The primary prey of wolves in Yellowstone National Park is elk. Elk are abundant large ungulates in the Greater Yellowstone ecosystem, and wolves commonly target them because they provide substantial food for an entire pack.
The importance of elk became especially visible after gray wolves were reintroduced to Yellowstone in 1995. Wolves restored a major predator to the ecosystem and influenced how elk moved, where they fed, and which animals were most vulnerable. Calves, weakened animals, and individuals affected by winter conditions can be especially susceptible.
Elk remain the main prey even though Yellowstone wolves eat a variety of animals. The National Park Service reports that roughly 90% of winter prey is elk, while deer become more important during summer. Wolves also kill or scavenge bison, but bison are dangerous opponents, so wolves often take calves, sick animals, or carcasses.
A common mix-up is assuming that the largest available animal must be the main prey. In Yellowstone, bison are larger, but elk are generally more numerous and more manageable for wolves to hunt cooperatively.