'System in flux': Scientists reveal what happened when wolves and cougars returned to Yellowstone

After wolves were reintroduced to Yellowstone National Park, cougars — that had only regained a foothold a few decades earlier — were able to coexist due to their diets changing and the varied landscape of the park, according to new research.

Run-ins between wolves (Canis lupus) and cougars (Puma concolor, also called mountain lions and pumas) in Yellowstone National Park happen when wolves steal prey from — and sometimes kill — cougars, and this dynamic becomes more harmonious when cougars shift to eating smaller prey, according to a new study published Jan. 26 in the journal PNAS. Successful wolf and cougar coexistence in Yellowstone, the findings suggest, depends more on the diversity of prey and the availability of escape terrain for cougars than it does on the overall abundance of prey.

"Yellowstone is a fascinating system because it's got the full complement of large carnivores and migratory ungulates that North America used to have," Chris Wilmers, a wildlife ecologist at the University of California, Santa Cruz who was not involved in the new study, told Live Science. "A lot of these species are coming back –– wolves were reintroduced, mountain lions and grizzly bear numbers have been recovering –– so it's also a system that's in flux. As these populations restore themselves, it's super interesting to look at these species' effects on each other."

Cougar and wolf habitats are increasingly overlapping in the western United States. Throughout the first half of the 20th century, both species were nearly eradicated from the U.S. due primarily to hunting. Cougar populations began to rebound in the 1960s under new protections, and wolf reintroduction began in the 1990s and benefited from expanded legal protection.

Both species are now prevalent throughout the western U.S., but scientists are still working to understand the animals' population dynamics and their impacts on the broader Yellowstone ecosystem.

The new study analyzed nine years of GPS data from collared wolves and cougars, combined with field observations at almost 4,000 sites throughout Yellowstone. The researchers found that wolves occasionally kill cougars, but cougars do not kill wolves.

These findings align with previous work that showed wolves were the more dominant large carnivore in this food web, even though the two species have similar body sizes. Wolves likely dominate because they move in packs, whereas cougars are solitary, which means wolves can run cougars off and steal their prey, said lead study author Wesley Binder, a doctoral student in the Department of Fisheries, Wildlife, and Conservation Sciences at Oregon State University.

"These interactions are very one-sided," Binder told Live Science. "But cougars have the ability to adapt in some ways."

Both cougars' and wolves' diets are changing, according to the new findings: Between 1998 and 2024, elk went from constituting 95% to 64% of wolf diets, and from 80% to 53% of cougar diets, likely because Yellowstone elk (Cervus canadensis) populations are decreasing more broadly.

This decline led to changes in wolf and cougar interactions. "If cougars kill larger prey like elk, that gives wolves more time to find the cougar sitting on that kill," Binder said. "We found that wolves and cougars were six times more likely to interact when cougars killed elk, compared to deer. Deer are less than half the size of elk, so cougars eat them a lot faster, and wolves have a lot less opportunity to discover those kill sites."

Shifting cougar diets from declining elk numbers led to fewer interactions with wolves overall. Instead of elk, cougars began eating smaller prey, like deer. Wolves, they found, started eating more bison.

"It's important to realize that's why the cougars switched, but in doing so, it made them less vulnerable to scavenging and potentially getting killed by wolves," Wilmers said.

The terrain, the findings showed, also shapes the animals' encounters. When surrounded by rugged terrain or trees they can climb, cougars had fewer dangerous encounters with wolves.

Yellowstone's diversity of both prey and landscapes seems to be a sweet spot for wolf-cougar coexistence. Both species' populations are currently stable. "Wolves and cougars prefer different habitat, and Yellowstone has different habitat that suits each of these carnivores," Binder said.

The findings reveal the ideal landscape and prey characteristics for the stable coexistence of two large carnivore species — and how clashes between predators can have a ripple effect on the whole ecosystem.

"We're always trying to understand what the impact is of large carnivores on prey [populations]," Wilmers said, "and what the interactions are between the large carnivores, and how they might combine or cancel out each other's influence on prey. … It's the beginning of unraveling that story between wolves and [cougars]."