Some like it hot: Warmer winters mean more rats in Boston

Boston’s booming rat population is likely linked to the warming climate, according to a new study that looked at rat population trends in 16 cities around the globe. The study found more rats corresponded to a rise in temperatures, especially in winter.

Of the 16 cities — which were chosen for their good record-keeping rather than any inherent rattiness — 11 had “significant” increases in rat numbers over about a decade, including Washington D.C., New York City and Boston.

Experts say there is no reliable estimate of the total number of rats in Boston. But the number of rat complaints in the city increased 53% between 2011 and 2021, according to the study.

While other factors contributed to rat booms — including more people in densely-packed neighborhoods, and dwindling urban parks and wild space — warming seemed to have the biggest effect. Cities that saw the greatest warming over time also experienced faster increases in rat numbers.

The study was published Friday in the journal Science Advances.

Rats are generally most active in summer. In frigid weather they tend to hunker down in their burrows, forage less and breed less. But warmer winters are changing their behavior in many cities, said the study's lead author Jonathan Richardson.

“Where you have a particularly warm winter, or even a warm week within a winter, those rats are likely to spend more time above ground, searching for food and bringing some of that food back to their colony,” said Richardson, an urban ecologist at the University of Richmond in Virginia.

Staying warmer and well-fed helps rats stay frisky all winter. That's great for the rats, not so great for people.

Rats damage infrastructure, chew through car wiring, contaminate food and spread disease. Plus, the study noted, "The very presence of rats also takes a measurable toll on the mental health of people living in contact with them."

Marieke Rosenbaum, an assistant professor at Tufts University’s Cummings School of Veterinary Medicine, said climate change may also be contributing to an increase in rat-transmitted diseases. Her recent research looked at rats in Boston carrying leptospirosis, a disease spread through rat urine that is more common in tropical and subtropical areas.

“As we see climates warming, we're expecting to see more rats in urban areas," she said. “And we are already seeing increases of leptospirosis in the Northeast in humans and in dogs.”

Three cities saw falling rat populations over the period of the new study: Tokyo, New Orleans and Louisville. Experts attributed these declines to better rat control measures.

“ Better trash handling, better sanitation, better awareness will drive the population down no matter what the climate,” said  David Power, a project coordinator in environmental health, and unofficial “rat czar,” for Cambridge.

Study author Richardson said the findings are “not an indictment of the efforts of hard working personnel,” trying to cull rat populations.

“Boston's a very ratty city, no doubt about it, but the team of people working on it knows that,” Richardson said, praising Boston’s Rodent Action Plan.

“There's really no silver lining to this finding where rat numbers seem to be increasing with warming temperatures,” he added. But he hopes cities can use the data for planning.

“You might as well go into it with a complete picture of the challenge you're facing,” he said.