011

If you were asked to think of a large mammal of the American prairie you might well say the bison, coyote or wolf, a measure of how much these species have settled in to our general consciousness. Yet there is one unique American animal which is less known but is perhaps the most charismatic of the Great Plains. Many refer to it as an antelope but that is far from the truth. A true American native, the Pronghorn has sojourned across the deserts and plains of North America for at least a million years: but an antelope it is not. Its closest relative is the giraffe.

The pronghorn is found nowhere else in the world except the interior western and central north of America. At first sight it certainly does resemble the antelope of the Old World but it is thought to be a classic example of convergent evolution. This is where species develop to inhabit at least two separate places in the world but which share features and behaviors which mean they may resemble each other despite no shared ancestry. To the untrained eye, their appearance would suggest that they are related species even when they are not.

Think of the wing.  Birds and bats both have them but their last shared ancestor did not. Yet the wing developed in two very different species - the same biological trait appearing in unrelated lineages. So it is with the pronghorn – the demands of its habitat meant that it developed features very similar to the Old World antelope despite having no relation to it in terms of ancestry.

The lone member of its family, the Antilocapridae, the closest relativeof the pronghorn is in fact the giraffids family, which itself now contains only two species, the okapi and giraffe. Yet they do resemble the bovids (to which the antelope belongs) much more than the giraffe. This is because on the plains of North America they filled a niche similar to that of the bovids that evolved in the Old World. They were highly successful too and for a million years there were more than a dozen species in the family. Then we arrived.

Though in decline in terms of the number of different species, when humans entered the New World there were still five antilocaprid species on the Great Plains. The pronghorn (or to give it its scientific name, Antilocapra americana which means American goat-antelope) is now all that remains (with a number of sub-species). Yet how lucky we are to have this remnant of a once thriving family of mammals among us!  The fastest animal in the western hemisphere, the pronghorn is quite remarkable.

Its speed and its astonishing eyesight are features which have adapted to suit life on the prairie. First the eyes – the lack of trees on the prairie for cover meant that it had to be able to spot its predators, today the wolf, bobcat, eagle and the coyote, from a great distance. This lack of cover also means that when it spots danger, it has to be able to leg it as quickly as possible.

And boy can it run! It can gallop away from danger at an incredible 60 miles per hour and is able to sustain 30 miles an hour longer than any other animal on the planet. Unlike that other speedy mammal, the cheetah, which tires after a couple of hundred meters, the Pronghorn can sustain speed over a long distance. It is unlikely a modern cheetah would ever catch a pronghorn, unless by stealth and surprise.

 It is thought that this amazing speed evolved as a defense against a number of species which are  now extinct, rather than those which still try their luck with this most speedy of species.

Unsurprisingly perhaps, the extinct predator which scientists believe encouraged this adaptation is the long departed American cheetah (left).

The pronghorn also has evolved quite an unusual technique to pass on the message to the others in the herd that danger is approaching.  It flares out the white hairs on its rump when it is scared and this serves as a warning to the others. When it gives birth (more often than not to twins) this is the time at which it is most susceptible to predators. However, it has a defense here too.  When they are born the young pronghorn do not have an odor. They remain motionless on the prairie floor for hours until they have the strength to move at speed.

Its extremely large lungs and heart as well as the fact that it has hollow hair allow great speeds to be attained. Yet the pronghorn is a poor jumper and this has meant that their ranges have been affected by ranchers’ fences. They also try to run under these barbed fences at great speed which can have unfortunate consequence for the animal. As such, ranchers are now encouraged to use barbless metal for the bottom wire. At one point along its length the migratory passage has been reduced to a mere one hundred meters in width so allowing these animals greater access to the prairie has become a real political issue.

As a migratory animal the barbed wire has proven a major obstacle to the pronghorn even though there are widespread measures being taken to ensure that it can migrate without hazard from humans. It is unfortunate for this animal that although the pronghorn eats many plants that sheep and cattle find unpalatable there is competition for food which has disinclined some ranchers to aid its plight.

The pronghorn was, until the 1920s, heading the way of the dinosaur, due to hunting. Its numbers were reduced to around ten thousand yet then the progressive habitat and hunting laws of the National Park System (introduced in 1916, just in time) came in, the species was able to recover. Although there is no completely accurate number it is thought that there are now up to a million pronghorn in the United States (yet three sub-species remain critically endangered).

Conservation measures mean that the future of the pronghorn is ensured, despite the fencing issue. It can only be hoped that this unique and remarkable species will continue to populate the Great Plains for another million years.


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