Now in his 70s, he has devoted his entire adult life to studying rats and our relationship with them. Last year, he and 10 other scientists published a paper describing the effect of global heating on rat populations. In 16 cities they found that rat numbers had increased dramatically, and there was a correlation between their numbers and rising temperatures.
The two most common types of rat – the black and the brown – do not hibernate, which means in places with regular winters their reproduction slows down or even stops during cold periods. Here lies the problem, says Corrigan. For many families of rats, the warmer temperatures may only offer a marginal advantage, but the problem escalates “when you start multiplying that little bit of extra rat production by hundreds of millions of rats”.
Humans have been trying to control rats for centuries. Often an invasive species, they are vectors for disease, decimate crop yields and destabilise ecosystems as they hunt other species to extinction. At the other end of the spectrum, domesticated rats, descended from wild Norway rats, are often kept as affectionate pets.
Now, more and more scientists and public servants around the world are trying to manage rats without filling their cities with harmful poisons. Methods for doing so range from contraceptives to robust public health regulations. The biggest issue with our current approach to rats – dumping poison on them and laying traps – is that we treat them like “any old vermin”, says Corrigan.
They are in fact “smart mammals with mammal brains”. Corrigan first got to know rats while working as an exterminator in the sewers of New York City. He worked in the sewers to save up for graduate school, where he studied rodent pest management.
There he then spent weekend after weekend camping out in barns to observe rats. I saw aggression: a bully would come around and attack other rats for no reason.” He describes his feelings towards rats as paradoxical: “Do I love that species and do I respect it as a scientist? On the other hand I know that they cannot cohabitate with us because they can kill us.” Rats can carry pathogens that spill over into human populations.
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