‘System in flux’: Scientists reveal what happened when wolves and cougars returned to Yellowstone
After the reintroduction of wolves to Yellowstone National Park, cougars – which had only regained a foothold a few decades earlier – were able to coexist thanks to their changing diet and the diversity of the park’s landscape, new research shows.
Clashes between wolves (Dog lupus) and cougars (Puma concolor, also called mountain lions and pumas) in Yellowstone National Park occur when wolves steal prey from – and sometimes kill – cougars, and this dynamic becomes more harmonious when cougars turn to smaller prey, according to a new study published Jan. 26 in the journal. PNAS. The results suggest that successful coexistence of wolves and cougars in Yellowstone depends more on prey diversity and the availability of escape grounds for cougars than on overall prey abundance.
“Yellowstone is a fascinating system because it supports the full complement of large carnivores and migratory ungulates that North America once had,” Chris Wilmersa 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 have been reintroduced, mountain lions and grizzly bears have recovered – so it’s also an evolving system. As these populations recover, it’s very interesting to look at the effects of these species on each other.”
Cougar and wolf habitats increasingly overlap in the western United States. Throughout the first half of the 20th century, both species were almost eradicated of the United States mainly due to hunting. Cougar populations began to rebound in the 1960s thanks to new protections, and wolf reintroduction began in the 1990s and received expanded legal protection.
Both species are now widespread throughout the western United States, 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 nearly 4,000 sites across Yellowstone. Researchers have found that wolves occasionally kill cougars, but cougars do not kill wolves.
These results are consistent with previous work showing that wolves were the most dominant large carnivore in this food web, even though the two species have similar body sizes. Wolves likely dominate because they move in packs, while cougars are solitary, meaning wolves can hunt cougars and steal their prey, the study’s lead author said. Wesley Binderdoctoral student in the Department of Fish, 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 certain ways.”
The diets of cougars and wolves are changing, according to new findings: Between 1998 and 2024, elk fell from 95% to 64% of wolves’ diets, and from 80% to 53% of cougars’ diets, probably because Yellowstone elk (Canadian Cervus) populations are decreasing more generally.
This decline has led to changes in interactions between wolves and cougars. “If cougars kill larger prey like elk, that gives wolves more time to find the cougar sitting on that prey,” 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 much more quickly, and wolves have far fewer opportunities to discover these kill sites.”
Changing cougar diets due to declining elk numbers have led to fewer interactions with wolves overall. Instead of elk, cougars began eating smaller prey, such as deer. The wolves, they discovered, began eating more bison.
“It’s important to understand that this is why cougars changed, but in doing so it made them less vulnerable to being searched and potentially killed by wolves,” Wilmers said.
The results showed that terrain also shapes encounters between animals. When surrounded by rugged terrain or trees they can climb, cougars have fewer dangerous encounters with wolves.
Yellowstone’s diversity of prey and landscape appears to be an ideal location for wolf-cougar coexistence. Populations of both species are currently stable. “Wolves and cougars prefer different habitats, and Yellowstone has different habitats that are suitable for each of these carnivores,” Binder said.
The results 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 across the entire ecosystem.
“We are still trying to understand what impact large carnivores have on their prey [populations]”, Wilmers said, “and what the interactions between large carnivores are and how they might combine or negate each other’s influence on prey. … This is the beginning of the discovery of this story between the wolves and [cougars]”.
