Rats Have Not Changed. We Have.

Sheltering in place produced a “natural experiment” for urban wildlife.

An illustration of rats among skyscrapers
Paul Spella / The Atlantic

After Chicago’s stores and restaurants shut down in March, Rebecca Fyffe, the director of research at a pest-control company, went on one of her usual evening “rat safaris.” Her employer, Landmark Pest Management, services many of the city’s high-end, Michelin-rated restaurants, which had been forced to close hastily, dumping piles of produce. Beside a dumpster near one such restaurant, Fyffe came across cases of past-prime avocados. “I saw this box of avocados just teeming with rats,” she says. As the city’s humans retreated home to avoid the coronavirus, its rats got “one big, final buffet.”

Then, however, came the famine. Restaurants remained closed, and nearby rat colonies started to starve. In restaurant-heavy neighborhoods, Fyffe began seeing more rats with wounds from fighting over food. She started finding more of the creatures in her traps too, probably because the bait had become a lot more enticing. Then, finally, the rat population in some areas started to drop. In the month from April 20 to May 20, her team trapped 19 rats outside a closed Chicago restaurant. In the two weeks following May 20, they trapped only two.

The coronavirus pandemic has upended human life in cities, which means it has also upended the habitat of city rats. In their millennia-long coexistence with humans, rats have acclimated to our changing ways of life and they are acclimating again to pandemic-stricken cities. “This is sort of a natural experiment,” says Maureen Murray, a wildlife-disease ecologist at the Lincoln Park Zoo in Chicago. Cities are now slowly reopening, and rats will adapt again—as they always have.

One thing is for sure: Rats aren’t going away. A female brown rat, the most prominent species in the U.S., can have litters of 10 pups every few weeks. Any rat populations that have been decimated by starvation in certain areas are likely to rebound as soon as their food comes back. Chicago’s rats are typically estimated to number in the hundreds of thousands.

A city is not homogenous, however, and the pandemic’s effects on its rats vary block by block too. “Rat-population changes are so hyperlocal that there is not one citywide answer,” Fyffe says. In residential areas, where rats might rely on household garbage, local populations have no shortage of food—in fact, there has probably been more household trash because people are sheltering in place.

But commercial areas, such as those Fyffe was monitoring on her rat safari, are a different story. Rats tend to stay within the same 150-to-200-foot diameter, happily gorging, for example, on one restaurant’s garbage their entire life. As those rats ran out of food in the aftermath of nonessential businesses shutting down, they’ve ranged more widely out of hunger. Fyffe says she’s worked with restaurants that have remained open for takeout and seen an influx of rodents. “Those clients have called us and said, ‘We need more control. We’ve never had this many rats!’ And they say, ‘Chicago’s rat population must be soaring,’” she explains. “No, that’s not the issue. You’re the only person on the block putting garbage in the dumpster, so all of the rats, instead of going to 20 dumpsters, they’re all coming to one.”

The Centers for Disease Control and Prevention has recently warned that hungry rats might be unusually aggressive. As rodent horror stories have spread during the pandemic—of rats swarming on New Orleans’s Bourbon Street, or nesting in car engines—Matthew Combs says he’s had to tamp down rat fears. Combs is an ecologist at Columbia University who has studied rats in New York City—where population estimates range from 300,000 to 2 million—and he stresses that, no, the animals are not evolving into aggressive super rats. Yes, they cannibalize their young, but they do that during non-pandemic times too. “Really, the rat behavior probably hasn’t changed so much. It’s really the way humans have altered our activity,” he says.

Other urban animals may be responding to changes in our behavior too. Traffic is one piece of it. “A lot of animals that live in the city have to constrain their movements to avoid getting hit by cars,” Murray says. Coyotes, she points out, tend to shift from being active at dawn and dusk to being nocturnal near humans. Christopher Schell, an urban ecologist at the University of Washington, Tacoma, who has studied coyotes with Murray, is using camera traps in parks and other green spaces to see how carnivores are changing their behavior during the pandemic. One hypothesis, Schell says, is that animals might shift their activity back to dusk and dawn with fewer humans around. But lately, the one species his camera traps have been capturing more of is humans, at least around the city and suburbs of Tacoma.

“There were sites that we barely saw a person,” he says, “and all of a sudden this year, we saw the number of people just skyrocket.” Coyotes or raccoons that used to show up during the day in certain places now seem to be coming around only at night. With so many people practicing social distancing in green spaces, some animals could actually become more nocturnal rather than less. Of course, adds Schell, patterns are likely to vary from city to city and state to state, given the patchwork of shelter-in-place restrictions. His project is part of the Urban Wildlife Information Network, which collaborates in about 25 different cities in North America that together might tell a more complete story of the pandemic’s effect on wildlife.

Working at home all day has certainly made many people more aware of the wildlife around them. In other words, it’s a busy time for pest control. “Right when the shelter-in-place order came, we got dozens of phone calls,” Fyffe says. People were seeing coyotes they had never noticed before in their yards and hearing squirrels in their attic that roused during the day. We’ve been sharing our homes this whole time.