Oh, great: Rat populations are surging as cities heat up

Gary Hershorn // Getty Images Rats are, in many ways, better adapted to cities than the humans that built them. While urbanites struggle with crowds, sparse parking spaces, and their upstairs neighbors stomping around at 4 a.m., rats are living their best lives. Huddled safely underground, they pop up at night to chew through heaps of food waste in dumpsters and hot dogs left on stoops.  Now, scientists have found yet another gnawing advantage for rats, Grist reports. A study published in January in the journal Science Advances found that as temperatures climb in cities, rat populations are growing, even as city dwellers suffer. “In cities that have experienced the fastest warming temperatures, they tended to have faster increases in their rat numbers as well,” said Jonathan Richardson, an urban ecologist at the University of Richmond and lead author of the paper. “Females will reach sexual maturity faster. They’re able to breed more, and typically, their litters are larger at warmer temperatures in the lab.” The analysis used public complaints about rats and inspection records from 16 cities between 2007 and 2024, which collectively served as a proxy for rat populations. In 11 of those cities, rat numbers surged during that period. The winner of the Most Rats Gained award goes to Washington, D.C., with a 390% increase according to the city’s last decade of data, followed by San Francisco (300%), Toronto (186%), and New York City (162 percent). Meanwhile, a few cities actually saw their rat populations decrease, including New Orleans, Tokyo, and Louisville, Kentucky, due in part to more diligent pest control. “It’s a first step at answering this question, that if you get a bunch of rat scientists into a room, we’re bound to ask each other: How might climate change play into rat populations?” said Kaylee Byers, a health researcher at Simon Fraser University in Canada, who wasn’t involved in the study. Beyond the physiological factors that influence breeding, rat behavior changes with temperature, too. If it’s too cold out, the rodents tend to huddle underground–in basements, sewers, and really anywhere else in the subterranean built environment. Once it warms up, rats emerge and gorge, but also bring food back to their nests to store in caches. Climate change is also altering the timing of seasons: If the weather stays warmer a week or two longer into the early winter, and if spring comes a week or two earlier, that’s more time to forage. “Rats are really well-adapted to take advantage of a food resource and convert that to new baby rats that you’ll see in your neighborhood,” Richardson said. While temperatures are rising globally, they’re getting particularly extreme in cities thanks to the urban heat island effect. Buildings and concrete absorb the sun’s energy, raising temperatures up to 27 degrees Fahrenheit higher than in surrounding rural areas and releasing that heat at night. That’s especially dangerous in the summer for urbanites during prolonged heat waves. But in the winter, that bit of extra heat could be helping rats. Rising temperatures were the dominant force helping rat populations grow, but they weren’t the only factor, the study found. Urban human populations are exploding around the world, and they’re wasting a lot of food for rats to find. As cities expand around their edges, they have to add new infrastructure, which rats colonize. And when cities build new sewer systems to handle more people, they often leave the old ones in place, providing a welcoming environment for rats. “The vestigial urban infrastructure that’s down there, it doesn’t really matter for us,” Richardson said. “But for a rat, that’s like a free highway.” The researchers also found that cities with fewer green spaces had higher growth of rat populations. It’s not clear yet why that might be, they said. No two green spaces are the same: A small urban park might teem with rats because office workers flock there to eat lunch, then drop their leftovers in trash bins, whereas the interior of a larger space like Central Park might provide less food and fewer places for rodents to hide from predators like hawks and coyotes. So how can a city control its rat population as temperatures rise? For one, by getting more data like the numbers found in this study. “You can’t manage what you can’t measure,” said Niamh M. Quinn, who studies human-wildlife interactions at the University of California Division of Agriculture and Natural Resources but wasn’t involved in the research. “We live in an infinite sea of rats, so you can’t just manage small pockets. You have to have municipal rat management.” New Orleans has succeeded by being proactive, Richardson said, such as with education campaigns teaching building owners how to rat-proof their structures, and insisting that if they do see rats to call the city for eradication. Cities can’t just poison their way out of this problem without hurting other animals, he said, because that poison makes its way into the stomachs of rat-eating predators. “Right now, our approach to rat management is very reactive,” Byers said. “We’re not thinking about the future at all. We need to do that if we’re actually concerned about rats, and if we want to manage the risks associated with them.” This story was produced by Grist and reviewed and distributed by Stacker.