It’s akin to a light switch; it’s that stark. One day mountain lions inhabiting the Southern Yellowstone Ecosystem are predominantly killing mule deer, and the next day they all switch to killing elk. And then they kill elk for five to five and a half months before they switch back to deer.
It happens on a slightly different date each year. In 2013, it was November 14th, two weeks earlier than in 2012. There hasn’t been a deer killed by the cats we are tracking as part of Panthera’s Teton Cougar Project since that date, and we don’t expect we’ll find a deer killed by a project cougar until May. Nowhere in their vast geographic distribution do cougars exhibit such distinct summer and winter diets as in our study area.
Ungulate (a fancy word for a “hoofed mammal”) migrations, driven by the seasonal availability of forage that sustains them, result in large-scale redistributions of resources for carnivores. In response, carnivores adapt their foraging behaviors in systems with migrating prey.
In summer, carnivores in North America are quick to seize opportunities presented by the birth pulse of numerous, vulnerable, young ungulates. Whereas in winter, carnivores take advantage of the numerous ungulates struggling to survive harsh weather on lesser-quality forage and suffering poorer health. In the Southern Yellowstone Ecosystem, cougars do something more. Not only do local mountain lions follow trends in changing prey vulnerability, but they also track changing prey numbers.
Through November and December each year, thousands of elk descend upon the National Elk Refuge and surrounding Bridger-Teton National Forest and Grand Teton National Park lands in northwest Wyoming (as well as thousands of tourists to ride in sleighs drawn by sturdy horses to view them, but cougars generally steer clear of the tourists). Recent winter elk counts in the area number around 11,500 animals. This boon in prey numbers for local mountain lions coincides with the migration of mule deer out of our study area into the flat lands surrounding Jackson, WY and further south. Thus, it is not surprising that cougars here switch their focus from deer to elk with the Christmas season.
Cougar populations across North and South America are primarily non-migratory and hunt non-migratory prey. Nevertheless, where mule deer exhibit seasonal migrations, some cougars follow, exhibiting seasonal ranges themselves; other cougars remain in winter deer range through the summer, where deer persist, but at lesser numbers (Pierce et al. 1999, Cooley et al. 2008). In contrast, cougars here remain relatively stationary as ungulates migrate across the Southern Yellowstone Ecosystem, and kill different prey in different habitats in different seasons.
As they hunt through the year, mountain lions in the Southern Yellowstone Ecosystem also distribute a wealth in carcasses for scavengers, decomposers and floral communities, in more diverse locations than they would if ungulates didn’t come and go. Thus ungulate migrations in the Southern Yellowstone Ecosystem are not just vital to the dynamic health of the area, but a cornerstone piece needed to understand why local cougars do what they do throughout the year.
Cooley HS, Robinson HS, Wielgus RB, Lambert CS (2008) Cougar prey selection in a white-tailed deer and mule deer community. J Wildl Manage 72: 99-106.