How rats are taking over – and why we should learn to love them

The habitat, Calhoun imagined, was sufficient to accommodate 5,000 rats, and to kick things off he introduced five pregnant females. Brown rats can produce litters of up to 14 pups, and average around half a dozen. Given their ability to fall pregnant almost immediately after giving birth and gestate while simultaneously lactating, he presumed the woodland floor would soon be teeming. However, despite the provision of a constant supply of food over two years of research, the population never exceeded 200 and ultimately levelled off at around 150.

A few years later, while employed at the National Institute of Mental Health, Calhoun repeated the experiment but this time in indoor pens. Once more he introduced rats and provided food, water and bedding. Calhoun called this experimental world a ‘rat utopia’. But there was one limit deliberately imposed on this world that was different to his outdoor experiment: space.

The rodents bred prodigiously and the pens soon heaved with animals. Then something strange occurred: the rat society started to collapse in upon itself. Dominant males formed aggressive packs that attacked females and the young. Some rats became hypersexual, attempting to mate with everyone they encountered. Mothers abandoned and even attacked their pups as infant mortality rose to 96 per cent. Cannibalism was rife. What became left of rat utopia was a group of terrified subordinate rodents huddled together for safety in the middle of the pen while the more feral inhabitants of the colony roamed the perimeter. The population crashed and did not recover.

Published in 1962, Calhoun’s study came at a time of rapid urban expansion. Calhoun termed his findings of the rats’ descent into vice and destruction a ‘behavioural sink’. This was a horror story that resonated deeply.

There are two species of rat in the UK: the brown rat (Rattus norvegicus) and black rat (Rattus rattus). While there is evidence of black rats present in Roman Britain, they are now restricted to just a few population clusters, having been gradually displaced by the larger brown rat. Both are invasive species (brown rats originated in China, and black rats in the Indian subcontinent) and have followed humans wherever they have gone. Now found on every continent except Antarctica, rats are one of the most populous and successful mammals on earth. Nearly the most destructive, too – although that particular mantle belongs to us. It would be disingenuous to suggest that rats are not a pest and, in the right circumstances, a threat. It would, in fact, be difficult to envisage a more effective harbinger of pestilence than the rat: muscular, ferocious, with incisors that are stronger than steel and bodies capable of squeezing through the tiniest of gaps to access food sources in our homes.

Rats, along with other rodents, pilfer one-fifth of the world’s food harvest and can devastate populations of animals and seabirds. They are also a reservoir of pathogens known to cause more than 70 diseases. Rats have been found to harbour bubonic plague, cholera, typhus, leptospirosis, cowpox and hantavirus infection. A 2014 study from Columbia University found that the average New York City subway rat carried 18 novel viruses not yet detected in humans, along with dozens of familiar pathogens.