Of Pronghorn and Predators
- 8:46 am |
Capable of reaching speeds exceeding 70 kilometers per hour, the pronghorn (Antilocapra americana) is one of the fastest mammals on earth. No large North American carnivore can match it for speed – some conservationists have go so far as to suggest importing cheetahs to special parks to reinstate the evolutionary race between pronghorn and extinct big cats – yet every year many pronghorn fall prey to a canid more often considered a pest than a consummate hunter. Wolves, cougars, bears, and even eagles all prey upon pronghorn from time to time, but it is the coyote that kills more individuals than any other, especially in the northern range of Yellowstone National Park.
While traveling through northern Utah and Wyoming last summer I saw many pronghorn, but, despite their apparent abundance in the area, the Yellowstone population is quite small. Consisting of less than 300 individuals – low enough to put them at risk of being extirpated locally – the Yellowstone population primarily occupies an area along the northern border of the park. As reported last year in Western North American Naturalist by a team of ecologists led by Kerey Barnowe-Meyer, the partially-migratory group often summers in the arid, shrubby land around Gardiner, Montana in the winter but some migrate into Yellowstone during the summer.
To understand the population dynamics of the Yellowstone pronghorn and which predators posed to greatest threat to them, Barnowe-Meyer and colleagues monitored the movements and mortality of adult females and newborn fawns. During the winters of 1999-2001 and 2004-2006 the team darted adult females and placed tracking collars on them (which would also notify the scientists when the animal died) and in the spring of 1999-2001 they similarly tracked new fawns (taking care to make sure they did not place the baby pronghorn at increased predation risk). When an individual was killed the team went out to inspect the carcass and recorded whatever useful information remained about what kind of animal had killed the pronghorn, thereby providing an outline of what kinds of predators were taking pronghorn and with what frequency.
The trouble with a carcass in Yellowstone is that it does not last for long. Beyond the damage done by the attacking predator, scavengers can quickly obscure clues about what kind of animal made the kill. Nevertheless, the team was able to authenticate the cause of death in 22 cases of adult mortality, with 13 of those being attributed to predators (eight were undetermined and one was due to complications during birth). Of those thirteen the predator breakdown looked like this: 5 coyotes, 3 cougars, 1 wolf, and 4 undetermined predators. The sample was small, but, based upon the incidents in which the killer could be identified, coyotes appeared to be the most significant predators of pronghorn.
The sample of pronghorn fawns was also small, but showed a similar pattern. Of 28 tagged fawns, four survived, eight disappeared, and two died of unknown causes, leaving 14 cases of predation. Of this subset, six were killed by coyotes, five were scavenged (and may have been killed by) coyotes, one was killed by a large bird of prey, and two were killed by an unknown predator. Once again, coyotes appeared to be the most significant predator of the pronghorn, especially since they frequented the kind of habitat which pregnant females preferred for giving birth to and raising their fawns. The other predators took pronghorn opportunistically as the herbivores migrated between Wyoming and Montana, but coyotes preyed on them consistently.
What remains unknown, however, is how the predation of pronghorn by coyotes has been influenced by the reintroduction of wolves to Yellowstone in the 1990′s. Coyotes are mesopredators – second-tier carnivores whose populations are controlled by apex predators – and it has been proposed that the existence of wolves in northern Yellowstone acts as a check on the number of coyotes in the area. Then again, it may be that coyotes avoid the rugged, more forested habitats which are home to other predators in favor of more open areas, thus placing adult female pronghorn and their fawns at increased predation risk. At present, the effect of wolves and top-level predators on coyotes in Yellowstone is still poorly known, but figuring out how coyotes have responded to the reintroduction of wolves may help conservationists manage what is left of the Yellowstone pronghorn population.
Female and infant pronghorn are not the only individuals to be killed by coyotes. Male pronghorn, despite their armaments, can also fall prey to the mesopredators, and one recent case identified a unique risk suffered only by the males. Much like elk, male pronghorn often fight with their horns, and every now and then two males become irrevocably stuck. As reported by Jennifer Chipault and Dustin Long in The Southwestern Naturalist, at about 8 PM on October, 2 of 2006 two male pronghorn in Vermejo Park Ranch, Colfax County, New Mexico were found locked together – the horn of one was stuck on the head or neck of the other such that they were nearly nose-to-nose. One was already in a bad state, lying on its side and breathing shallowly, and the other made frequent attempts to free himself.
The naturalists observed the pronghorn intermittently throughout the night, but they were not the only ones watching. At about 2 AM on October 3rd several coyotes were seen in the vicinity of the stuck pronghorn. The coyotes did not immediately attack, perhaps being deterred by the presence of the human observers, but when the researchers left and checked back at the site at about 6:30 AM there was little left of the pronghorn which had been lying on the ground. What remained of it was still attached to the other male. As the researchers described the scene:
The pronghorn on the ground had been partially consumed; all that remained was the head and four limbs held together by dorsal skin, backbone, pelvis, and ribcage. The head of the carcass was still attached to the head of the live, standing pronghorn, which pulled, twisted, and within ca. 1 min freed himself from the carcass.
Is the great speed of the pronghorn attributable to an evolutionary “arms race” with the extinct, fleet-footed cat Miracinonyx? Perhaps, but it would be a mistake to consider the relationship between pronghorns and their predators as only a matter of speed. Infant and female pronghorn are vulnerable to much slower predators during the fawning season, and male pronghorn may inadvertently grapple their way into very vulnerable positions. Pronghorn are not withering away while waiting for a long-lost superpredator to reappear and kickstart their evolution – they continue to be actors in the Darwinian “struggle for existence” in which the rules, and competitors, are subject to change at any time.
Kerey Barnowe-Meyer, P.J. White, Troy Davis, and John Byers (2009). Predator-Specific Mortality of Pronghorn on Yellowstone’s Northern Range Western North American Naturalist, 69 (2), 186-194 DOI: 10.3398/064.069.0207
Chipault, J., & Long, D. (2010). Pronghorn (Antilocapra americana) Locked in Fight Becomes Prey of Coyotes (Canis latrans) The Southwestern Naturalist, 55 (2), 283-284 DOI: 10.1894/TAL-07.1