PronghornsSurvivors of the American
Savanna
Article by Alex Hawes
Lions roamed Los Angeles. The jungle kings’ fossilized
jaws now rest in the Rancho La Brea tar pits along Wilshire
Boulevard, past the bus depot and the local Sav-on. Mastodons,
saber-toothed cats, the aptly named yesterday’s camel,
and an unassuming ungulate known as the American pronghorn
also lie entombed in this Pleistocene-epoch asphalt. The visitor
to La Brea may find it hard to visualize these Ice Age beasts
inhabiting the Bel-Air or Burbank of yesteryear. Yet in sinkholes
and fossil beds across the continent, the remains of zebras,
ground sloths, giant marmots, and other seemingly exotic creatures
further spotlight the star-studded cast of mammals that graced
the North American stage more than 10,000 years ago.
A changing climate and prehistoric human hunters each likely helped draw the curtains on this mega-mammal extravaganza. The pronghorn (Antilocapra americana), however, keeps performing. The fastest mammals this side of Africa, pronghorns have outrun extinction. These graceful animals can today be seen streaking across the grasslands, deserts, and sagebrush flats of 15 western states and adjacent areas of Canada and Mexico.
Although known commonly as antelopes, pronghorns are not closely related to the true antelopes and gazelles of Africa and Asia. Rather they belong to a distinctly American family of hoofed ruminants, the Antilocapridae, which arose on this continent about 20 million years ago. Pronghorns have filled antelopes’ savanna-sprinting niche admirably. At least 12 pronghorn genera—and dozens of species—are known from the fossil record. Only the lone member of the genus Antilocapra outlived the Ice Ages, however. Scientists now recognize five pronghorn subspecies, or races, within this single surviving species. These include Antilocapra americana americana, the most abundant, found across the western U.S. as well as Alberta and Saskatchewan; A. a. oregona, from southern Oregon, northern California, and Nevada; A. a. mexicana, from Texas, New Mexico, Arizona, and northern Mexico; A. a. peninsularis, from the deserts of Baja California; and A. a. sonoriensis, which clings to a fragile existence along the Arizona-Mexico border [see sidebar, Borderline Survival].
More than 800,000 pronghorns persist—a mere fraction of the tens of millions that once pranced North America, yet a much healthier population than the estimated 13,000 that remained in the 1920s after decades of hunting and plowing by European settlers. Pronghorns’ mere presence today stands as testament to the adaptability of this extraordinary species—and inspires our dream of a Western prairie reborn.
Pronghorns are truly creatures of the range. Once described as “dainty foragers,” pronghorns snack on dozens of forb and shrub species without stripping any one area bare. A typical pronghorn salad might include sagebrush, dandelion, alfalfa, fescue, bluegrass, rabbit brush, and even cactus. The animals’ small mouths and lips make detailing their diet difficult, however. “When they get their noses down to the ground and start eating, you can’t tell what it is,” says Patryce Avsharian, a University of Idaho biologist studying pronghorns in Montana. Being ruminants, pronghorns swallow their food quickly and then regurgitate and chew it more slowly later. This process makes tough, fibrous plant parts easier to digest—and easier to eat on the run.
Pronghorn demographics fluctuate with the availability of forage. In November and December, large herds of as many as several hundred animals coalesce in winter ranges where snow doesn’t accumulate too deeply and where tall, hardy vegetation like sagebrush peeks above the ubiquitous blanket of white. With new plant growth in the spring and summer, pronghorn herds fragment into smaller groups and disperse. Females strike out alone in May and June to find isolated spots to give birth. Then, as the summer progresses, males begin herding harems of females in preparation for the fall rut.
Pronghorns mate during an intense two- or three-week period each year in which bucks attempt to chase rivals away while keeping the females in their harem from shopping around for other mates. According to scientists, a buck has little actual chance of preventing a doe from straying. Yet females may judge the value of a prospective mate based on the vigor of his corralling efforts as well as his success fighting off rivals. Once the rut has passed and their prominent horn sheaths are shed, males that once fought fiercely begin congregating side by side for the long winter ahead.
Almost from birth, pronghorns vie for rank. Fawns born early in summer have a leg up on those born only a few days or weeks later, and hierarchies formed within their first year can last a lifetime. While yearling pronghorns may be physically capable of reproduction, females don’t usually bear offspring until their second year. Young males have little chance of out-competing older bucks for mates until their third or fourth year of life.
Maturing pronghorn males develop dark scent glands located at the back corners of their jaws. As the rut approaches, bucks thrash their heads against conspicuous shrubs to release this pungent scent, marking territory. While females’ horns range from tiny nubs to sharp, three-inch spikes, pronghorn bucks develop a pair of horns extending as much as 15 inches high above a shorter “prong” that juts forward. During jousts, these prongs may act much like a sword’s cross-guard, redirecting a rival male’s horn tip before it can inflict a potentially fatal jab.
Despite living in herds, pronghorns are not terribly vocally interactive. They do employ a few calls to communicate basic messages. Perturbed male rivals may emit a “snort-wheeze” to intimidate one another—Klamath Indians named the pronghorn cha-oo after the sound of this call. Frightened pronghorns also make a similar snorting sound—often while flaring their bright white rump hairs—to signal other group members of danger. A herd flees perceived threats in a tight ellipse or single-file line (which Captain Meriwether Lewis, in 1804, compared to a “rappid flight of birds”). Like a zebra’s stripes, the mottled tan and ivory banding on pronghorns’ necks and sides may create a hypnotic effect within a running herd, making it difficult for a pursuer to home in on an individual pronghorn.
Not that any modern predator can catch a pronghorn in full sprint.
Along the western slope of Wyoming’s Bighorn Mountains, an 85-foot carst sinkhole preserves in frightening detail the remains of the pronghorn’s ancient foes: the North American cheetah (Miracinonyx trumani) and American lion (Panthera leo atrox), each larger than their modern African cousins; the giant short-faced bear (Arctodus simus), a creature larger—and faster—than a Kodiak brown bear; as well as other dangerous carnivores like the dire wolf (Canis dirus) and a ferocious hyena-like creature, Chasmaporthetes ossifragus.
Pronghorns evolved their tremendous speed—they’ve been clocked at upwards of 60 m.p.h.—to escape these extinct Pleistocene predators, scientists believe. More than 400,000 pronghorn now inhabit Wyoming, symbolic of their victory over the beasts lingering at the bottom of Natural Trap Cave. Only the modern African cheetah (Acinonyx jubatus) could today best the pronghorn in a short sprint, and no animal on Earth could likely outrun America’s antelope in footraces longer than 800 meters. Coyotes and mountain lions may kill an occasional pronghorn through ambush attacks rather than extended chases. But by sticking to open grasslands and deserts with clear vistas, healthy adults rarely get eaten.
A pronghorn running on all cylinders is a thing of beauty. Like a Porsche sliding through highway traffic, a pronghorn can shift gears between a trot, gallop, and full sprint with remarkable fluidity. Studying videotape of pronghorns running, scientists at the University of Lethbridge in Canada detected at least 13 distinct gaits, including one reaching nearly eight yards per stride. Pronghorns rely on light foot and ankle bones and slender legs—which can withstand up to 45,000 pounds of pressure—for propulsion. Their circulatory system provides the fire, thanks to mighty lungs, hearts four times larger than those of a sheep, and a mesh of thick blood vessels that act as a radiator to keep their brains from overheating. Pronghorns run with their mouths agape to suck in as much oxygen as possible, further fueling their epic endurance and speed.
With eyes as large as those of a 1,200-pound horse, pronghorns also employ keen sight to keep a close watch over the range. Oregon biologist Arthur Einarsen, in 1948, wrote that pronghorns could spot objects moving three to four miles away; their vision has been compared favorably to someone using 8-power binoculars. Pronghorn eyes are set deep into their sockets, perhaps as protection against the thorny brush into which the animals often stick their faces. These sockets also sit laterally on the pronghorn’s skull to allow for wide-angle views. Sportsmen claim pronghorns can see behind them, and this isn’t entirely untrue. But the phantoms they seek may be invisible.
In his landmark book, American Pronghorn: Social Adaptations and the Ghosts of Predators Past, University of Idaho biologist John Byers argues that a host of pronghorn traits—their speed, their vision, and even their gregariousness—evolved under the threat of predatory species long since vanished. “Nothing that lives today in North America is nearly as dangerous as a cheetah, large hyena, a lion, or a huge long-legged bear,” writes Byers. Pronghorns had to employ extraordinary vigilance, speed, and endurance to survive these predators—far more than is necessary to escape predators alive today.
Such anachronisms can exact a price. Pronghorn bucks remain sleek and swift year-round, although they could conquer and defend harems more easily if they bulked up for the rut. Furthermore, by choosing speed over strength, bucks enter winter with less stored body fat than females, and some starve. By living in large groups—whose main benefit is the increased ability to spot predators—pronghorns also needlessly encounter competition with one another for food, Byers argues.
Pronghorns are far from alone in their anachronisms. Science writer Connie Barlow, in her recent book, The Ghosts of Evolution, highlights a host of living organisms whose partners in evolution have been upstaged [see “Books, Naturally,” September/October 2001 ZooGoer]. Take the lonely avocado, a fruit whose seeds evolved to be dispersed by now extinct goliaths, like the giant ground sloth, that could swallow and stomach its massive pits. Such fruits today must rely on human assistance—in the form of farming—to keep their populations going. “A wild avocado is Ginger Rogers without her Fred Astaire,” Barlow writes. “Jerry Lewis is now her only suitor, and he keeps tripping over his own feet.”
Nonetheless, speed continues to serve the pronghorn well. The animal’s overwhelming edge discourages carnivores like coyotes (Canis latrans) from even attempting a frontal assault, saving both predator and prey wasted energy. Yet, on rare occasions, a coyote will attempt a foolhardy dash at a pronghorn herd. “The ensuing interaction is almost comical,” writes John Byers. “The chase is very much like those depicted in the famous Roadrunner cartoons: The coyote, tongue lolling and clearly in an anaerobic condition, is about to close on Roadrunner, who suddenly smiles, says ‘Beep-beep,’ and streaks out of sight over the horizon.”
A novice tracker, I spotted my first pronghorn buck last July on a sandy sagebrush plain north of the Rabbit Ears Mountains in Colorado’s Arapaho National Wildlife Refuge. Within minutes, a lone coyote trotted into view 50 yards away. Eying me warily, the coyote crossed head-down in front of the buck before drifting away through the scrub. The two animals ignored each other, for the coyote was likely after easier prey—pronghorn fawns.
Pronghorns and coyotes both give birth during late spring and early summer, an unfortunate coincidence for protective pronghorn mothers. A single fawn represents about two days worth of energy for an adult coyote. Fawns in the first weeks of life thus offer a bonanza for coyotes and their newborn pups, according to John Byers, who has watched pronghorns and coyotes tango in Montana’s National Bison Range for nearly two full decades.
Pronghorn mothers, which average 105 pounds prior to pregnancy, usually deliver twins averaging a hefty eight pounds each at birth. Among the most precocious of newborn mammals, pronghorn fawns can stand within 20 minutes and can outrun even the most determined field biologist by the end of their first week. Predators can again be thanked for encouraging this extreme adaptation.
Nearly invisible in the tall brush and almost completely odorless, a pronghorn fawn lies motionless for much of its first month of life. The mother keeps her distance—an average of about 75 yards, Byers reports—to prevent lurking coyotes from guessing her fawn’s whereabouts. If a coyote approaches the general vicinity, the mother may pretend to forage or rest while subtly keeping an eye on the area where her fawn is hiding. If a coyote should wander too close—or if it should appear to be searching deliberately for fawns—the mother will chase after the hunter or kick at it with her hoofs. Coyotes normally dart off rather than fight back, although a small number of does are killed defending their young. Once the coyote is a safe distance away, the mother will lead her fawn or fawns to a new location.
Yet the wily coyote has hit upon a clever counter-strategy. A coyote will sometimes feign indifference to a pronghorn mother with a hidden fawn while ostensibly searching for mice or other small prey. The coyote may then stumble upon the pronghorn fawn by chance, or may wait for the mother to wander out of sight before searching more intensively. If it strikes out, at least the coyote will have eaten a few mice in the meantime. Byers found that over the course of a 15-year study period, 87 percent of pronghorn fawns at the National Bison Range in Montana perished during their first year of life. While coyotes leave little trace of their kills, Byers presumes that they are indeed the culprits in the large majority of cases. But by producing twins year after year, pronghorns can sustain and even expand their populations in spite of such heavy predation—assuming that there remains room to roam.
One sunny summer morning, field biologist Kate Nittinger and I venture into Yellowstone National Park’s Lamar Valley in search of a missing doe. Nittinger has spent three summers tracking 18 radio-collared pronghorns in the northern reaches of Yellowstone to compare the success of fawns here with those at the National Bison Range. The news so far is bleak: Only a tiny percentage of Yellowstone’s fawns survived the previous summer, and this year isn’t shaping up any better.
Pronghorns in the Lamar Valley move among elk and mule deer, thundering herds of bison, the occasional bear (black or grizzly), and—following their April 1996 reintroduction—a pack of now more than 20 gray wolves. This realization makes census-taking somewhat unnerving for the newly initiated.
Our doe—female #852—was spotted with a single fawn several days earlier. However, as Nittinger takes a telemetry reading from the side of her jeep, the doe’s signal goes undetected. We must head out on foot. After a few hundred yards, we reach the rushing Lamar River. Just as we’re about to shed our boots, Nittinger sees a bear blithely digging in the dirt on the opposite shore. “I think it’s a grizz—it’s got a hump alright,” she whispers from behind her binoculars. We hurry upstream until the bear’s a safe distance away, then ford the thigh-deep Lamar and continue on.
Spotting a group of pronghorns, Nittinger extracts her radio-tracking signal box and turns the dial to the frequency for the doe’s collar. The contraption hisses static, offering no guidance. “This doe is being very mysterious,” she mutters. Nittinger thinks the doe may have wandered up and over a ridge called Jasper Bench. And so, zigging and zagging around grumpy bison, we climb uphill until we reach the top of a crest overlooking the full sweep of the valley. Nittinger takes a telemetry reading again. Only when she aims the antenna back downhill toward the pronghorn herd we just passed does the machine chirp with promise. Perhaps #852’s signal had been blocked by a small rise in the undulating valley. We head back down.
Nittinger finally gets #852 in her sights and focuses her spotting scope on the animal’s udder, which is still bulging—a promising sign. We watch the doe intently while slapping at the biting flies. Several minutes and bug bites later, we observe a fawn approach #852 and begin nursing. “It’s alive!” cries Nittinger with glee. The hungry youngster is soon nudged away by its mother. At about 50 days of age, the fawn is ready for weaning. More important, it has matured enough to escape coyotes, golden eagles (Aquila chrysaetos), bobcats (Lynx rufus), and other predators. Odds are, the fawn will reach adulthood. This, in itself, is exceptional. Of the 36 fawns born this summer to the does in Nittinger’s study, only five have survived their first month of life.
The difficulty of radio-tracking animals across the Lamar Valley’s ridges reveals part of the problem. Yellowstone, in truth, is not at all ideal pronghorn habitat. Its steep, broken terrain hampers mothers’ ability to protect fawns as well as the ability of all pronghorns to detect and evade predators. Pronghorns here occasionally hang out “smack dab in the middle of the woods,” says Nittinger. As a result, even adult pronghorns may fall victim to not just mountain lions and wolves but also to the normally cautious coyote.
Pronghorn numbers in Yellowstone plummeted mysteriously from a high of 591 in 1991 to 235 in 1995. Only 209 pronghorns were counted last year. The demise of pronghorns in Yellowstone has coincided somewhat with the return of wolves. However, little evidence exists of wolves killing many pronghorns—these pack hunters prefer the meatier offspring of bison (Bison bison) and elk (Cervus elaphus). Instead, pronghorns are perhaps being foiled by continuing development north of the park that has cut off traditional migration routes.
Such impediments may extend throughout pronghorn habitat. National park creation out West has tended to follow the spine of the Rockies—from Glacier in Montana to Yellowstone and Grand Tetons in Wyoming to Rocky Mountain National Park in Colorado. A recent U.S. Geological Survey study found that the majority of federally protected reserves across the nation occupy scenic middle- to high-elevation ground characterized by low soil productivity. Much flat, pronghorn-friendly habitat, on the other hand, spans private land. There, pronghorns often surrender to the obstacles of rural civilization, appearing hesitant to cross major roads—and utterly unwilling to leap over livestock fences.
Biologists Hall Sawyer and Fred Lindzey have tracked the annual migration of Jackson Hole’s resident pronghorn herd from summer range in Grand Tetons National Park as much as 170 miles to the southeast—the longest known exodus for a pronghorn population. They found that the herd moves through tight bottlenecks in the terrain less than a half-mile wide that, if blocked, could single-handedly stop these historic treks. Unlike deer, which routinely clear hurdles six or more feet high, pronghorns for unknown reasons rarely jump over barriers of any size. The animals will scramble under fences that have sufficient room beneath their bottom strands. Yet many pronghorns, cut off from winter range, have starved at the foot of simple sheep fences whose wires extend nearly to the ground.
Pronghorns outnumber people in Wyoming, but so do cattle (by a margin of four to one) and sheep. “There’s a real range of attitudes toward antelope,” says Bill Rudd, a wildlife coordinator for the Wyoming Department of Game and Fish. “Some people really dislike them, while others are much more open to having antelope as part of the landscape.” Hesitant ranchers contend that free-ranging pronghorns chew up their pastureland. However, pronghorns have coexisted for millennia with bison, which, like cattle, forage primarily on prairie grasses, while pronghorns focus on forbs and shrubs. Broad-leafed forbs thrive in exposed sunlight, and so pronghorn herds have prospered in the wake of bison clearing away dense grass cover. Conversely, bison—and therefore cattle—are unlikely to find their pastures overgrazed by picky pronghorns.
State wildlife officials are encouraging cattle ranchers and federal overseers of Bureau of Land Management property to erect pronghorn-friendly fencing—namely ones with smooth, unbarbed bottom wires at least 16 inches off the ground. Officials are further exploring ways to facilitate pronghorns trying to cross dangerous highways. The claustrophobic creatures rarely use road underpasses, so overpasses may prove a more feasible, albeit expensive, alternative. Moving livestock fences back from roadsides could also keep pronghorns trapped between these fences from panicking when vehicles pass by. If viable habitat returns, so might the momentous pronghorn herds of centuries past.
As dawn warms the Lamar Valley, two pronghorn bucks stand sentinel amid a herd of more than 50 bison. In the distance, three wolves dart playfully among the buffalo and the gigantic crickets. A hawk launches itself into the azure sky, while meadowlarks serenade the valley in songs of morning. This is the Wyoming waltz, a dance performed in this small corner of the American West today—and perhaps on a bigger stage tomorrow.
“With sufficient open space, they will always be there,” writes John Byers of pronghorn, “to remind us of just how fast a North American cheetah could run.”
MORE! Borderline Survival
—Alex Hawes is Associate Editor of ZooGoer.
ZooGoer 30(6) 2001. Copyright 2001 Friends of the National Zoo. All rights reserved.
