Rats Are as Bad as Human Beings in Some Ways

In October of 1942, Baltimore’s mayor, Howard B. Jackson, provided $5,000 to hire a team of workers, and the Office of Scientific Research and Development in Washington, DC, provided ANTU — alpha-naphthylthiourea — and bait mix for the Rodent Ecology Project at the Johns Hopkins School of Hygiene and Public Health to run a large-scale trial spanning 200 city blocks near the Johns Hopkins Hospital. (ANTU was one hundred times more toxic than any other rodenticide available at the time and that had only a weak odor.) Results were encouraging, with kill rates often in the 80–95% range. Mayor Jackson was impressed enough to increase his office’s funding of the work: “There are no strings attached to the appropriation,” Jackson told the Johns Hopkins team. “Anything you suggest will be done. We are giving you $25,000, and all we ask is that you kill $25,000 worth of rats.” Over the next few years, the City of Baltimore would use one hundred thousand pounds of ANTU-poisoned bait to deal with rats infesting 4,160 city blocks. 

By 1944, close to a million rats had met their demise by ingesting food from ANTU-baited traps. In that sense, the project had been extraordinarily successful. But there was a problem. Within months after a population on a block was culled by ANTU traps, with kill rates between 50 to 90%, the population would return to its earlier size: ANTU was a phenomenally good predator of sorts, but apparently there were other ecological forces in play.

The more time that John Bumpass Calhoun, a key member on the Rodent Ecology Project team, spent on the streets of Baltimore, the more he realized that social behavior, which to that point had largely been overlooked by the Johns Hopkins team, might be important in understanding population growth in rats. City blocks really did demarcate rat populations; rats virtually never migrated from one block to another. For all practical purposes, each rat had all its behavioral interactions with a relatively small number of others on its own block — and with no other rats. Those behavioral dynamics might very well have had something to do with why populations tended to settle at population sizes well below what the food on a block could sustain.

The only way to know was to set up an experiment.

As Calhoun’s thoughts on an experiment to understand the population dynamics of rats began to gel, he made a strategic decision. Aside from one small pilot study, all the work by the Rodent Ecology Project involved using ANTU to decrease population size and then measure what happened. Calhoun decided to come at the problem from the other end and “override …  rat clannishness” by establishing a “second layer of rat society in an already established society.” The idea was to embed that new layer within “the less used interstices of [that] established society.” All of which is to say that to understand why rat populations seemed to settle at about 150 (far below what a city block could sustain), rather than decimate the population with ANTU, Calhoun would increase population size and see if it headed back down, and if it did, he would try to determine why. 

Calhoun’s experiment involved three adjoining city blocks that he labeled the northern, central, and southern blocks, each of which had a population in the 100–150 range. “Rat man,” as Baltimore residents on those blocks came to call Calhoun, first trapped and then marked rats from all three blocks. He then introduced 112 marked alien rats onto central block, where he had marked 75 rats, and he used the northern and southern blocks as controls: nearby blocks where aliens were not added. 

Calhoun used live traps in the central block to look at how the population of resident and alien marked rats was faring. As he did so, the resident humans on the block, who knew rat man but not the details of the experiment he was doing, told Calhoun that the rat problem on their block was getting worse, as he knew it would be given the design of his experiment. People naturally assumed Calhoun’s goal was to get rid of rats, and they tried to be as helpful as they could. Sometimes that kindness backfired on Calhoun (and the rats). “They tried to help me,” Calhoun wrote. “Thinking that I shared their belief that every good rat is a dead rat, every so often someone would gather up the traps containing tagged rats that I wanted to record and release, pour boiling water over them, and meet me smiling.” 

Calhoun’s results from the central block were not pretty. Many rats were found dead, and data from who was and who was not found in live traps indicated the rat body count was even higher than the number of corpses Calhoun saw. Mortality was anything but random. Calhoun estimated that, because of attack by other rats, aliens suffered mortality rates three times higher than central block residents. As to whether the population started to drop back to preinvasion levels, what Calhoun discovered was that just a few days after the release of the aliens onto the central block, the total population there dropped 15%, on its way back to the preinvasion level of near 150. 

The ANTU work had shown that if the 150 or so rats that were living on a Baltimore block were poisoned down to a much lower number, they soon began heading back toward 150, though there was food for plenty more. Now, Calhoun had shown that if the number of rats on a block was experimentally increased, it soon started heading back down to 150. That, in and of itself, was an important finding with respect to the dynamics of population stability, but Calhoun wanted to know why his experimental population started to decline so quickly. For that he had to make some educated guesses, which he was not shy about doing. Calhoun had only anecdotal information on the behavioral interactions between aliens and residents, but from what he did know from his observations, he thought that competition, in the form of dangerous fights between residents and aliens, was why “all hell broke loose” on central block.

Calhoun was becoming convinced that “survival required some essential modicum of social stability” and that his introduction of alien rats had shattered that social stability, leading to violence and population decline. Ecologist David Davis, head of the Rodent Ecology Project, was starting to see similar things when he looked at rats in the laboratory. “Psychological turmoil” was how Davis described what happened when aliens were added to a laboratory rat population with an already-defined social structure. When asked what that turmoil entailed, and how it would affect population growth, Davis told an audience full of biologists to imagine adding a very large “rat into a colony which, up to that time, was quite happy.” A colony where “everybody knew who was married to whom, and whose children were whose.” It seemed likely to Davis that “there would be a considerable amount of commotion” and that the new rat “would try to establish himself as the dominant.” Davis thought it was “entirely possible that factors of that type would explain [subsequent population] decline.” 

Although Calhoun published his results on central block in a technical, rather specialized, journal called the Journal of Wildlife Management, this experiment marked the first time his work was picked up by the national press. “Rats are as bad as human beings in some ways,” was how Time magazine opened its June 14, 1948, article, “Displaced Rats.” What Calhoun was up to, the readers of Time learned, was studying “the troubles which refugee rats have to put up with when they emigrate.” There was a plethora of those troubles, Time wrote, and “at once there was social strife.” Time interviewed Calhoun for the article and summarized what rat man said: “In a long-established rat community, says Dr. Calhoun, there is very little fighting. Every rat, having tested its strength against its neighbors, knows its social position and stays in it. Newcomers must battle for places in this set order. But since (like human immigrants) they do not know the new country, they are at a disadvantage. When danger threatens, they do not know where to hide … The native sons, familiar with local conditions, win most of the battles for position in the social order.” Whether Calhoun actually said “country” and “like human immigrants” or whether Time was paraphrasing is not clear, but given the language in Calhoun’s later writings, it is not a stretch to imagine the words were his.

The streets of downtown Baltimore provided Calhoun with a natural laboratory, but there was only so much control he could have there, because people lived in this laboratory. Residents killed rats in his live traps or moved the traps, and empathetic youngsters occasionally released a rat from a trap. What’s more, block residents might take garbage out one day but not the next. They might put out rat poison when they deemed it necessary. They might tidy up the alley at an inopportune time, right in the middle of an experiment. Of course, Calhoun didn’t begrudge residents the right to do any of those things and more, but they could make interpreting the results of experimental manipulations difficult. 

To have the control he would need to truly understand the private lives of rats, Calhoun decided he would need to construct “a habitat that simulated the Baltimore row-house rat habitat” and to design a study “to determine the manner and extent to which social interactions might influence population growth.” 

The first question for Calhoun was where to build this large rat world. His house in Towson, Maryland, was surrounded by hundreds of acres of fields and forest. In the fall of 1946, as John O’Donovan, owner of the surrounding property, was taking his dogs out for a walk, Calhoun joined him and asked whether he might build his experimental enclosure on a field a hundred yards from the Calhoun home. “Not knowing really what I meant by a ‘rat pen,’” Calhoun recalled, “[O’Donovan] told me to go ahead and make use of his land … After settling a few legal problems, he left me in peace to become a peeping Tom of the sex and other life of rats.” 

Calhoun began by sitting down and writing a list. He’d need to build an observation tower atop of which he could watch his rats. The enclosure had to be large enough that it could eventually house many groups of rats but small enough that, once he marked all the inhabitants, he could observe anything they did above ground. The enclosure would need to have a tall fence around it, with posts every ten feet and barbed, electrified wire at the top, so vertebrate predators could not enter. He would need to make the fence ratproof, so that the residents could not leave and strangers could not enter. He would need one location for food and water. Rats would need to be able to build burrows where they liked. Finally, he would need to divide the enclosure into four parts by yet another short fence, which would “enable the formation and delimitation of local colonies” but also have passages that allowed movement into the sections. 

With that daunting to-do list in hand, Calhoun hired two laborers and started building. He settled on a 100-by-100-foot structure — roughly a quarter of an acre — as that was about the size of some of the smaller Baltimore city blocks where he had done his work. As for the rodent dining room in the enclosure, Calhoun decided it should be 20 feet in diameter and surrounded by a shorter fence that allowed access from passages on the north, south, east, and west. The feeder would be stocked with an abundance of Purina rat chow (used to feed laboratory rats everywhere), and on occasion some garbage, so the rats didn’t grow bored with the same menu every day; it and the watering hole were constructed so that a dozen or more rats could feed at a given time. To make the rats feel even more at home, Calhoun added nine nesting boxes in each corner of the enclosure. 

By January 1947, the basic structure was in place. Next came the bells and whistles that would allow Calhoun to monitor the rats even when he could not be perched, six-power binoculars in hand, atop the 20-foot-tall tower built just outside of the enclosure. To do that, Calhoun got a company in New York City to donate recording devices that he could set so that whenever a rat broke a beam of light emitted by the recorder, it triggered a pen to create a mark on a roll of recording paper. 

Calhoun didn’t have the resources to film the rats, but the United States Army did, and they wanted some footage of his work to use in a film they were making about pest control and epidemiology. And so it was that Major Lown and Lieutenant Strickland, along with a team consisting of a dozen camera operators, a sound technician, electricians, and more, paid a visit to the Towson enclosure to do some filming. When they were finished, they gave Calhoun a copy of everything they had filmed, providing him details on the private lives of rats that he otherwise would not have had. Though the army movie was about rats as pests and disease vectors, after spending some time with Calhoun’s rats, the director decided to have the narrator open with “the rat is a social animal” and then to close with a denouement of “the life of this so-called lower mammal is quite complex.”

Reprinted with permission from Dr. Calhoun’s Mousery: The Strange Tale of a Celebrated Scientist, a Rodent Dystopia, and the Future of Humanity by Lee Alan Dugatkin, published by the University of Chicago Press. © 2024 by Lee Alan Dugatkin. All rights reserved.

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