Why So Fast, Pronghorn?

Quick! What’s the second fastest land animal in the world?
Everyone knows that the Cheetah is the fastest. But who’s number 2?
Give up?

It’s America’s own: The Pronghorn.

Pronghorn-buck-watches-over-harem-1024x768

And what a strange – and unique – animal it is.

It looks like an antelope. But it isn’t.

Its scientific name is Antilocapra Americana, which means, American antelope-goat. But while the pronghorn shares many of the features of deer, antelopes and other ruminants: a four-chambered stomach, cloven hooves, and a body shape similar to that of antelopes; it is not an antelope, nor a deer; not a goat, not a sheep. It is unique. It is the sole remaining member of the family antilocapridae. Its closest taxonomic relatives are the giraffids (giraffes and okapi).

Once, back in the Pleistocene, there were twelve different species of antilocapra in North America.[1] The other species differed in size and horn type from the Pronghorn: Ramoceros had long forked horns, and was much smaller than the pronghorn; Hayoceros had pronged horns above its eyes, like the Pronghorn, but also had another pair of straight horns behind those; Stockoceros had four horns.

But they are all extinct – only the Pronghorn survives.

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They do look like antelopes. Pronghorns range in size from 75 to 130 pounds, and stand about three and half feet high as the shoulders. They are predominantly a rusty-reddish color, with white patches on their bellies and rump, and white stripes on the throat. They flash the white patch on the their rumps to signals others of danger. They mate in the fall, when males fight rivals to corral a harem. And they fight using their odd pronged horns – again something unique to the Pronghorn.

You may know that antlers grow each season, and then are shed, only to grow again the following year; while horns on cattle or bison are permanent. The Pronghorn falls somewhere in between. It is the only animal in the world with branched or forked horns; and the only animal in the world to shed its horns. The horn begins with a bony growth, and then grows up over that; and then is shed annually. The horns of the male pronghorn are much larger than those of the female, and can grow to fifteen inches in length. The horns of the females are smaller, and rarely have prongs.

In addition to the unusual pronged horns that give the animal its name, the pronghorn has another unusual anatomic feature. It has extraordinary vision. Its eyes are the largest, relative to its size, of any North American ungulate; and it has a nearly 300 degree arc of vision, without moving its head or eyes. Pronghorns see movement more clearly than stationary objects, and can detect movement up to four miles away.

1024px-Antilocapra_americana_male_(Wyoming,_2012)

And that remarkable speed: The Pronghorn is the greatest runner in the world. Its speed is second only to the cheetah. A pronghorn can run at speeds up to sixty miles per hour; a cheetah, can run – for short distances – at speeds up to seventy miles an hour. But the cheetah flames out after a few hundred yards. The Pronghorn does not. True, they can’t sustain speeds of sixty miles an hour, but one source says that they can sustain speeds of 57 miles per hour for nine miles.[2]  And they can run at speeds in excess of forty miles per hour for over thirty minutes. As one author put it, “if pronghorn ran marathons, they would complete the course in 40 minutes.”[3]

The pronghorn is designed for speed, and endurance. Its bones are lightweight, relative to its size; the trachea and lungs are enlarged; and it runs with its mouth open, to take in more air. And the pronghorn’s capacity to utilize oxygen is incredible. In one test, scientists found that pronghorns use five times more oxygen per minute while running than other mammals of comparable size. But remarkably, the scientists who have studied the pronghorns and their adaptation for speed have found nothing else, really.

“[T]he researchers discovered that the pronghorn is just a little bit better at everything. Compared with the goat, it has bigger lungs with which to absorb oxygen, slightly more blood hemoglobin with which to transport the oxygen from the lungs to the muscles, and slightly bigger and leaner muscles containing a higher concentration of mitochondria–the cellular organelles that burn oxygen to provide power for muscle contraction. In other words, there are no tricks to the pronghorn antelope. It has simply perfected the same equipment that all mammals have.”[4]

So why this speed? Why has the pronghorn evolved this unequaled capacity to run at very high speeds for very long times? Why that incredible vision?

In a word: predation. Evolution is driven by adaptation. Something drove the pronghorn to develop this unmatched capacity to run and run and run, at high speed, over enormous distances. The question then is this: What nightmare creature had the speed and endurance to drive pronghorns to evolve their exceptional combination of speed and endurance?

The answer is: No one is really sure.

It wasn’t wolves, and it wasn’t coyotes. Newborn or injured pronghorns are vulnerable to predation by wolves and coyotes. But adult pronghorns are much, much faster than coyotes: They can outrun them easily. Wolves are pursuit predators. They hunt in packs and employ relay strategies where some of the wolves chase a prey animal, and then others of the pack take up the pursuit, eventually tiring and wearing down the prey so that it can be taken. And this perhaps was an effective strategy against the pronghorn when wolves roamed the same country as the pronghorns do. But for the most part wolves do not prey on pronghorns. They have other prey – elk, for example, and – today, at least – are not commonly found in the type of country where pronghorns roam.

Ambush predators like mountain lions don’t have enough cover to successfully stalk and kill pronghorns regularly.

So if it wasn’t coyotes, nor wolves, nor mountain lions, what animal was it? What nightmare creature had the speed and endurance to drive pronghorns to evolve their exceptional combination of speed and endurance?

There are two primary contenders: The American cheetah, and the hunting hyena.

One theory is that the pronghorn developed its remarkable running ability in response to predation by the American Cheetah. The name is not entirely accurate. Although they were fast – very fast – the two species of American Cheetahs were not cheetahs at all, but more closely related to mountain lions (cougars). Nonetheless they showed evolutionary adaptations designed for speed. One of them – Miracinonyx trumani – lived on the prairies and plains. It may well have been the pronghorn’s partner in the evolutionary race for speed and endurance.

But recently, Miracinonyx fossils have been found in the “wrong” habitat – in steep, rocky hillside areas – leading to some speculation that the American Cheetah lived more like a modern day snow leopard, than its African namesake.[5]

And cats tend to be sprinters rather than marathoners. Since the pronghorn can sustain its incredible speeds for such a long time, it seems likely that the running ability evolved in response to pressure from some predator that could run at high speed for long distances. And, in fact, there is another possible explanation for the pronghorn’s speed and endurance: The American hyena, Chasmaporthetes ossifragus, called the hunting hyena.

Not much is known about the hunting hyena. It is the only species of hyena to cross the land bridge from Eurasia to North America. “Fossil remains of Chasmaporthetes have been found at 4 sites in Florida, 3 sites in Arizona, 2 sites in north Texas, 2 sites in Mexico, and 1 site in New Mexico. . .[6]  Chasmaporthetes is called the hunting hyena because it, too, like the America cheetah, shows adaptations for speed. Its legs were long and slender, and it probably hunted in packs on open grassland. Today, African hyenas are tenacious pack predators, often chasing prey for long distances. Thus is seems possible that the American hunting hyena might have been the pronghorn’s partner in the evolution of this speed and endurance.

The American cheetah is long gone now, and the hunting hyena went extinct roughly seven hundred eighty thousand years ago. Still, the pronghorn survives; and still it carries the adaptations it evolved: the vision to see danger from far away, and the speed to run and run and run, as long and as fast as possible, to escape from whatever it was that pursued it – the American cheetah, or the hunting hyena. Whatever it was, it haunts the pronghorn’s dreams still.

Links to sources and articles of interest:

http://animaldiversity.org/accounts/Antilocapra_americana/

http://www.desertusa.com/mag99/may/papr/pronghorn.html

http://www.nwf.org/wildlife/wildlife-library/mammals/pronghorn.aspx

http://library.sandiegozoo.org/factsheets/pronghorn/pronghorn.htm

http://www.rollinghillswildlife.com/animals/p/pronghorn/

Yoon, C. “Pronghorn’s Speed May Be Legacy of Past Predators,” NY Times, Dec. 24, 1996. http://www.nytimes.com/1996/12/24/science/pronghorn-s-speed-may-be-legacy-of-past-predators.html

http://en.wikipedia.org/wiki/American_cheetah

http://www.prehistoric-wildlife.com/species/m/miracinonyx.html

http://twilightbeasts.wordpress.com/2014/08/08/the-surprising-cheetah/

Barnett, R., et al. “Evolution of the extinct Sabretooths and the American cheetah-like cat. 2005. Elsevier Pub. http://www.cell.com/current-biology/pdf/S0960-9822(05)00836-5.pdf

Switek, B. Did False Cheetahs Give Pronghorn a need for Speed? Phenomena, January 8, 2013. http://phenomena.nationalgeographic.com/2013/01/08/did-false-cheetahs-give-pronghorn-a-need-for-speed/

http://en.wikipedia.org/wiki/Chasmaporthetes

http://markgelbart.wordpress.com/2013/10/01/the-american-hyena-chasmaporthetes-ossifragus/

Switek, B. “The Hyena Who Saw the Canyon,” Laelaps, Wired.Com
March 3, 2011.
http://www.wired.com/2011/03/the-hyena-who-saw-the-canyon/

Photos:

Top photo by Stacy Dunn,http://www.molecularecologist.com/2012/12/qa-stacey-dunn-chases-pronghorn-fawns-to-measure-batemans-slope/

Middle Photo by Yashin S. Krishnappa, http://en.wikipedia.org/wiki/Pronghorn#mediaviewer/File:Antilocapra_americana_male_(Wyoming,_2012).jpg

Bottom photo by David Tremblay, http://www.discoverruidoso.com/Pronghorn-Antelope

Footnotes:

[1]Wikipedia reports that there were still five species when humans first entered the Americas, although the statement does not have a citation or reference.

[2] San Diego Zoo, Pronghorn, Antilocapra Americana, May, 2009. http://library.sandiegozoo.org/factsheets/pronghorn/pronghorn.htm

[3] Nowak, Rachel. “The Pronghorn’s Prowess,” Discover, December 1, 1992. http://discovermagazine.com/1992/dec/thepronghornspro172

[4] Nowak, Rachel. “The Pronghorn’s Prowess,” Discover, December 1, 1992. http://discovermagazine.com/1992/dec/thepronghornspro172

[5] See, for example, http://twilightbeasts.wordpress.com/2014/08/08/the-surprising-cheetah/

[6]http://markgelbart.wordpress.com/2013/10/01/the-american-hyena-chasmaporthetes-ossifragus/