Pronghorn: A Bison Associate

Except for a lone red-tailed hawk flying high above, the high plains appear to be scrubbed of animal life.  Yet, a sense of being watched cannot be helped.  A scan of the surroundings does not reveal anything at first.  Suddenly, a flash of brown darts across the vista and just as suddenly stops.  Two big black orbs stare back.  The watcher is being watched.  In the distance, barely perceptible among the flora stands a pronghorn, assessing the newcomer to its territory for potential danger.  Discerning none, yet it remains still, vigilant. 

The pronghorn, commonly referred to as the American pronghorn antelope [1] or just American antelope, is not an antelope.  Its scientific name is Antilocapra Americana which means American antelope goat.  However, it is neither antelope nor goat.  The pronghorn has no close relative, being the surviving descendent of an ancient family of herbivores extending back 20 million years [2].  The more common, shortened name of pronghorn derives from the pronged horn which can grow to a foot long on males.  Different than antlers of a deer or elk, pronghorn horns grow around a bony, skin-covered core that is not shed. The horns of pronghorn are also different than buffalo horns.  For the pronghorn the outer sheath of keratin that makes up the horn is shed and regrown [3].

The Pronghorn

When Lewis and Clarke made their way across the Great Plains in their journey to the Pacific Ocean, pronghorn numbered in the millions much as the bison did.  The range of this smallest ruminant in North America, extended from the plains of south-central Canada—Saskatchewan and Manitoba—south through a large part of the western United States and into Mexico.  Great herds ranged from the Mississippi River to the Pacific Ocean lapping the shores of central California, and from the Gulf of Mexico to the Pacific Ocean in southern California.  Now, their numbers are much less, and their distribution restricted to pockets of isolated habitat within the pronghorn’s original range.  The presence of highways, railways, fencing, and other human imposed barriers prevent movement of the pronghorn [4].

Pronghorn share the same range, foraging and watering with the bison, yet they do not compete.  The bison consume mostly grasses while the pronghorn graze on forbs (broadleaf plants such as wildflowers, milkweeds, etc.) and shrubs which benefit from bison activity.  Bison disturb dominant grass communities by trampling, wallowing and grazing, which result in greater production of forbs and shrubs favored by pronghorn.

Pronghorn are picky eaters.  Small bodies need less food but better food.  The smaller a warm-blooded animal is, the more of its body warmth is lost to the air, and to compensate, the higher its metabolism must be, requiring a higher quality of food.  Grasses that have been clipped off retain the growing portion—close to the root, where most of the protein resides.  This is a source of high quality sustenance [5]. Bison do not eat grass down to the root.  Rather they clip the grass off just above the ground, which leaves the younger growing portion for other grazers such as the pronghorn.  Bison also provide for the pronghorn, as well as other species, during the winter.  In areas of deep snow, bison grazing activity push away the snow with their massive heads, exposing forage for others [6].

The forbs, however, make the life of the pronghorn possible.  Forbs contain more protein and less lignin [7] than grasses.  Since forbs grow in patches among the grasses, pronghorns have a different relationship to the grasslands than the bison’s.  For bison the grasses go on forever, which renders selective grazing rather moot.  For the pronghorn, however, grazing is much more selective, which necessitates moving from patch to patch [8]. 

Unlike the bison pronghorn are devout territorialists due to their grazing needs.  Again, it is about the grassland.  For bison the grassland is more or less grassy everywhere.  But for the pronghorn it’s about the patches of forbs.  A bison bull needs only worry about laying claim to the cows. It does not have to worry about staking out a grazing territory.   The pronghorn buck, on the other hand, needs to worry about food sources to carry on its progeny.  This small associate of the bison, then, expends much effort in marking its territory by urinating and defecating in strategic spots to ward off other bucks. 

Bucks guarding territory is not the only worry of pronghorn.  Bucks, does and fawns keep a careful eye and a watchful curiosity on their surroundings, allowing them to detect danger early on.  The most inquisitive of all the bison’s grassland associates, the pronghorn—with its large, dark eyes—has a keen, wide-ranging vision, and is able to spot predators at great distances.  Movement can be seen up to 4 miles away [9].  If an approaching predator is spotted, near-by pronghorn are signaled.  In time of danger the stiff hair on the rump bristle and rise, becoming a broad patch of white.  These rump displays telegraph signals across the terrain, warning other pronghorn of the impending danger [10]. If the danger is warranted, an adult pronghorn can break into a run at speeds reaching 55 miles per hour and sustaining that speed up to a half mile, making it the fastest land animal in North American.  In terms of peak speed, the only other faster animal is the cheetah of Africa, which can reach 80 miles per hour.  In regard to sustained speeds, however, the pronghorn can maintain speeds greater than the cheetah over longer distances.  For instance, the pronghorn is able to run several miles at over 30 miles per hour [11].    An adult pronghorn can easily outrun any of its predators.  Some scientists believe the pronghorn developed such in speed long ago to escape the American Cheetah, a fast ambush predator, which went extinct about 12,000 years ago [12].

Pronghorn Running

A pronghorn’s predators include coyotes, bobcats, mountain lions, golden eagles and wild dogs, which primarily target fawns.  The pronghorn’s main defenses include its keen eyesight and its speed.  The best defense for a fawn is to find cover, lie down and remain motionless while its mother or other adult pronghorn lure the predator away [13].

Pronghorn Fawn

Two hundred years ago pronghorn numbered in the millions but by the 1920s they had suffered the same fate as the buffalo.  Unregulated hunting, drought, and human activity had caused those numbers to plummet, resulting in less than twelve thousand pronghorn.  Since the early 19th century, however, conservation measures and the elimination of hunting seasons have brought back the pronghorn.  One measure particularly effective was the modification of fencing.  Pronghorn are highly migratory and such constructs as fencing can present a devastating barrier. Unlike other species, pronghorn do not jump fences.  Rather, they will attempt to crawl under.  In the past this resulted in getting hung up in the fencing, causing injury and possibly death.  Today, however, ranchers construct fencing which allows the pronghorn to go under and continue their migratory journey [14].

The restoration of the pronghorn, a unique species, has been a marked success in the overall efforts to preserve the Great Plains.  Their comeback is proof that concerted conservation endeavors can succeed, providing inspiration for other Great Plains restoration undertakings.

End Notes:

[1] Urbigkit, Cat. 2010. Path of the Pronghorn. Boyds Mill Press. Honesdale, PA 7.

[2] Rickel, Bryce. 2005. Large Native Ungulates. Chapter 2 in USDA Forest Service Gen. Tech. Rep. RMRS-GTR-135-vol. 2. 27.

[3] US National Park Service. Pronghorn: Racers of the Great Plains.  Pronghorn: Racers on the Great Plains (U.S. National Park Service) (nps.gov).  Retrieved 28-Nov-2020.

[4] Rickel.

[5] Lott, Dale F. 2002. American Bison: A Natural History. University of the California Press. Berkeley, CA. 122.

[6] Schuette, Keith.  “Bring Back the Buffalo Nation—Part 3.” February 2019.  www.bisonwitness.com.

[7] Organic polymers that form key structural materials in the support tissues of vascular plants.

[8] Lott. 120-122.

[9] Rickel. 29.

[10] McHugh, Tom. 1972. The Time of the Buffalo. University of Nebraska Press. Lincoln, Nebraska. 219.

[11] Urbigkit, 8. See also National Wildlife Federation. Pronghorn. Pronghorn | National Wildlife Federation (nwf.org). Retrieved 4-Dec-2020. 

[12] US National Park Service. Pronghorn: Racers of the Great Plains.  Pronghorn: Racers on the Great Plains (U.S. National Park Service) (nps.gov).  Retrieved 28-Nov-2020.

 [13] Rickel.

[14] Urbigkit. 32.