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How 10,000 cougars could prevent hundreds of automobile accidents

Deer kill a lot of people in car crashes. Cougars eat deer. Problem solved? 

The hero we need?
Micheal J /Flickr
Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

Don’t be fooled by their wide-eyed cuteness, their passivity, or their vegetarianism: Deer kill, injure, and maim.

To be fair, it’s not intentional. But every year, deer are involved in 1.2 million motor vehicle collisions, resulting in around 200 deaths, according to the Insurance Institute for Highway Safety. That makes deer deadlier than predators like the North American grizzly, great white sharks, and all venomous snakes combined.

Deer populations have exploded over the past decades, but there may be a solution to end the carnage: Bring back predators that eat deer.

“Large carnivores, or ‘nature's deer control,’ could provide a much-needed solution,” says Sophie Gilbert, a professor of wildlife ecology at the University of Idaho and co-author of a new paper in the journal Conservation Letters.

And the solitary, sleek cougar might be the perfect animal for the job.

How deer became a big problem

For one, there are likely more deer than ever before running across our roads.

According to Nature, although there are no hard national numbers, “in many states deer populations continue to rise well beyond historical norms.” This chart, of trends in deer population in Missouri, matches a similar trend for the rest of the Eastern United States.

University of Missouri Extension

To explain the sharp increase in population in the past half-century, we should back up to bans on deer hunting that began in the 1920s (after hunters put a huge dent in the population in the 18th and 19th centuries). Those deer found an ideal environment in growing green leafy suburbs. “The most obvious factor contributing to the rapid growth of deer populations is increased forage,” according to a 2004 paper in the Annual Review of Ecology, Evolution, and Systematics.

Meanwhile, driving became more and more popular among humans. It’s been a deadly mix.

Despite local efforts to control the deer population — like the sharpshooter program in Rock Creek Park in Washington, DC, and fewer people hunting overall — it doesn’t appear that the deer population is declining.

“Current deer control practices fail because they are ... very expensive per deer and therefore hard to implement across large areas,” says Gilbert.

Meanwhile, deer-car collision rates by state are either rising or holding steady. According to auto insurer State Farm, the costs of insurance claims for deer-related accidents increase year over year.

The case for letting cougars kill deer

Putneypics / Flickr

They go by many names — cougars, mountain lions, pumas, panthers — but they are all the same animal. The ones that used to be common in the Eastern United States are called Eastern cougars. So we’ll stick with cougars. But whatever you call them, these big cats are receiving renewed attention because of what they love eat: other large mammals, like deer.

The new Conservation Letters paper crunches some numbers and lays out of vision for an Eastern US where a reestablished cougar population decreases the number of deer-vehicle accidents by 22 percent. The estimate is based on the assumption that “a single cougar would kill 259 deer over an average 6-year lifespan,” the paper notes. Each cougar, then, would yield a $37,600 savings in auto insurance claims. In total, the paper argues, a reestablished cougar population would save about five human lives a year and prevent 680 injuries.

That all sounds great. But wait a second: How many more cougars are we talking about?

“We estimated that in the large, contiguous forested areas of the 19 Eastern states we modeled, there would be roughly 10,300 cougars total by the time deer and cougars reach a balance with each other after 30 years,” Gilbert says. (The distribution of the cougars would vary by state.)

While 10,300 may sound like a lot, it’s what their model came up with — taking into account the population levels that could be best supported by the natural environment while culling the deer population.

(Though it isn’t clear where these 10,300 cougars will come from. Gilbert suggests increased conservation efforts and stricter hunting guidelines could help bring more of them back. Remember: This paper is more of a theoretical framework for how cougars could control deer populations than an actual plan.)

But would we be replacing a deer problem with a cougar problem?

Springfield — home of The Simpsons — had a problem with pigeons. A horde of Bolivian tree lizards (which had been accidentally introduced) took care of them in short order. Lisa, always wise, asked this question: “What happens when we’re overrun by lizards?”

What happens when there are 10,000 more cougars in the country?

The good news is cougar attacks on humans are incredibly rare. According to a 2011 paper in Human–Wildlife Interactions, there have only been 29 recorded fatal cougar-on-human attacks since the 1890s (out of a total of 343 attacks). “Relative to other large carnivores with a history of attacking humans, cougars are among the least lethal,” the paper concluded, reassuringly.

The Conservation Letters paper estimates the increased cougar horde would only injure around five people a year, and maybe cause one death.

Human-Wildlife Interactions

The bigger concern might be risks for pets and livestock.

“Yes, carnivores definitely come with costs, which could include attacks on humans, pets and livestock, reductions in deer hunter success, and increased anxiety for rural residents and outdoor recreationists,” Gilbert writes. But the potential gain of fewer traffic fatalities might make up for those risks.

And Gilbert and her co-authors have some small evidence that that plan could work. Cougars have been making a comeback in South Dakota in recent years, and their increasing numbers have been correlated with a decrease in deer-on-vehicle collisions, preventing $1.1 million in collision costs each year.

Gilbert stresses, though, that her analysis is just a start down the road of thinking of how to use cougars to prevent car crashes. It’s not conclusive.

“While our analysis of South Dakota is based on real world data, the model for the Eastern US is just that — a model — and not data-driven since cougars are not there yet,” she writes. More work has to be done thinking of the consequences of a cougar surge.

It’s time to think more about how we can (and should) coexist with carnivores

Anthony Trumbo / Flickr

The Conservation Letters paper posits a novel, eyebrow-raising idea to solve the problem of motorists hitting deer.

But it’s also arguing something more provocative: that we can coexist with carnivores and possibly benefit from their presence.

It’s easy for communities to be afraid of animals like the cougar and grizzly bear — which has spurred recent debates as its populations recover. It’s one reason the numbers of cougars dwindled and dispersed as they were forced out by human development and hunting.

But the truth is that these fearsome animals are much less deadly to humans than they appear. And some non-predators, like the deer, are potentially more dangerous.

The bottom line is we can encourage the recovery of carnivore populations without fearing for our own lives.

“I think the biggest thing determining if Cougars will recolonize or not is human tolerance,” Gilbert writes. “In the modern world, large carnivore populations will succeed only if we let them. That's why figuring out and communicating the benefits that they can provide to us, as well as the costs, could be a valuable tool for conservation efforts.”

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