Global Heating Drives Rat Populations Up, Study Finds; Poison May Not Be the Answer

The study, published last year by Corrigan and 10 other scientists, analysed rat numbers in 16 cities and found a clear link with rising temperatures. The two most common rat species, black and brown, do not hibernate, so cold winters normally slow their breeding. Corrigan explained that repeated warm winters give rats an advantage, and even a modest increase in reproduction can multiply across hundreds of millions of rats. "When you have repeating warm winters, one after the other, the rats benefit," he said.

Corrigan, now in his 70s, began his career as an exterminator in New York City's sewers to save for graduate school. He later spent weekends camping in barns to observe rats. "I saw things back then," he said. "I saw kindness: that rat just brought food to another because it looked sick. I saw aggression: a bully would come around and attack other rats for no reason." He described rats as "smart mammals" rather than "any old vermin", but acknowledged they can transmit diseases such as Weil's disease, hantavirus and new coronavirus variants.

Despite decades of relying on poison, Corrigan calls it "a band aid". Rodenticides kill rats slowly and cruelly, harm other wildlife, and cause thousands of accidental human poisonings each year in the US alone. Most experts, including Corrigan, accept poison as a temporary corrective measure, but they say prevention through stricter public health regulations often works better.

Fertility controls are gaining attention as an alternative. Richard Parr, director of the UK charity Center for Wild Animal Welfare, said developing new forms of birth control is key to managing soaring numbers of poison-resistant rats compassionately. He added that outdated EU biocide regulation is "stifling innovation" and deters investment.

Not all experts are convinced. Steven Belmain, professor of ecology at the University of Greenwich, said people are "sometimes quite naive" to think fertility control can solve the problem. "If you've got a rat in your house, you're not going to feed it contraceptives and just let it live there with you," he argued. He warned that restaurants, hotels, hospitals and food warehouses would collapse without some form of poison management.

Back in New York, Corrigan is trying to gather more evidence that fertility controls are a viable alternative to poisons. In his view, the fight against rats requires understanding them as intelligent beings that cannot cohabitate with humans.