Up the valley of Yellowstone National Park’s Lamar River there was a sight we had never recalled witnessing. Across the river, on the edge of a herd of bison, was a group of pronghorn (also known – incorrectly — as antelope). They were many miles from their winter range near the north entrance at Gardiner, and barely within the outer limits of a summer range map drawn by wildlife biologists.
We wondered why so many pronghorn had drifted this far east, and if maybe they hung out with bison as some protective precaution. In the old days, pronghorn would follow the bison herds, feeding on native forbs and shrubs left ungrazed by the bison. Here, the same players were reenacting their ancient roles.
Yellowstone's current pronghorn population ranges around 300, far below levels seen in the last century. (Patrick Dawson/WyoFile — click to enlarge)
We watched the group of pronghorn cut through the middle of the bison herd, angling through the sagebrush toward the river, just as a female radio-collared wolf turned back from watching the bison, swam the river, and trotted across the road past groups of amazed human wolf-watchers and up the mountain slope to her den.
Of course – this was all wolf territory now, hostile ground for coyotes, which had roamed here unchallenged for decades until wolves returned in 1995. Which apparently made it safe for pronghorn to range about, as coyotes would not dare enter wolf turf looking to prey on new pronghorn fawns. Coyote numbers in parts of Yellowstone have plummeted since wolves came back.
It was mid-May, just before fawning season, when most pronghorn does drop twins. Heading west back down the road towards Tower Junction, we came upon more pronghorn, close to the road, with separate groups of males and females bounding back and forth on the right-of-way, apparently heedless of human traffic in their habitat.
Fluctuating Populations
Were Yellowstone’s pronghorn expanding their range? Were they growing in number? After all, they had hovered right around 200 to 300 animals for decades, according to annual spring counts, a number considered too small by some biologists for long-term viability. Until about the late 1800s, they could still stretch out on a winter range downstream from the north entrance, into Montana’s Paradise Valley along the upper Yellowstone River, all the way to Livingston. That range shrunk, so pressured by human settlement and fences (that don’t deter elk or deer but are insurmountable for pronghorn) that the National Park Service in 1932 added an adjacent 7,600 acres of former National Forest land as pronghorn range. But it appears to be inadequate, as the northern range’s pronghorn numbers climbed to about 800 during World War II, dropped to a low of 102 in 1981, rose to about 600 by 1990, then crashed down to its current levels below 300 ever since.
According to a NPS report, coyote predation of fawns was a major factor in the quick decline of Yellowstone's pronghorn in the early 1990s. But the reintroduction of wolves has caused shifts in the coyote locations, subsequently creating pronghorn growth. (Patrick Dawson/WyoFile — click to enlarge)
“The reason for the sudden population decline in the early 1990s remains unclear, but fawn survival is low due to coyote predation, and development of private land north of the park has reduced available winter range,” the Park Service reported last November in a status report on Yellowstone’s wildlife. “The pronghorn range in the park is former agricultural land infested with non-native vegetation of low nutritional quality.”
As the continent’s fastest land animal, adult pronghorn don’t need to worry about predators in open country. Most threats are to the newborn, easy pickings for coyotes, bears, eagles and sometimes wolves. In Yellowstone, studies have shown that most predated pronghorn fawns have been taken by coyotes. But something has changed in recent years: Reintroduced wolf packs are dominating the scene, displacing the coyote’s once-exclusive territory and its predation of pronghorn young. Could it be that more wolves mean fewer coyotes which equals more pronghorn?
Pronghorn rebound in Grand Teton
A Wildlife Conservation Society (WCS) three-year research study of pronghorn mortality in the Grand Teton National Park concluded that in sectors where there were no wolves, but a high density of coyotes, pronghorn fawn survival rate was only 10 percent.
In areas of known wolf habitat, however, 34 percent of fawns survived. More than 100 pronghorn fawns were radio-collared and monitored over the summers.
“People tend to think that more wolves always mean fewer prey,” said WCS researcher Dr. Kim Berger, lead author of the study. “But in this case, wolves are so much bigger than coyotes that it doesn’t make sense for them to waste time searching for pronghorn fawns. It would be like trying to feed an entire family on a single Big Mac.”
WCS biologists explained that wolves reduce coyote numbers by killing them outright or by causing them to shift to safer areas of the Park not utilized by wolves. After wolves were reintroduced in 1995, according to the WCS study finished in 2008, Grand Teton’s pronghorn population increased by about 50 percent.
WCS concluded that de-listing of wolf protection in the region could affect long-term viability of other species, notably pronghorn. “This study shows just how complex relationships between predators and their prey can be,” said Berger. “It’s an important reminder that we often don’t understand ecosystems nearly as well as we think we do, and that our efforts to manipulate them can have unexpected consequences.”
Not long after the WCS Teton report came the results of an intensive research project on Yellowstone’s northern range pronghorn which found that coyotes accounted for 56 percent of adult predation and up to 79 percent of fawn predation.
Pronghorn predators studied
Between 1999 and 2005, research biologists from the National Park Service at Yellowstone and University of Idaho remotely monitored pronghorn does that they had captured in the winters and fitted with radio collars equipped with mortality sensors. They also electronically tracked 28 fawns that had been fitted within four days after birth. From 1999 to 2006, they visually investigated several non-wired fawn mortality incidents.
Early research on coyote predation showed evidence that wolves had displaced the coyote population in certain areas. Later studies showed pronghorn fawn were more likely to survive in areas with a higher wolf population. (Jon Sullivan — click to enlarge)
They recorded 13 predator kills of adult pronghorns: five coyotes, three cougars, one wolf and four undetermined predators, as well as one death “due to birthing complications.”
“We witnessed one instance,” the researchers reported, “in which a coyote attacked and killed an instrumented pronghorn mother on the day after she had given birth. Two of the cougar predation events on adults occurred within migration corridors in the period immediately following birthing.”
Researchers reported that they personally witnessed two of the six coyote-killed fawn events. “Also, we recovered four of the five coyote-scavenged fawn ear tags within 30 meters of coyote dens; all showed bite marks.
“Finally, a black bear was observed opportunistically killing a bedded, 10-day old fawn in 2004. The bear quickly carried the non-instrumented fawn into a timbered area while being observed by a small group of alarmed adult female pronghorn nearby.”
Predators most responsible for pronghorn adult and fawn mortality were led by coyote, followed by cougar, Golden Eagle and black bear. Wolf predation of pronghorn was considered very low, usually “opportunistic,” or by random chance. The 18-square-mile, semi-arid pronghorn winter range between Gardiner and Mammoth Hot Springs is not known wolf turf, but has plenty of coyotes, while the pronghorn’s spring and summer range is wolf territory – now hazardous zones for coyotes.
The Yellowstone researchers, careful not to draw conclusions on the wolf-coyote-pronghorn relationship, diplomatically observed, “There is speculation that the reintroduction of wolves may indirectly contribute to increased recruitment of migrant Yellowstone pronghorn by reducing the coyote population in areas of high wolf use… Further work is needed to investigate the extent to which wolves, habitat characteristics and social factors influence the predation risk posed by coyotes on Yellowstone’s northern range.”
Wolves helping pronghorn thrive?
A succeeding report, in which the Yellowstone researchers analyzed their work and published their findings in The Journal of Mammalogy in 2010, was less equivocal, concluding that, after several seasons of wolf presence, the park’s pronghorn are exhibiting uncharacteristic behavior of heading for higher ground to seek refuge from coyotes while birthing. They also state that in winter, coyotes limited in their movements by deep snow allows for high-elevation sanctuary for young pronghorn.
A map of pronghorn birth sites along the northern border of Yellowstone National Park, as related to areas with wolf populations. (American Society of Mammalogists — click to enlarge)
The researchers had already chronicled incidents of pronghorn predation that mirror those of the past tens of thousands of years: That when pronghorn are away from open country, their protective advantages of visual acuity and mobility quickly diminish, and they become ambush prey for cougars and bears, notably in timbered migration corridors.
There is the obvious risk of becoming ambush prey in the mountains, but the pronghorn are now taking that chance, despite the danger, and so far it is paying off, leading, the researchers explained, “to improved reproductive success … our results suggest that wolf reintroduction has been of net benefit to the Yellowstone pronghorn population. This is further supported by the higher overall rate of fawn survival in relatively wolf-rich migrant areas compared to areas inhabited solely by non-migrants and an increase in the proportion of migrants from pre-1995 to the current study period.”
Pronghorn as living fossils
Together, bison and pronghorn represent living specimens of survivors of large mammals going back to the Pleistocene, or North American Ice Age, at least 15,000 years ago.
Back during the Pleistocene, pronghorn evolved its great speed, endurance and bulged-eyeball vision as it adapted to a couple of the fiercest, swiftest predators of the era, the American cheetah (Miracinonyx trumani) and the giant short-faced bear (Arctodus simus). Those mega-predators are long gone, and yet pronghorn remains, a living relic, running fast as ever, its evolutionary adaptation apparently not relaxed by millennia of slower predators.
Fossilized pronghorn of Wyoming’s Natural Trap cave. The pronghorn species has existed on Earth since the Pleistocene Era. Many of it's predators of that era are long since extinct. (Patrick Dawson/WyoFile — click to enlarge)
“The late Pleistocene extinctions of North America decimated an ungulate and carnivore savanna fauna that was more diverse than the [contemporary] fauna of east Africa,” according to J.A. Byers in his book, “American Pronghorn.”
Byers, also a lead author of the Yellowstone pronghorn research cited above, explained, “The living pronghorn thus is a fair representative of its family. In most respects, the pronghorn is not a specialized end product, but, rather, the fortunate survivor of late Pleistocene extinctions that wiped out nearly all species of a specialized lineage… Although it is impossible to know whether running adaptations of pronghorn have declined under relaxed selection, the running ability of the species today suggests that decline has been slight. Pronghorn are ridiculously too fast for any modern predator.”
In Wyoming’s Natural Trap cave east of Lovell, paleontologists in the 1970s discovered what is considered “one of the more reliable records” of fossilized pronghorn, closely resembling today’s living Antilocapra americana, dated at between 17,000 and 20,000 years ago. Traces of more than a dozen pronghorn were found down there in the layers of sediment along with scattered remains of others that unwittingly dropped in through a hole in the cavern’s ceiling: eleven Giant American cheetah, plus short-faced bear, mammoth, saber-toothed lion, camel, sloth, Ice Age wolf and little stilt-legged horse.
As Byers observed, “The behavior of living pronghorn is, in part, a ‘historical document’ that illuminates the fossil-written text of the North American savanna fauna. … Many aspects of pronghorn behavior also read as historical documents, reflecting an earlier time of intense predation.”
The pronghorn population, according to the spring flyover count from April 2011, was not up at all over recent years – probably skewed due to dispersal of many of them down the upper Yellowstone valley due to that winter’s above-average snowfall. The biologist in the Piper Super Cub counted 242 last year, compared to 297 the previous year. Park naturalist data suggest that they are adapting, on the move, dispersing from lower to higher elevations, ranging wider, using the wolf’s territory as kind of a “no-man’s-land” buffer to protect their newborn from coyotes. The 2012 flyover count will come later this summer.
The “relict herd” of Yellowstone pronghorn that outlasted extinct American cheetah by millennia only to be stymied by fences and European settlement, still runs through the hills and valleys, always alert for predators, and helped for now, it seems, by the return of the wolf packs.
Banner photos by Michael Wifall/Flickr and Dennis Matheson/Flickr.
REPUBLISH THIS STORY: For details on how you can republish this story or other WyoFile content for free, click here.
If you enjoyed this article and would like to see more quality Wyoming journalism, please consider supporting WyoFile: a non-partisan, non-profit news organization dedicated to in-depth reporting on Wyoming’s people, places and policy.