Crowded Rat Syndrome

Magpie's picture

In an earlier thread, I mentioned that I felt that there is increasing factionalization of young men and women into separate, isolated, social enclaves. I have been doing some reading of older scientific literature and thought I might share some of my findings, which are relevant to the above observation.

John B. Calhoun was a man concerned with the increasing human population, and ran a number of experiments from the 1940's through the 1970's, looking at the effect of overpopulation on mice and rats. His most famous experiment was published in 1972--the results of his so called "Universe 25". The set up was sort of a rat apartment block, with256 rooms capable of housing 15 mice each, stacked high and surrounding a courtyard below. Unlimited food and water were provided, so the typical effects of starvation etc were taken out of play--the only limiting factor was space.

The experiment was started with a mere four breeding pairs of mice. After an adjustment period, the mice began to breed--and in those early 'golden days', the mice had numerous litters and feasted on the unlimited food provided them. However, with the population doubling every 55 days, things began to become crowded. After day 315, population growth slowed (with over 600 mice at this time). Because of the overcrowded conditions, normal mouse behavior started to break down--male mice had increasing difficulty defending their territories, reducing their success in attracting females, and also leaving pregnant and nursing females open to attack from marauders. Some frustrated female mice began killing or abandoning their young, or giving up on sexual activity altogether.

A significant proportion of females ended up holing up in the highest mouse apartments, alone. And two new castes of males developed. The first were the "losers", listless bachelors who would huddle together in the courtyard, occasionally erupting in bursts of violence. A small segment of these showed unusually high creativity and innovation (and inspired Mrs. Frisby and the Rats of NIMH). The second group were what Calhoun called the "beautiful ones"--males that spent all of their time grooming and eating, never seeking female attention. The remaining mice became increasingly pansexual, violent, and cannibalistic.

This was the beginning of the end. By day 560, the mouse population peaked at 2,200 mice. Fewer and fewer females gave birth, and few pups survived through to weaning. Mouse society had become segregated into uninterested females and the two types of uninterested males, while the reproductive segment of the population had all but disappeared. The population fell, and fell, but even with less crowded conditions, the mice did not resume normal mouse behaviors.

The breakdown of mouse and rat society was highly reproducible: all of Calhoun's "rodent utopias" became rodent hells. Calhoun strongly believed that this was the path humanity would be heading down, and I think that some current social phenomena reflect his findings.

Also, have you heard of Bruce Alexander's rat park experiments? Those are worth a look too.

This is fascinating! I must express some distaste at the methods--although can I really claim that he was mistreating the rats by providing them with utopian conditions?--but the results are worth some thought.

Fascinating as well are the reactions people have. I did a bit of searching around, and a handful of people snicker and say, "Ha! That's why society sucks," (A surprising number even claim to be "beautiful ones" themselves), but most people seem to immediately jump to something like, "That couldn't relate to humans because humans have choice!" This is rather strange since the mice didn't lack choice. Quite the opposite, in fact, what the study noted was that mice in these conditions made extremely strange and destructive choices that were in no way forced by the scientists or even by other mice. Not one person in my quick survey of blog posts and youtube comments seemed noted the ecological dimension. The experiment has straightforward ecological implications: Mouse behavior is influenced by population density and relies on the ubiquitous presence of disease, predation, and limited resources for proper regulation.

Ecologically and evolutionary, it couldn't be otherwise. Mice that are best suited for the world they actually live in, complete with disease and limited resources, are going to reproduce more than mice that would theoretically be better suited to a world that doesn't actually exist. Likewise, any predators or diseases that were TOO effective at reducing mouse population would quickly see their own population crash (see the boom and bust population cycles that rabbits and predators go through in Europe and America) or would evolve to a less virulent form because the more virulent forms kill their hosts before the hosts can reproduce and spread the virus (see the introduction of myxomatosis to rabbit populations in Australia). The real research results then are not IF disease, predation, and limited resources are necessary for proper functioning (although it is nice to see a clear illustration of that), but rather WHAT aspects of behavior are specifically influenced by population density, which understandably seems to be those aspects that are related to mating and intra-species aggression.