Mountain lions or cougars, such as this one in the Santa Monica Mountains of Los Angeles, rely on deer as their main source of food. (National Park Service)

In 1921, an article in the quarterly journal of the California Department of Fish and Wildlife made plain the prevailing feeling about one native inhabitant of the state: “The one predatory animal for which practically no good can be said is the mountain lion,” it began.

The big cats’ main crime? Having caused, the article said, a “heavy natural drain on the deer supply.”

Nearly 100 years later, researchers have made a case in another journal, Conservation Letters, that mountain lions’ deer-killing skills could be lifesaving to people on the other side of the country, where vehicles regularly crash into highway-hopping deer. If mountain lions returned to their eastern U.S. range, the study found, they could prevent 708,600 deer-vehicle collisions, 155 human deaths and 21,400 human injuries over 30 years. That would save at least $2.13 billion, the authors said.

The mountain lions’ return to the east, where people long ago killed them off, is certainly possible. The lions — also known as cougars, pumas or panthers — once lived across the entire hemisphere. While they’re now mostly in the west, crowding is causing them to expand their range. There are now breeding populations in Nebraska and South Dakota, for example, and one male cougar even made it to Connecticut in recent years.

In most of the eastern half of the country, deer have proliferated in the absence of cougars, destroying vegetation and contributing to so many car crashes that they’ve become the most dangerous large mammal to humans in North America. To control them, officials have resorted to culling and more expensive efforts, including contraception and special highway crossings.


Two bucks and a doe stand in a field under a flock of Canada geese in Huntingtown, Md. (Linda Davidson/The Washington Post)

Laura Prugh, a wildlife ecologist at the University of Washington, said she and her co-authors wanted to bring a consumer-friendly, “‘Freakonomics’ kind of thing” to her research on large carnivores’ roles in ecosystems. They examined well-established data on deer-vehicle collisions, cougar predation on deer, available cougar habitat and deer populations in 19 eastern states. They assumed each cougar would kill 259 deer over an average six-year life span; to be conservative, she said, they assumed about 75 percent of those ungulates would have died anyway from other causes such as starvation, which is happening more often as deer strip eastern vegetation.

To backstop their findings, they looked at South Dakota, where about half the counties on one side of the Missouri River have been recolonized by cougars in recent decades. Before the cougar repopulation, deer-vehicle collisions were increasing at a steady rate each year. But in the counties where cougars set up shop, Prugh said there was a “very dramatic change” within eight years: Deer-vehicle collisions dropped by 9 percent, preventing 158 such collisions annually that cost $1.1 million every year.

During that time, Prugh said, there was no big change in vegetation that might have led to a deer population decline, and deer hunting actually decreased, which might have led to a rise in collisions — if not for the cougars.

“That was very striking,” she said. “When I looked at it, I was like, that looks like made-up data.”


A mountain lion, or cougar, in the Santa Monica Mountains of Los Angeles. (National Park Service/AP)

Michelle LaRue, a University of Minnesota wildlife ecologist who is executive director of the Cougar Network, which tracks cougar recolonization in the Midwest, said she thought the study was “a really interesting step forward in understanding the benefits that we don’t think about when we think about mountain lions.”

At the rate they’re spreading, however, LaRue said she doesn’t expect cougars to repopulate the east anytime soon. Young males, which need to find their own territory to avoid being killed by other males, are the pioneers moving east more rapidly, she said. But females typically migrate in a slower “steppingstone” pattern, moving to the next open patch and settling down there. You need both genders, of course, to get a self-sustaining cougar population.

LaRue said research indicates that females will begin recolonizing new parts of the Midwest in the next 25 years.

“That’s not very far in relation to the entire continent,” she said. “If it happens [in the east], it’s going to take a long time.”

That’s probably a good thing, given that people, Prugh said, might not be psychologically ready for cougars in the eastern woods.

“What I would hope is that by making this fairly substantial benefit more concrete, people might be a little bit more accepting of them when they do show up,” she said of the study.

Prugh and her colleagues considered some of the costs of new cougar populations, including about $2.35 million worth of lost livestock and an unknown number of lost pets (though cougars, she noted, are “deer specialists,” and probably wouldn’t consider a house cat worth their time). Fewer than 30 people would be killed by cougars over 30 years, their study estimated.

“Having cougars in the east looks like it would actually save about five times as many lives through reducing deer-vehicle collisions as they would actually kill,” she said. “People don’t stay awake at night worrying about crashing into a deer the next day, even though it could easily happen. But they might stay awake at night worrying about a cougar jumping on their back as they’re walking through their neighborhood.”

The chances of that happening are extremely low, she said. But, she added, “People’s fear doesn’t really track the statistics well.”

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