Universe 25 was the twenty-fifth of its kind, and the mice were a late choice. For twenty years before them John Calhoun had worked with rats, and everything the public knew about his mice in 1970 it had learned from his rats in 1962. This piece is about the two rat experiments: the pen in the woods at Towson, Maryland, and the room in a barn at Rockville, where the behavioural sink was made and named.

Baltimore and Towson

In 1946 Calhoun joined the Rodent Ecology Project at the Johns Hopkins School of Hygiene and Public Health, whose business was getting rats out of Baltimore. The city’s method was poison. Treating a block with ANTU, the project found, cut its rats by 50 to 85 per cent, and within 15 to 44 months the number was back where it had been. Each block, in other words, held a fixed number of rats, and held it although the garbage cans were open and there was room for more burrows. Calhoun’s hypothesis was that the rats’ own behaviour towards one another set the number, and in 1947 he built a place to test it.

The place was a quarter of an acre of partial clearing near Towson, a hundred yards from his own house, which mattered because rats are nocturnal and the watching had to be done at dusk, before dawn, and on chosen nights under 2,000-watt lamps. It was modelled on a Baltimore row-house block. A rat-proof fence kept the rats in; an electrified fence a few feet outside it and a ring of traps kept predators out; a tower twenty feet high beside the pen let one person with six-power binoculars see every rat in it. Food and water were plentiful. Each rat carried a fish tag in its ear and a pattern of spots clipped from its fur, 43 combinations for each sex, so that it could be known by sight. The founders were five pairs of wild Norway rats trapped in February 1947 on Parsons Island, 150 acres in Chesapeake Bay cut off from the mainland since the turn of the century, chosen because an island population that had been through bottlenecks of fewer than 150 animals would be nearly uniform in its heredity, and whatever the rats did could then be put down to their surroundings. The Army Signal Corps came and filmed the pen in April and May 1949. Calhoun watched it for twenty-seven months.

The rats sorted themselves at once. The heavier, dominant animals, who also bred best, lived in the south-east of the pen; of a litter born on the north-west side only three survived, and they grew more slowly and ranked lower than the thirteen survivors of the south-east litters. As the pen filled, males from the south-east moved to the north-west, and no rat ever moved the other way. “At one end of the social spectrum there was a colony consisting of a single dominant male and a harem of 13 females, most of whom were reproducing successfully,” he wrote. “At the opposite extreme there were all-male aggregates whose members rarely engaged in any sexual activity.” A female in heat in the north-west was followed by packs of males and mounted several hundred times a night. Those females stopped building nests and burrows, conceived less often, and reared fewer young. Rank set the hours at which a rat went to the single central food and water station: the high-ranking kept the twice-nightly schedule of the colony’s early days, the low-ranking went at dusk and dawn, when their superiors were asleep. The rats wore trails through the vegetation between burrows and food, and then stopped making them: “Expansion of the meshwork of trails was limited by some response which precluded the minimum diameter of the polygon enclosed by a group of trails becoming less than 5 feet. Once the pen became filled to this extent, further increase in the number of rats merely increased the frequency of contacts between rats inhabiting the same locality.”

Ten thousand square feet of pen, Calhoun calculated, could have held 5,000 healthy rats at the two square feet a laboratory gives each one. It held 150 adults at the end, and never more than 200. “The obvious explanation is that under free-ranging conditions the rats expressed genetically determined and culturally modified behavioral potentialities which would be impossible under caged conditions.” The report ran to 288 pages, was finished in 1956 with help from Walter Reed and the National Institutes of Health, and was published in 1963.

One thing in it mattered more to him later than the numbers. The most disorganised rats in the pen, a group of males who had lost the ability to court and could not keep their burrows in repair, dug a new burrow in a way no rat had been seen to dig one. Instead of pushing the dirt out as they went, they made it into forty or fifty wads, formed the wads into a ball underground, and rolled the ball out. Calhoun never saw it done; he saw the ball beside the fresh burrow, and saw them do it again. He called it something akin to the discovery of the wheel, and the lesson he drew from it in 1970 was the one he would spend the rest of his career on: that invention comes from the pressed and the peripheral, and that “we must be very careful about what we call mental health.”

The barn at Rockville

From 1954 Calhoun was at the National Institute of Mental Health, which leased him a barn on a farm at Rockville, and there, between 1958 and 1962, he ran six populations of domesticated albino rats, in two series of three, under conditions he could control. Each population lived in a room ten feet by fourteen, divided by electrified partitions into four pens. Each pen was a complete dwelling: a food hopper, a drinking fountain, and an elevated burrow reached by a winding staircase, with five nest boxes in it. Twelve adults is the size of a wild rat group, so the room should have held 48 in comfort; each population was allowed to grow to 80 and held there by removing litters, and watched for sixteen months through a window in the ceiling.

The room was built to be unfair. Ramps crossed three of the four partitions, so that pens 2 and 3 could be reached from either side and pens 1 and 4 from one side only, which loaded the odds in favour of crowding in the middle. In the first series the food was hard pellets behind wire mesh, which made eating slow. A rat eating slowly was likely to be joined by another, and the association of food with company grew until “the rats would rarely eat except at hoppers already in use by other animals.” Then it fed on itself: as the crowd at one middle hopper grew, the other hoppers emptied, and the rats at them, “finding themselves deserted by their groupmates, would transfer their feeding to the more crowded pen.” In time three-quarters of the population ate at one hopper, and by the end half or more of them slept in that pen as well. “Eating and other biological activities were thereby transformed into social activities in which the principal satisfaction was interaction with other rats.”

This was the behavioural sink. The name was geological before it was anything else: a sink is a hollow with no drainage, where water and organic matter collect and stagnate. The sentence that defined it in Scientific American in February 1962 was chosen with care: “The unhealthy connotations of the term are not accidental: a behavioral sink does act to aggravate all forms of pathology that can be found within a group.” In the pens where it developed, infant mortality ran as high as 96 per cent. In the second series, where powdered food in an open hopper let a rat eat and go, and a slow fountain made the animals social drinkers instead, no sink developed, and 80 per cent of the infants in the middle pens died anyway.

What the females did is the part everyone remembers. In the crowded middle pens they built flat nests, then incomplete ones, then none, and bore their litters on the sawdust. A mother alarmed by a pack of males at the burrow would start to carry her pups to safety and be distracted on the way; “they would move only part of their litters and would scatter them by depositing the infants in different places or simply dropping them on the floor of the pen. The infants thus abandoned throughout the pen were seldom nursed. They would die where they were dropped and were thereupon generally eaten by the adults.” Nearly half the first- and second-generation females in the sink died of disorders of pregnancy and birth by the sixteenth month. In the end pens, where one dominant male slept at the foot of the single ramp and let no other male come down it, his harem of females built proper nests, nursed, and carried their young to safety one at a time; half the infants born there lived. “The brood pens, as we called them, appeared to be the only healthy ones, at least in terms of the survival of the group.”

The males were sorted into types, and the types were given names. The dominants, “the most normal males in our populations,” who still went “berserk” at intervals. The pansexuals, who mounted anything. The probers, hyperactive and uninterested in rank, who broke into burrows after females and in time became cannibals. And the ones who had withdrawn entirely, who “moved through the community like somnambulists,” ignoring and ignored: “To the casual observer the passive animals would have appeared to be the healthiest and most attractive members of the community. They were fat and sleek, and their fur showed none of the breaks and bare spots left by the fighting in which males usually engage. But their social disorientation was nearly complete.” Those were the beautiful ones, eight years before Calhoun called any mouse that. When the experiments ended he kept the four healthiest males and four healthiest females from each of two populations, six months old, and gave them room. They bred less than normal rats in the next six months, and none of their young lived to maturity.

The phrase

The article was called “Population Density and Social Pathology.” It became one of the most cited papers in psychology and is reprinted to this day in a textbook called Forty Studies That Changed Psychology. Its names travelled further than its numbers. In 1968 Tom Wolfe put the sink at the end of The Pump House Gang, in a chapter about New York, and Hunter S. Thompson wrote to congratulate him on a phrase he took to be Wolfe’s own. A California comic of 1970 called its stories “Tales from the Behavioral Sink.” British journalists were writing of “sink estates” by 1972. The ecologists at Johns Hopkins and the barn at Rockville had been asking what sets the number of rats on a block. The phrase that came out of the barn was about cities, and it was in print and in common use before the first mouse went into Universe 25.

Sources

• John B. Calhoun, “Population Density and Social Pathology,” Scientific American 206 (2), 139–148, February 1962. The room, the pens and ramps, the hoppers and the fountain, the 96 and 80 per cent, the females, the types of male, and the eight survivors. In copyright; quoted briefly. The sentence defining the sink is quoted at second hand from Ramsden and Adams, below, because the page that carries it is illegible in the copy read. gwern.net
• Maya Pines, “How the Social Organization of Animal Communities Can Lead to a Population Crisis Which Destroys Them,” in Julius Segal, ed., Mental Health Program Reports 5, National Institute of Mental Health, December 1971. Baltimore and ANTU, the Towson pen and its tower, Parsons Island, the Signal Corps films, the south-east and north-west, the trails, the 5,000 and the 200, the ball of dirt, and “what we call mental health.” files.eric.ed.gov
• Edmund Ramsden and Jon Adams, “Escaping the Laboratory: The Rodent Experiments of John B. Calhoun and Their Cultural Influence,” working paper, LSE, 2008, for the Towson pen’s dimensions, Wolfe, Thompson, Insect Fear, and “sink estate.” researchonline.lse.ac.uk
• John B. Calhoun, The Ecology and Sociology of the Norway Rat, US Public Health Service Publication 1008, 1963, the 288-page report of the Towson pen, cited by Pines; not read.