Yellowstone elk have strategies for avoiding both wolves and mountain lions in the park/NPS

Viewed as prey by both wolves and mountain lions, elk in Yellowstone National Park have figured out how best to avoid those predators, according to a new study.

While the elk aren't always successful, the ability of many elk to dodge the predators on a daily basis explains why elk, mountain lions, and wolves all are "thriving on the Yellowstone landscape," said Dan Stahler, a Yellowstone biologist who participated in the research.

According to the just-released study, mountain lions prefer to hunt at night in forested areas, whereas wolves usually hunt in the morning and early evening hours on flat, grassy areas of the park's Northern Range. 

"Elk sidestepped both cougars and wolves by selecting for areas outside these high-risk domains, namely forested, rugged areas during daylight when cougars were resting, and grassy, flat areas at night when wolves were snoozing," explained Michel Kohl, the research paper's lead author who now is an assistant professor at the Warnell School of Forestry and Natural Resources at the University of Georgia in Athens.

The team produced its results by revisiting global positioning system (GPS) data from 27 radio-collared elk that had been collected in 2001-2004 when numbers of wolves and cougars were highest. Kohl and Dan MacNulty, a Utah State University researcher and the paper's co-lead author, combined the elk GPS data with information on the daily activity patterns of GPS-collared cougars and wolves and the locations of cougar- and wolf-killed elk to test if elk avoided these predators by selecting for 'vacant hunting domains,' places and times where and when neither predator was likely to kill elk.

Recognizing that cougars and wolves hunted in different places and at different times allowed the researchers to see how elk could simultaneously minimize threats from both predators.

"Had we ignored the fact that these predators were on different schedules, we would have concluded, incorrectly, that avoiding one predator necessarily increased exposure to the other," said MacNulty. "Movement out of the grassy, flat areas and into the forested, rugged areas to avoid wolves did not result in greater risk from cougars and vice versa because these predators were active at different times of the day."

Despite the compatibility of elk spatial responses to cougars and wolves, Toni Ruth, executive director of the Salmon Valley Stewardship in Salmon, Idaho, cautioned that "some adult elk still end up on the cougar and wolf menu, with those in poor condition during winter being most at risk."

The study highlights that where prey live with more than one predator species, attention to one predator that ignores the role of another may lead to misunderstandings about the impact of predators on prey populations and ecosystems. It also offers new insight into how prey can use differences in hunting behavior among predators to maintain safety from all predators simultaneously.

Still, the researchers were surprised to learn that mountain lions, not wolves, exerted the most pressure on elk habitat selection.

"Wolves are often the presumed or blamed predator for any change in a prey population, numerical or behavioral," said Doug Smith, who leads Yellowstone's wolf program. "Our research shows that this is not necessarily true, and that other large predators in addition to wolves need to be considered."

"Despite the fact that most prey species live in habitats with multiple predators, the majority of research on predator-prey interactions focuses on a single predator species," added Betsy von Holle, program director for the National Science Foundation's Division of Environmental Biology. "The novelty of this research is the simultaneous study of multiple predator species, revealing the complexity of predator avoidance behavior by the prey."