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Rats can’t vomit. This may be a function of their anatomy—their stomachs are “not well structured for moving contents towards the esophagus” is how one study delicately put it—or it may have something to do with their brain circuitry, or it may be a combination of the two. Whatever the cause, the result is that rats, contrary to their popular (or unpopular) image, are fussy eaters. Even as they pick through the trash, they’re hesitant to try new foods. This makes poisoning them complicated; quite often—and quite literally—they won’t take the bait.
In 1942, a Johns Hopkins biologist named Curt Richter discovered a new poison that rats apparently couldn’t taste. His breakthrough caught the attention of the United States Office of Scientific Research and Development, the Second World War equivalent of DARPA. The agency, among its many worries, feared that the Axis powers were at work on biological weapons that would use rats as vectors. (In fact, the Japanese did try to spread plague during the war, with some success.) The O.S.R.D. had the poison—alpha-naphthyl thiourea, or ANTU for short—tested in the back alleys of Baltimore. The city was so pleased with the resulting carnage that it appointed Richter to lead a new rodent-control office, based in City Hall. By 1946, ANTU-laced corn had been spread over more than fifty-five hundred blocks and, according to Richter, “well over a million rats” had been killed.
By that point, however, ANTU was starting to lose its efficacy. Apparently, rats were learning to associate adulterated corn with unpleasant consequences and becoming bait-shy. New measures, it was realized, would be needed, and an even more ambitious research effort was born—the Rodent Ecology Project.
The project was funded by the Rockefeller Foundation, which tapped another Johns Hopkins professor, David E. Davis, to lead it. Davis thought that the best way to control rats was to understand their habits. He set about studying how Baltimore’s rats spent their days, or, really, nights, since the animals in question—Norway rats, which actually come from Asia—are nocturnal. He and his assistants trapped rats on the streets and marked them, usually by clipping off some of their toes. They released the digit-poor rodents back onto the streets, then tried to recapture them. In dry weather, they put out food infused with blue dye and tracked the tinted droppings that resulted.
These labor-intensive routines revealed that rats live in small groups of about fifteen individuals. They tend to stick close to home, and they don’t like to cross roads. Davis’s team also found that rat numbers were remarkably stable. About ten groups, or a hundred and fifty individuals, lived on an average block. If some of the rats on a block were killed, either by ANTU or by predators, the population quickly rebounded, levelling off again at about a hundred and fifty rats.
Such stability was hard to explain. Clearly, the rats’ numbers were not limited by resources, as there was always more garbage to be plundered. So why didn’t some blocks have a whole lot more rodent residents? One of Davis’s assistants, a young ecologist named John B. Calhoun, suggested an experiment. What if additional rats were introduced on a street? Would the population increase? Calhoun trapped more than a hundred rats, marked them, and released them on one particular block. When he and his colleagues tried to recapture the imported rats, they couldn’t find any. Meanwhile, it seemed, the block’s original rat population had declined. As one account of the experiment put it, “It looked like the most effective rat killer was more rats.”
Both Richter and Davis eventually moved on from the study of street rats to pursue other projects. But Calhoun was hooked. He would spend the rest of his life investigating what controlled rats’ numbers, with results that many experts interpreted as ominous for humanity.
Two new books take up the subject of Calhoun and his rats. The authors of the first, “Rat City: Overcrowding and Urban Derangement in the Rodent Universes of John B. Calhoun” (Melville House), are a pair of British researchers, Edmund Ramsden and Jon Adams, who for a time both taught at the London School of Economics. The second, “Dr. Calhoun’s Mousery: The Strange Tale of a Celebrated Scientist, a Rodent Dystopia, and the Future of Humanity” (University of Chicago), is by Lee Alan Dugatkin, a historian of science at the University of Louisville. Both books cast Calhoun as a visionary. Both also portray him as eccentric to the point of crankdom.
Calhoun, who went by the nickname Jack, was born in 1917 in rural Tennessee. His father was a school administrator and his mother a teacher. As a child, he was passionately interested in nature, particularly in birds. In 1933, his father lost his job, a development that might have prevented Calhoun from attending college, had ornithology not intervened. One day that summer, while visiting the University of Virginia, Calhoun ran into a dean who happened to be an avid birder. After just a few minutes of conversation, the dean offered him a scholarship to U.V.A. In 1943, Calhoun completed a Ph.D. at Northwestern University; a few years after that, he landed the position with the Rodent Ecology Project.
Calhoun’s translocation experiment convinced him that there was still a lot to learn about the social lives of rats. But, he decided, he could no longer work on the streets of Baltimore; there were too many variables he couldn’t control. Nor did it make sense to work with lab rats; their lives were too artificial. What he needed, Calhoun thought, was an urban setting only he had access to. With the blessing of the Rodent Ecology Project, he constructed a simulacrum of a city block on an empty lot in Towson, Maryland, about ten miles north of Baltimore. (Though it was smaller than an actual block, the setup replicated the typical layout of Baltimore’s back yards and alleyways.) To keep the rats in and predators out, he erected an elaborate series of fences around his SimCity, and, to monitor the goings on there, he built himself a little observation tower. He placed ten wild rats—five males and five females—inside the fences, and then, for two years, he watched.
The Towson experiment produced reams and reams of data. Every six weeks, Calhoun would conduct a census of the enclosure’s population by capturing every rat. (Individual rats were marked with metal ear tags.) Sometimes, before releasing the rats, he would anesthetize them so that he could record their size, their weight, and the number of their wounds. He tried to register every birth in the enclosure, and every death. In the process of all this, Ramsden and Adams write, Calhoun came to know “more about the behavior of the Norway rat than anyone else alive.”
The Towson rats were supplied with essentially limitless food, and for a while they took advantage of this by increasing their numbers. At the end of a year, ten rats had become thirty. By the eighteen-month point, there were a hundred and fifty in the enclosure. Then the population abruptly levelled off. For the last six months of the experiment, it never rose above a hundred and eighty.
Having observed the rats so closely, Calhoun now had a pretty good idea of what was limiting growth. The rats had divided themselves into eleven clans. Four had burrows conveniently located at the center of the enclosure, near where Calhoun had placed the food bins. In these privileged clans, a few dominant male rats mated with (and protected) a larger number of females. Although the high-status mothers successfully raised many pups, this wasn’t enough to offset the losses in a population that was aging and, increasingly, brawling.
The rats from the banlieues, for their part, lived under constant stress. When they attempted to get to the food bins, the fat rats in the middle tried—often successfully—to repulse them. Along the edges of the enclosure, packs of low-ranking males roamed from burrow to burrow, harassing the females. The outer-burrow females were so exhausted that they rarely conceived, and, when they did give birth, they often abandoned their pups.
Calhoun published his results in a two-hundred-and-eighty-eight-page monograph, “The Ecology and Sociology of Norway Rats.” As Ramsden and Adams point out, the use of the word “sociology” in the title was daring, as this term is normally reserved for the study of humans. Toward the end of the volume, Calhoun made explicit his intention. “Animal subjects,” he wrote, “may be of value in elucidating some of the social problems which confront man today.”
Calhoun’s Rodent Ecology Project contract ended in 1949. It took him almost a decade to get another major rat study up and running, but, when he did, it was an extravaganza. The new experiment was financed by the National Institute of Mental Health, which had just been created. At a cost of a hundred thousand dollars—more than a million dollars in today’s money—Calhoun had a ten-foot-tall rat enclosure constructed in a barn in Gaithersburg, Maryland. The enclosure was divided into six rooms, each of which was further divided into four cells. This time around, Calhoun planned to control the enclosure’s population himself, by removing pups when there were more than eighty rats per room.
The experiment got under way in January, 1958. For the first few months, the rats seemed content in their apartment-like dwellings. But then, once again, things took a dystopic turn. Calhoun had laid out the rooms asymmetrically. The two cells in the center each had two entrances; those on the ends had just one. Dominant males assumed control of the easier-to-defend cells and allowed only a select group of females to enter them. This forced the other rats into the central cells, where order gradually broke down. Dispensing with the courtship rituals that usually precede mating, mid-cell male rats took to simply trying to mount females, or even other males. Aggression increased; at times, Calhoun wrote, “it was impossible to enter a room without observing fresh blood splattered about.” Central-cell females basically gave up on mothering. They built inadequate nests or none at all. When disturbed, they would start to move their babies, only to then abandon them. The pup mortality rate in the crowded cells rose to as high as ninety-six per cent. Calhoun came up with a new term to describe the process he had witnessed. The rats, he said, had fallen into a “behavioral sink.”
With the barn experiment, Calhoun again cast his work as a form of sociology. In an article he published in Scientific American, in 1962, he observed that research like his could, “in time,” offer insights into “analogous problems confronting the human species.” He didn’t specify what the analogous problems were, but he didn’t have to. In the early nineteen-sixties, fears of overpopulation and urban decay were rampant. At about the time Calhoun wrote his article, a group of researchers at the University of Illinois decided to calculate what would happen if the number of people on the globe continued to increase along the trajectory it had followed for the previous two millennia. The researchers concluded, with a mathematical version of tongue-in-cheek, that the population would approach infinity on November 13, 2026. In the meantime, the planet would become so crowded that there would be no room to move. “Our great-great-grandchildren will not starve to death,” they wrote in Science. “They will be squeezed to death.”
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