JACKSON HOLE, WYO – Mule deer numbers are down in western Wyoming. That’s not disputed. But a recent study may cast some light onto reasons for the population decline—most often attributed to severe winters of late.

A study by cat conservation group Panthera found evidence that could debunk game managers’ popular belief that hunting pressure on mountain lions is good for mule deer. In fact, the opposite could be true.

Study area in the Greater Yellowstone Ecosystem, located northeast of the town of Jackson, Wyoming. The rectangle indicates the area where we caught and monitored pumas intensively. (L. Mark Elbroch)

Looking at the Greater Yellowstone Ecosystem for the past 17 years, the study’s authors found, at best, conflicting evidence that controlling pumas (mountain lions) aids ungulate populations.

“Food webs and predator-prey dynamics are complex, however, and the assumption that limiting predators results in increased prey does not always play out in natural systems,” said the study’s lead author L. Mark Elbroch. “Our results suggest that the current controversial strategy of puma culling to aid mule deer in multiprey systems, and more broadly, the strategy of culling any predator to aid one prey type in multiprey systems, may result in unexpected consequences.”

What research found is intuitively predictable. Younger cats eat smaller animals like beavers and porcupines. As the lions get older (about 3-years-old and up), they take on larger prey like deer and elk. Hunting mountain lions removes some of the oldest, largest prime toms that are experienced enough and large enough to take elk as their preferred prey.

Too much hunting pressure on big cats can leave a skewed demographic and disproportionate amount of mid-adult pumas in the 2-3 age range that have graduated from eating small game but are not quite up to elk. These cats prey mainly on mule deer.

“Humans have been controlling carnivore numbers for centuries. Predator hunting, however, may indirectly influence predator-prey dynamics unintentionally by influencing the age- and sex-structure of predator populations that exhibit intraspecific (IS) variation in prey selection,” Elbroch stated. “In short, the older the animal, the larger the prey it specialized upon. Our provocative results suggest that the current controversial strategy of increasing puma culling to aid mule deer, as currently underway in Colorado, may in fact exacerbate problems for mule deer by changing the age-structure of the puma population to predominantly younger animals that are more likely to hunt deer over elk.”

The study’s author stressed that larger sampling and more research needs to be done to fully understand the intricacies involved in ecosystems that include several apex predators and carnivores like mountain lions, wolves, grizzlies, coyotes, as well as numerous potential prey like deer, elk, pronghorn, sheep, and other ungulates.

Adult cougar (NPS, Dan Stahler)

What do mountain lions eat?

Between April 2012 and December 2016, researchers visited 3,261 clusters for 13 pumas in the field, documenting 1,398 prey items and 120 incidents of scavenging.

From most frequently detected to least, prey included: 564 elk, 488 mule deer, 85 North American porcupines, 43 bighorn sheep, 41 American beavers, 37 pronghorn, 26 coyotes, 12 ruffed grouse, 10 red foxes, 10 striped skunks, 9 moose, 9 Yellow-bellied marmots, 8 snowshoe hare, 8 trumpeter swans, 7 Northern raccoons, 5 white-tailed deer, 4 Canada geese, 3 American martens, 3 dusky grouse, 2 American badgers, 2 Barrow’s goldeneye, 2 North American red squirrels, 1 gray wolf, 1 American black bear, 1 puma, 1 domestic sheep, 1 northern pocket gopher, 1 unknown small mammal, 1 unknown ungulate, 1 Great gray owl, 1 Great horned owl, 1 Common raven, 1 Sandhill crane, 1 unknown bird, 1 Black-billed magpie, 1 Dark-eyed junco, 1 Common flicker, 1 Pine siskin, 1 American coot, 1 Mountain bluebird, 1 American robin, 1 Red-naped sapsucker.