An Unorthodox Strategy to Stop Cars From Hitting Deer

Try wolves.

A deer stands in the middle of a road.

The thousand or so wolves that live in Wisconsin may inadvertently be doing a service to humanity, saving the lives of dozens of people. On average, 19,757 Wisconsinites collide with deer every year, leading to about 477 injuries and eight deaths. But according to Jennifer Raynor, a natural-resource economist at Wesleyan University, more would do so if wolves weren’t around. “Some lives are saved, some injuries are prevented, and a huge amount of damage and time are saved by having wolves present,” she told me.

These predators tend to prowl along human-made corridors such as trails and roads. By killing deer near these areas, or simply intimidating them into staying away, wolves could keep the animals far from cars. By analyzing 22 years of data, Raynor and her colleagues found that Wisconsin’s wolves have reduced the frequency of deer-vehicle collisions by a quarter. They save the state $10.9 million in losses every year—a figure 63 times greater than the total compensation paid for the loss of livestock or pets. “The icing on the cake is that wolves do this work all year long at their own expense,” says Liana Zanette, an ecologist at Western University, in Canada, who was not involved in the study. “It all seems like a win-win for those wolf counties.”

But wherever wolves go, controversy follows.

By the 1960s, European settlers had all but eradicated the gray wolf from the continental United States. Once the species gained legal protection, it began bouncing back. Wolves have naturally recolonized some places, such as Wisconsin, and been deliberately reintroduced to others, such as Yellowstone National Park. But their influence on the ecosystems around them has been hotly contested. They’ve been repeatedly delisted from and relisted to the Endangered Species Act. In November, Colorado voted to bring them back, but narrowly. They are, as my colleague Michelle Nijhuis once wrote, “the most political animal.”

Raynor’s findings, if replicated and confirmed, could add three important dimensions to the long and fractious wolf debate. First, most wolf research takes place in national parks, such as Yellowstone and Isle Royale, which are largely free of human influence. Such studies say little about how these animals might behave in unprotected areas full of towns, farms, and roads. If wolves are to thrive in the U.S., or even be reintroduced in more states, they’ll have to coexist with people in places that look little like Yellowstone but a lot like Wisconsin.

Second, “the people who value the existence of wolves are often not in the same communities where wolves are present,” Raynor told me. Urban wildlife lovers may be happy to know that wolves exist out there, but rural people have to stare at the carcasses of livestock and pets. Wolves’ benefits to the former are abstract and nebulous; their costs to the latter are tangible and bloody. But deer-vehicle collisions “are happening in both urban and rural areas,” Raynor said. “No one is avoiding this problem,” which means that rural people are also benefiting from wolves, unbeknownst to them.

Third, the “public discourse focuses on the potential negative economic impacts of wolves on people,” says Rebecca Niemiec, a social scientist at Colorado State University who studies attitudes to conservation. But Raynor’s study, she says, provides “compelling evidence” that wolves can also be a positive economic force, saving dollars by averting crashes.

Skeptics could argue that there must surely be easier ways to stop cars from hitting deer than, oh, introducing wolves. There are, Raynor says, but they all have problems. Cheap measures such as standard warning signs for drivers don’t actually work. Effective measures such as overpasses for deer are so expensive that “they can really only be implemented at really severe deer-vehicle-collision hot spots,” Raynor said. “What do you do everywhere else? Wolves are cost-effective compared to multimillion-dollar investments that affect one intersection.” (Notably, cars rarely hit wolves; just 21 such collisions were recorded from April 2019 to April 2020, in contrast to an annual average of 20,000 deer-vehicle collisions.) It sounds almost ludicrous to say, but deer-vehicle collisions are a civil-engineering problem that might best be solved by adding wolves.

To make that claim, Raynor and her co-authors, Corbett Grainger and Dominic Parker, gathered several lines of evidence. They showed that since the 1990s, when Wisconsin’s wolf populations started taking off, deer numbers plateaued in 29 counties where wolves are present, but rose in the 34 wolf-less counties. Whenever wolves first entered a county, the proportion of road accidents involving deer tended to fall. And although deer-vehicle collisions are specifically rarer in wolf-colonized counties, other types of collisions are not. These trends suggest that wolves really have made Wisconsin’s roads a little safer, irrespective of other factors. “It’s beyond the scope of the study to really nail causation, but the evidence is extremely compelling in favor of wolves being an underlying cause,” Zanette told me.

But Guillaume Chapron at the Swedish University of Agricultural Sciences, who studies large carnivores, says the team has not provided enough information about their statistical methods, the degree of uncertainty in their results, or details on how to replicate their analysis. “It may be that they found a new dimension to the role played by wolves, but their paper makes a critical evaluation of their findings impossible,” he told me. “I’m sure it will be loved by wolf advocates, but much less by statisticians.”

Adrian Treves from the University of Wisconsin at Madison also studies large carnivores and reviewed the paper; he told me that it’s a “valuable contribution, but needs to be replicated and validated.” For example, the team could focus on a few counties that differ in wolf abundance but are carefully matched in other variables, such as traffic patterns and deer numbers. “This is only the first step in understanding a potentially very important biological and economic relationship,” Treves said.

If wolves truly are making roads safer, a crucial question remains: how? Raynor’s statistical model suggests that the wolves are reducing collisions out of all proportion to the number of deer they eat. Most of their influence, she argues, arises through dread, not death. Their very presence creates a landscape of fear, which pushes deer away from roads and other heavily prowled areas. “This makes tremendous sense,” Zanette said. “Predators always scare way more prey than they can actually kill. That’s becoming established as a fundamental principle in ecology.”

But Matthew Kauffman of the University of Wyoming isn’t convinced, because of his experiences at Yellowstone. Since 2001, some scientists have famously claimed that reintroduced wolves benefit the park’s aspen and willow trees by scaring off the elk that overeat them. A decade later, Kauffman complicated that narrative. Using GPS collars, he showed that elk only rarely and fleetingly flee from wolves, and that elk behavior has no bearing on the survival of aspen. Wolves, he realized, change their prey’s behavior in more subtle ways. In Wisconsin, they could be culling the least risk-averse deer, which would have been disproportionately prone to running in front of cars. The only way to know for sure, Kauffman told me, is to actually measure the behavior of deer and wolves, using collars, which Raynor’s team hasn’t done.

The team’s economic analysis comes with caveats, too. It estimates that every year, wolves avert $10.9 million of damages related to deer collisions, while costing $174,000 in compensations related to injured pets and livestock—a 63-fold difference. But these figures aren’t directly comparable, as Raynor acknowledges. The collision costs are calculated comprehensively, while the compensations ignore “the sadness of seeing your animals killed, the effects on livestock productivity because animals are stressed, and the money farmers spend on protecting their animals,” she said. Wolves’ economic benefits seem 63 times higher than their costs, but “I don’t know what the ratio would be if we had a more comprehensive accounting.”

Even if the benefits and costs were the same, the former would still be high, and would involve saving human lives. Raynor adds that wolves could have other economic benefits. Through fear or predation, they could keep deer away from agricultural land. As part of an ecological chain that also involves coyotes, foxes, small rodents, and ticks, they could potentially reduce the incidence of Lyme disease. These are just hypotheses, but they should be tested, especially “when we think about places where wolves could potentially be reintroduced in this country,” Raynor added.

"There is a great need to conduct this kind of research,” says Peter David of the Great Lakes Indian Fish and Wildlife Commission, which represents 11 Ojibwe tribes in Wisconsin and neighboring states. “It provides insights into a whole new realm of previously undocumented benefits that having ma'iingan (‘wolves’) on the landscape provides.” That includes landscapes heavily used by humans. David added that these findings speak to an Ojibwe teaching about humility: “We are wise to proceed conservatively, protecting and valuing all parts of the ecological landscape—even those we still poorly understand.”