Around a year ago​ a rat died in my kitchen. The first thing I noticed was the cloud of bluebottles flying drunkenly against the window. Then there was the smell, which was fetid and slightly sweet. It took me a while to find the source, and when I eventually pulled up the floorboards there was nothing left of the rat but a shrivelled sack of skin and fur, its tail a mangy question mark. I donned washing-up gloves, picked it up by the tail and put it in the bin. I blocked all the holes I could see and hoped that would be the end of the matter. But for months afterwards I was haunted by thoughts of rats. At night, I imagined I could hear the patter of their feet beneath the floor.

No one seems to know the origin of the claim that you’re never more than six feet away from a rat. It persists because it feels true. (According to one estimate, the actual average figure in the UK is 164 feet.) One possible source is The Rat Problem (1909) by W.R. Boelter, a book that is part natural history, part anti-rat screed. Boelter estimated that there were forty million rats living in the UK – one for every acre of arable land, and roughly equal to the human population at the time. His great enemy was Rattus norvegicus, the Norwegian or brown rat, which was believed to have arrived in Britain on Scandinavian trading ships in the 18th century. In fact, Rattus norvegicus probably came to Europe from northern China. Its ferocity and adaptability meant that it soon displaced its smaller and more timid cousin Rattus rattus, the black or house rat. (There has always been a degree of xenophobia in the naming of rats: in Wales, Boelter reports, the black rat is called ‘LLyodun Ffrancon’, the French mouse; the French, for their part, call it the ‘English rat’.) For Boelter, rats were a scourge with no redeeming features. They spread disease, ate crops and caused electrical fires with their gnawing. He calculated that they cost farmers £1 million a day. Even if rats were eradicated from the country, he warned, their fecundity is ‘known to be so enormous that, unless such preventative measures are of permanent character, the progeny of … rats imported on ship-board or having come over the border, would soon rise to such a number as to constitute a new plague’.

Rats are unnerving in part because of their elusiveness, which makes the rat in the head seem more real than the rat in the sewer. Those who keep them as pets know them to be clean, gregarious creatures with a highly developed social life, but for most of us rats inspire only revulsion. This might be one reason they have become such popular subjects for scientific experiments, it being easier to sacrifice animals to the pursuit of knowledge if you’re not particularly fond of them in the first place. The first true lab rats were an albino strain of Rattus norvegicus developed in 1915 at the Wistar Institute, a biomedical research centre in Philadelphia. Rats had already been used for some time to teach dissection in European medical schools, and Henry Donaldson, a neurologist at the Wistar Institute who had been searching for an animal for his own experiments, thought Rattus norvegicus might be suitable.

In 1909 Donaldson’s colleague Helen King began to breed rats, selecting offspring for their docility and fertility. After six years, 28 generations and tens of thousands of rats, King had bred what she considered the ‘perfect specimen’: a stable strain that, due to intensive inbreeding, amounted to what Jon Adams and Edmund Ramsden in Rat City call ‘a race of incest clones’. With the development of the Wistar, the lab rat became a branded product. According to The Rat: Reference Tables and Data for the Albino Rat and the Norway Rat (1915), a guide by Donaldson and King, the Wistar rat was ‘clean, gentle, easily kept and bred, and not expensive to maintain’. Its lifespan was about three years, and it reached sexual maturity in three months. It bred all year round, producing relatively large litters (between six and twelve pups). The Wistar rat’s homogeneity, Donaldson hoped, would ensure that the results of experiments conducted in different laboratories at different times could be more objectively compared. It was the biological equivalent of standardised units in physics, like the kilogram prototype or the mètre des archives.

Several million Wistar rats have died for science since Donaldson and King published their guide, and descendants of that first colony are still used in experiments today (when I tried to buy one, from a lab which said that their longevity and ‘high rate of spontaneous tumours’ made them ‘an ideal choice for ageing studies’, I was told I would need a permit from the Home Office). More specialised strains have also been bred: the spontaneously hypertensive rat develops high blood pressure at six weeks old, while the Royal College of Surgeons rat suffers from congenital retinal degeneration. There are chronically obese rats, rats doomed to develop early onset Alzheimer’s, even rats that are thought to display behaviour associated with anxiety and depression. Yet a study in 2007 found that testing drugs on rats produces reliable results – in terms of their efficacy for humans – no more than half the time. Other studies have shown that, at least when assessing the potential toxicity of new drugs in humans, rats may be worse than useless. One analysis found that ‘results from tests on animals (specifically rat, mouse and rabbit models) are highly inconsistent predictors of toxic responses … little better than what would result merely by chance – or tossing a coin.’ Relying on rodent models to draw conclusions about human behaviour seems even less sound. How might one recognise depression in a mouse? Who’s to say how an autistic rat will behave?

John Bumpass Calhoun, the subject of these two books, was one of the most famous advocates of rodent experimentation in 20th-century psychology. In both Rat City and Lee Dugatkin’s Dr Calhoun’s Mousery he is treated as an eccentric visionary, whose insights might still have something to teach us not just about rats, but about ourselves. The son of teachers, Calhoun grew up in rural Tennessee, where he developed an interest in small mammals and birds. A chance encounter with Ivey Lewis, a fellow twitcher and dean at the University of Virginia, led to his being offered a scholarship to study biology. After completing a doctorate at Northwestern on cyclical behaviour patterns in rats and voles, he found a position working on Johns Hopkins’s Rodent Ecology Project, which was trying to eradicate rats in Baltimore using a range of novel rodenticides. Boy Scouts and air-raid wardens distributed corn and apples laced with poison across the city, and nearly a million rats were killed.

Despite the success of the poisoning campaign, the total rat population remained remarkably stable over time. Since poisoning tended to target old, infirm or otherwise less capable rats, leaving their younger and more fecund colleagues alive, rat numbers bounced back quickly after each round of poisoning. But the population also appeared to have a natural upper limit (of around 150 rats per city block, far fewer than the environment should have been able to support), which was more difficult to explain. Calhoun found that even if you artificially increased the population of one block by importing rats from another – he kept track of the newcomers by marking their fur or feeding them dyed food so he could identify them by their droppings – the population would soon correct to the lower level. He began to think that some invisible mechanism must be preventing the rat population from growing beyond a set limit.

In 1947, having left the Rodent Ecology Project, Calhoun set about building a quarter-acre rat city, mimicking the layout of downtown Baltimore, in a field near his home in Towson, Maryland. The enclosure was divided into sections connected by alleyways, with one large ‘dining room’ in the middle providing unlimited food and water, and nest boxes in each corner. Calhoun released ten rats into his city – five males and five females – and for the next 27 months observed their behaviour from a watchtower. He noted every birth and death, and many thousands of social interactions. He conducted autopsies on the dead and periodically recorded the weights of his rats and the number of wounds on their bodies. It took him a decade to collate his data, but when The Ecology and Sociology of the Norway Rat was published in 1962 it was recognised, Adams and Ramsden write, as ‘the most comprehensive and complete account of rodent behaviour ever produced’.

As the title suggests, Calhoun was as interested in the ‘sociology’ of the colony as he was in the rats’ biology. This was controversial among biologists and ecologists at the time, who regarded his approach as overly anthropomorphic. By the end of the experiment, descendants of the first ten subjects had splintered into distinct groups, which Calhoun thought represented distinct personality types. Some had chosen to live in the nesting areas set aside for them; others occupied the alleyways. The groups that had been pushed to the corners of the enclosure, Calhoun observed, became weaker and had fewer social interactions than those in the middle. Roving packs of isolated males, which he called ‘imcasts’, harassed lone females who didn’t have the protection of dominant males. Calhoun had calculated that his enclosure should be able to accommodate thousands of rats, but their numbers never rose above two hundred. It was social pressure rather than the availability of resources, he concluded, that acted as a natural check on the population.

In 1954, with funding from the National Institute of Mental Health, Calhoun built a new rat city in a barn belonging to a local farmer. This enclosure was the size of two shipping containers and comprised six different rooms, each divided into four sections connected by ramps. The sections were equipped with artificial burrows, raised on plinths, and in the corner of each cell a hopper provided food and clean water. Calhoun released sixteen pregnant females into the enclosure. He and his assistants observed the rats from a gantry. For the first few generations, life seemed pretty good. But things soon started to go wrong. Most of the rats congregated in the central cells. Dominant males set up camp near the edges and took to sleeping defensively on the ramps, guarding – or imprisoning – groups of females. Rats don’t usually like sharing space with strangers, but over time those that had congregated in the middle of the enclosure seemed to prefer feeding when surrounded by others. What Calhoun called a ‘pathological togetherness’ infected the colony, leading to the emergence of all sorts of unusual behaviour. Subservient male rats which had been unable to secure a burrow fell into two distinct camps. One group became what Calhoun called ‘probers’: timid ‘delinquents’ that would avoid the dominant males but harass any unfortunate female they encountered and often fight with one another. The other group Calhoun called ‘the beautiful ones’. These rats withdrew entirely, spending their days obsessively grooming and eating, but never interacting with the others. The beautiful ones were ‘phlegmatic animals’, Calhoun observed, ‘blobs of protoplasm’ that were ‘physically healthy but socially sterile’. He later suggested that they might be suffering from the rat equivalent of autism.

Calhoun’s​ best-known experiment, which began in new facilities at NIMH in 1968, was called Universe 25. By then he had turned his attention from rats to mice. Universe 25 (it’s not quite clear what the previous 24 universes consisted of – much of Calhoun’s research remains unpublished) was designed as a large enclosure, divided into sections lined with high-rise ‘apartment blocks’, each containing several nest boxes. At first, the mice thrived. The population doubled every 55 days for the first ten months until it numbered six hundred. Around day 315, however, growth suddenly slowed, then, in the second year, crashed altogether. Calhoun thought the enclosure could comfortably support almost four thousand mice, but the population peaked at half that.

The colony also began to show signs of the social decay Calhoun had observed in his previous studies. Some mice retreated to the highest nest boxes; others began to demonstrate the same ‘aberrant behaviours’ he had diagnosed in his rats – homosexual mating, rejection or eating of the young, loss of sex drive, indiscriminate aggression. By the end of the experiment, fertility rates had crashed. More worrying was the fact that the ‘aberrant behaviours’ seemed to persist when mice were removed from Universe 25. Many of their offspring, Calhoun reported, were unable to breed successfully or to look after their own young. His conclusion was bleak: overpopulation in rodents, even when resources were plentiful, led not to mere stasis but to societal collapse and, eventually, species extinction.

Calhoun was probably a better communicator of his ideas than he was a scientist. In an article for Scientific American in 1962 he coined the phrase ‘behavioural sink’ to describe the process by which his rodent communities had fallen apart – caused, he believed, by the stress of living in close proximity with others. Calhoun’s thesis became popular with journalists and writers (even if it was challenged by some of his peers). Fears of human overpopulation were in the air. In The Population Bomb (1968), Paul Ehrlich warned that in the near future hundreds of millions of people would starve to death from famine. Calhoun’s experiments appeared to suggest that psychological and societal breakdown would be at least as significant an outcome of the ‘population bomb’ as material deprivation. Just as unwanted social encounters had left Calhoun’s rats and mice in ‘a near permanent state of fight or flight’, so too might humans descend into the behavioural sink, leading to the extinction of the species.

One of the reasons Calhoun’s theories proved so attractive was that he was never shy about spelling out their implications. ‘I shall largely speak of mice,’ he begins his most famous paper, ‘Death Squared: The Explosive Growth and Demise of a Mouse Population’, ‘but my thoughts are on man, on healing, on life and its evolution.’ When a journalist asked whether he was ‘maybe seeing the phenomenon of the beautiful ones … in the dropout, drug culture?’ Calhoun didn’t think the question ridiculous. The Daily Telegraph published an article on some of Calhoun’s findings with the headline: ‘Mice Point Way to Doom in 1984, Says Scientist.’ Yet despite encouraging a certain amount of journalistic sensationalism, he didn’t himself fully support the narrative of decline and despair. As Adams and Ramsden point out, during the second half of his career, when he was increasingly dismissed by other scientists, Calhoun wasn’t hopeless about the future of humanity. He tried to drum up interest in his proposed solutions to the problems of overcrowding, advising architects on urban regeneration and prison design, and courting visiting politicians and town planners. Tom Wolfe borrowed the phrase ‘behavioural sink’ for his essay collection The Pump House Gang (Hunter S. Thompson, believing it to be Wolfe’s coinage, wrote to him calling it ‘a flat-out winner’). In 1971, two years after visiting Universe 25 for National Geographic, the journalist Robert Conly published the popular children’s book Mrs Frisby and the Rats of NIMH, which was partly inspired by Calhoun’s research.

Calhoun’s ideas seem to have become even wilder towards the end of his life, though all three authors are too polite to say so. Sidelined by NIMH and unable to get funding for his research, he began speculating on the future of humanity in ever more gnomic ways. He spent years writing an unfinished sci-fi novel, 317 P.H.: A Satire on a Future Multiple ‘Utopia’, about a ‘Posthomo’ species bred by mankind to survive the apocalypse. He also devoted himself to trying to invent and promote a kind of prosthetic ‘global brain’, a vague idea that his supporters have claimed anticipated the development of the internet.

The accuracy of Calhoun’s observations about rat sociology, still less their relevance to human society, remains a matter of debate. Even at the time he was criticised for the methodologies of his experiments (one colleague at NIMH told him that ‘there are so many variables in your research that you can’t possibly draw any conclusions about anything’). Adams and Ramsden, and Dugatkin, want to be clear-eyed in their assessments of his work, but they do accept some of his more speculative findings. Ramsden and Adams compare Calhoun’s ‘beautiful ones’ to the phenomenon of hikikomori in Japan: young people who refuse to work or engage with society. Dugatkin treats him more as an inspirational mad scientist who went on to inspire generations of social psychologists and biologists. But these approaches downplay the shakiness of Calhoun’s observations. Many of the behaviours he considered ‘aberrant’ – cannibalism, aggression, homosexual mating – are not uncommon in rodent society. As both books acknowledge, his experiments have never been successfully replicated by other researchers, and wild colonies of rats don’t seem to display any of the personality types he so vividly described. He rarely published in mainstream scientific journals, saying that his concerns were so pressing that he didn’t have time to wait for peer review.

It seems plausible that Calhoun was responsible for creating the conditions for population collapse. He only cleaned out Universe 25 every six to eight weeks; disease and parasitism could account for many of the phenomena he thought were due to mere proximity. It can’t have helped that, other than the ladders and ramps, the Universes didn’t provide much in the way of stimulation. Calhoun thought of his rat cities as utopias, but they seem more like well-stocked prisons. No wonder they didn’t bring out the best in the inmates.