Crowding into the Behavioral Sink
Calhoun, J. B. (1962). Population density and social pathology. Scientific American, 206(3), 139-148.

The effects of crowding on our behavior is something that has interested psychologists for decades. You have probably noticed how your emotions and behavior change when you are in a situation that you perceive as very crowded. You may withdraw into yourself and try to become invisible; you might look for an escape; or you may find yourself becoming irritable and aggressive. How you react to crowding depends on many factors.
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One way behavioral scientists can study the effects of density and crowding on people is to observe places where crowding already exists, such as Manhattan, Mexico City, some housing projects, prisons, and so on. The problem with this method is that all these places contain many factors that can influence behavior. For example, if we find high crime rates in a crowded inner-city neighborhood, there's no way to know for sure that crowding is the cause of the crime. Maybe it's the fact that people there are poor, or that there's a higher rate of drug abuse, or perhaps all these factors combine with crowded conditions to produce the high crime rates.
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Calhoun especially wanted to explore the effects of high density on social behavior. It may seem strange to you to think of rats as social animals, but they do socialize in various ways in their natural environment. To appreciate what led Calhoun to the study being discussed in this chapter, it is necessary to back up several years to an earlier project he conducted. Calhoun had confined a population of rats to a quarter-acre of enclosed, protected outdoor space. Plenty of food was available; there were ideal protected nesting areas; there were no predators; and all disease was kept to a minimum. In other words, this was a rat's paradise. The point of Calhoun's early study was simply to study the population growth rate of the rats in a setting free from the usual natural controls on overpopulation (predators, disease, etc.). After 27 months, the population consisted of only 150 adult rats. This was very surprising since with the low mortality rate of adult rats in this ideal setting, and considering the usual rate of reproduction, there should have been 5,000 adults in this period of time! The reason for this small population was an extremely high infant mortality rate. Apparently, reproductive and maternal behavior had been severely altered by the stress of social interaction among the 150 rats, and very few young rats survived to reach adulthood. Even though this number of rats (150 in a quarter-acre) does not seem to be particularly dense, it was obviously crowded enough to produce extreme behavioral changes. These findings prompted Calhoun to design a more controlled and observable situation inside the lab in order to study more closely what sorts of changes occur in the rats when they are faced with high population density. In other words, he had observed what happened, and now he wanted to find out why.
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RESULTS
It is important to keep in mind that the density of the rats was not extreme; in fact, it was quite moderate. If the rats wanted to spread out, there would only have to be 20 or so per pen. But this is not what happened. When the male rats reached maturity, they began to fight with each other for social status as they do naturally. These fights took place in all the pens, but the outcome was not the same for all of them. If you think about the arrangement of the room, the two end-pens only had one way in and out. So when a rat won a battle for dominance in one of these pens, he could hold his position and territory (the whole pen) simply by guarding the entrance and attacking any other male that ventured over the ramp. As it turned out, only one male rat ended up in charge of each of the end-pens. However, he was not alone. The female rats distributed themselves more or less equally over all four pens. Therefore, the masters of pens 1 and 4 each had a harem of 8 to 12 females all to themselves. And they didn't take any chances. In order to prevent infiltration, the males took to sleeping directly at the foot of the ramp and were always on guard.

On occasion, there were a few other male rats in the end-pens, but they were extremely submissive. They spent most of their time in the nesting burrows with the females and only came out to feed. They did not attempt to mate with the females. The females in these pens functioned well as mothers. They built comfortable nests and nurtured and protected their offspring. In other words, life for most of the rats in these end-pens was relatively normal and reproductive behavior was successful. About half of the infant rats in those pens survived to adulthood.

The rest of the 60 or so rats crowded into the middle two pens. Since these two pens each had central feeding and watering devices, there were many opportunities for the rats to come in contact with each other. The kinds of behaviors observed among the rats in pens 2 and 3 demonstrate a phenomenon that Calhoun termed the behavioral sink. A behavioral sink is "the outcome of any behavioral process that collects animals together in unusually great numbers. 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" (p. 144). Let's examine some of the extreme and pathological behaviors he observed:

1. Aggression. Normally in the wild, male rats will fight other male rats for dominant positions in the social hierarchy. These fights were observed among the more aggressive rats in this study as well. The difference was that here, unlike in their natural environments, top-ranking males were required to fight frequently in order to maintain their positions and often the fights involved several rats in a general brawl. Nevertheless, the strongest males were observed to be the most normal within the center pens. However, even those animals would sometimes exhibit "signs of pathology; going berserk; attacking females, juveniles, and less active males; and showing a particular predilection—which rats do not normally display—for biting other rats on the tail" (p. 146).

2. Submissiveness. Contrary to this extreme aggression, other groups of male rats ignored and avoided battles for dominance. One of these groups consisted of the most healthy-looking rats in the pens. They were fat and their fur was full, without the usual bare spots from fighting. However, these rats were complete social misfits. They moved through the pens as if asleep or in some sort of hypnotic trance, ignoring all others, and were, in turn, ignored by the rest. They were completely uninterested in sexual activity and made no advances, even toward females in heat. Another group of rats engaged in extreme activity and were always on the prowl for receptive females. Calhoun termed them probers. Often, they were attacked by the more dominant males, but were never interested in fighting for status. They were hypersexual and many of them even became cannibalistic!

3. Sexual deviance. These probers also refused to participate in the natural rituals of mating. Normally, a male rat will pursue a female in heat until she escapes into her burrow. Then, the male will wait patiently and even perform a courtship dance directly outside her door. Finally, she emerges from the burrow and the mating takes place. In Calhoun's study, this ritual was adhered to by most of the sexually active males except the probers. They completely refused to wait and followed the female right into her burrow. Sometimes the nests inside the burrow contained young that had failed to survive, and it was here that late in the study the probers turned cannibalistic. Another group of male rats was termed the pansexuals because they attempted to mate with any and all other rats indiscriminately. They sexually approached other males, juveniles, and females that were not in heat. This was a submissive group that was often attacked by the more dominant male rats, but did not fight for dominance.

4. Reproductive abnormalities. Rats have a natural instinct for nest building. In this study, small strips of paper were provided in unlimited quantities as nest material. The females are normally extremely active in the process of building nests as the time for giving birth approaches. They gather the material and pile it up so that it forms a cushion. Then they arrange the nest so that it has a small indentation in the middle to hold the young. However, the females in the behavioral sink gradually lost their ability (or inclination) to build adequate nests. At first they failed to form the indentation in the middle. Then, as time went on, they collected fewer and fewer strips of paper so that eventually the infants were born directly on the sawdust that covered the pen's floor.

The mother rats also lost their maternal ability to transport their young from one place to another if they felt the presence of danger. They would move some of the litter and forget the rest, or simply drop them onto the floor as they were moving them. Usually these infants were abandoned and died where they were dropped. They were then eaten by the adults. The infant mortality rate in the middle pens was extremely high, ranging from 80% to 96%.

In addition to these maternal deficits, the female rats in the middle pens, when in heat, were chased by large groups of males until they were finally unable to escape. These females experienced high rates of complications in pregnancy and delivery. By the end of the study, almost half of them had died.
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http://www.edmondschools.net/Portals/3/d...AD-Crowding.pdf

Interesting stuff.

It's not hard to see the effects in certain environments.