Cougars Lower the Risks of Car Crashes by Hunting Deer

If you’re worried about car accidents caused by deer, you might want to look into bringing some cougars into town. A study published Wednesday in the journal Current Biology finds that car accidents involving deer in the US could go down as much as 76% in areas where there is a healthy population of the big cats. The presence of cougars around roads cutting through forests or around a development edging up to the woods, the researchers found, made deer avoid these areas, leading to fewer accidents. Cougars—also called mountain lions, panthers, or pumas—are native to the Americas. But due to habitat loss and hunting their range in North America dropped off considerably following European colonization. Now, cougars are mostly found in the western US states and a small part of Florida. Deer, on the other hand, have flourished. With fewer predators and limits on their hunting—as well as land being cleared for agriculture—their habitat and populations have greatly expanded across the US over the past century. As deer populations have grown, though, so have traffic accidents. According to State Farm, there were 1.8 million auto insurance claims filed between 2022 and 2023 involving crashes with animals; deer are the most common cause. The Insurance Institute for Highway Safety, an industry nonprofit, tracks that deadly collisions with animals and cars have steadily risen since the 1970s; last year, more than 200 people died in car crashes involving animals. Deer are a hazard in part because there are so many of them and in part because they frequently forage on the side of roadways. The new study looks at how mountain lions can play the role of a traffic cop, using data collected on the Olympic Peninsula, where the research team has run a cougar monitoring program since 2018. That work, run by the nonprofit Panthera, built on the work of tribal biologists who had been observing the cats for decades, says Justin Suraci, one of the study’s lead authors and a senior scientist at Conservation Science Partners, another wildlife nonprofit involved in the research. The study, he says, aimed to look at the “costs and benefits” of humans coexisting with predators. While the costs ranchers can incur when predators kill cattle and other economic impacts have been well-documented, the benefits have been less studied, says Suraci. To look at how cougars affected deer behavior, the researchers compared data selected from more than 500 cameras across the peninsula with data gathered from a year of car accidents that involved deer. Cougars “are ambush predators,” Suraci says: “They prefer these sort of edgy, moderately fragmented habitats—roads cutting through the forest or lower density development, pushing up against a forested area.” This type of habitat “gives them opportunity to conceal themselves and attack the deer that's foraging just outside the forest.” The deer, accordingly, have decided to stay away from those areas. “They're actually moving away from the habitat edges deeper into the woods and therefore are less exposed to the roads and development that would be associated with traffic and vehicle collisions,” Suraci says. These avoidances can add up to safer conditions for drivers. The researchers found that deer were 15% more likely to avoid areas with roads where cougars were known to hunt, and were 86% more likely to spend time in more wooded areas. They were also more active during the day. The number of collisions in a five-year span was greatly reduced, going down more than 76% compared to areas with a low number of cougars. Prior studies looking at how carnivore populations could affect deer-related traffic accidents helped the researchers develop their methodology, Suraci says , including one modeling potential cougar expansion and one studying the reintroduction of wolves into parts of Wisconsin. These enabled the team to look at behavioral patterns of deer in areas where predators hadn’t been newly introduced and helped to rule out fewer accidents due to the decline in deer numbers caused by cougars hunting them. Laura Prugh, a professor of wildlife sciences at the University of Washington, praised the new research, saying it helps “fill some gaps” left by other studies. (Prugh was the coauthor of the 2016 study that modeled the effects that cougar recolonization may have on car accidents.) She says looking at summer behavior limits the study, particularly given how deer behavior can change during the fall mating season, known as the rut, when there is a “big spike in deer vehicle collisions.”. “I would be really curious to see if that behavioral effect would also extend into that fall rut when the deer are kind of running around a little more distracted, maybe not paying attention to risks quite as much,” she says.