If scorching heat waves, destructive storms, prolonged droughts and rising seas aren’t enough to make some folks fear the consequences of climate change, perhaps this will do the trick: The warmer it gets, the faster rats multiply in cities that already struggle to contain them.
That is sure to be unwelcome news to Americans, who collectively endure well over $27 billion worth of property damage each year at the hands — and teeth — of rats. That doesn’t include the cost of the diseases the animals spread, such as hantavirus, murine typhus and bubonic plague, nor the mental health toll of living among them.
The new findings, reported Friday in the journal Science Advances, are based on records of rat sightings in 16 cities around the world. Unfortunately for humans, 11 of those cities saw their rat populations expand over the course of the study, while two cities held steady and only three achieved measurable declines.
That the rodents are thriving should come as little surprise. They’re perfectly suited to urban environments, where they make their homes in walls, basements and subway stations and feast on garbage, sewage, dog poop and abandoned pizza slices. The only continent they have yet to conquer is Antarctica.
“This species is really well-adapted to take food and convert that into new baby rats that are scampering around your neighborhood,” said Jonathan Richardson, a biologist at the University of Richmond who studies wildlife in cities and their impact on human health. “They do that really efficiently.”
One of the few things that slows rats down is cold weather. And with climate change, we have less of it.
Global warming causes average temperatures to rise, which reduces the number of wintry days. In cities, the trend is compounded by the fact that the built environment absorbs and retains more heat than the rural area around it, a phenomenon known as the urban heat island effect.
To investigate a possible link between rat populations and rising temperatures, Richardson and his colleagues searched for reliable data in the country’s 200 most populous cities. Conducting a thorough rat census was impractical — if not impossible — so they used municipal inspection logs and rat sightings reported to government agencies.
They found 13 cities that had kept consistent records for at least seven years. Then they widened their search and found three more cities overseas. The final group had rat data going back for an average of slightly more than 12 years.
Since the cities used different sources of data collected over different periods of time, the researchers came up with a standardized way to measure the change in rat sightings. They found that rat reports increased the most in Washington, D.C., followed by San Francisco, Toronto, New York City, Amsterdam, Oakland, Buffalo, Chicago, Boston, Kansas City and Cincinnati.
Three cities — New Orleans, Louisville and Tokyo — managed to reduce their rat populations during the study period. There were no significant changes in Dallas or St. Louis.
Los Angeles wasn’t included in the analysis because systematic rat records weren’t available. L.A. routinely ranks among the top three in pest control companies’ annual lists of America’s “rattiest cities,” but Richardson said the perennially large volume of rodent complaints had more to do with the city’s sprawling size than a uniquely rat-friendly environment.
Next, the researchers used statistical methods to see which factors might account for the differences in the cities’ rat control outcomes. About two-thirds of the variation could be explained by five things, including human population density and the amount of area covered by vegetation.
The most important factor was the change in a city’s average temperature — the more it rose, the more the rat population grew.
The change in a city’s minimum temperature had no bearing on rats. Richardson said the team initially was surprised by that, since cold weather extends the time it takes for female rats to become fertile and reduces the number of pups in a litter.
In more hospitable weather, a rat can become pregnant when she’s just two months old, and that pregnancy will last only three to five weeks. The researchers realized that if rising average temperatures caused winter conditions to arrive a week or two later and wrap up a week or two earlier, it could buy a rat enough time to squeeze in an extra reproductive cycle each year, Richardson said.
Santtu Pentikäinen, a researcher at the University of Helsinki who was not involved in the work, said the study authors made a convincing case that global warming is good for rats.
“The results just make sense,” said Pentikäinen, a member of the Helsinki Urban Rat Project.
Coauthor Maureen Murray, a wildlife disease ecologist at Chicago’s Lincoln Park Zoo and leader of the Chicago Rat Project, said she hoped the findings will “motivate people to care that climate change could exacerbate their rat problems.”
But Richardson said he wasn’t sure the prospect of “more rats scurrying around” will be any more galvanizing than pictures of “the sad polar bear floating on ice.”
