The Behavioural Sink

...all animals, including birds, seem to have a built-in, inherited requirement to have a certain amount of territory, space, to lead their lives in. Even if they have all the food they need, and there are no predatory animals threatening them, they cannot tolerate overcrowding beyond a certain point. No more than two hundred wild Norway rats can survive on a quarter acre of ground, for example, even when they are given all the food they can eat. They just die off.

But why? To find out, ethologists have run experiments on all sorts of animals, from stickleback crabs to Sika deer. In one major experiement, an ethologist named John Calhoun put some domesticated white Norway rats in a pen with four sections to it, connected by ramps. Calhoun knew from previous experiments that the rats tend to split up into groups of ten to twelve and that the pen, therefore, would hold forty to forty eight rats comfortably, assuming they formed four equal groups. He allowed them to reproduce until there were eighty rats, balanced between male and female, but did not let it get any more crowded. He kept them supplied with plenty of food, water and nesting materials. To the human eye the pen did not even look especially crowded, but to the rats, it was crowded beyond endurance.

The entire colony was soon pluged into a profound behavioural sink. "The sink." said Calhoun, "is the outcome of any behavioural process that collects animals together in unusually great numbers. The unhealthy connotations of the term are not accidental; a behavioural sink does act to aggravate all forms of pathology that can be found within a group."

For a start, long before the rat population reached eighty, a status hierarchy had developed in the pen. Two dominant male rats took over the two end sections, acquired harems of eight to ten females each, and forced the rest of the rats into two middle pens. All the overcrowding took place in the middle pens. That was where the 'sink' hit. The aristocratic rats at the ends grew bigger, sleeker, healthier and more secure all the time.

In the 'sink' meanwhile, nest building, courting, sex behaviour, reproduction, social organisation, health, all of it went to pieces...

No more than three males - the dominant males in the 'sink' - kept up the old customs. The rest tried everything from satyrism to homosexuality or else gave up on sex altogether. Three or four might chase one female at the same time and...

Homosexuality rose sharply. So did bi-sexuality. Some males would mount anything, males, females, babies, senescent rats, anything...

Females in the 'sink' were ravaged physically and psychologically. Pregnant rats had trouble continuing a pregnancy. The rate of miscarriages increased significantly and females started dying from tumours and other disorders of the mammary glands, sex organs, uterus, ovaries and Fallopian tubes.

Child rearing became totally disorganised. The females lost the interest or the stamina to build nests and did not keep them up if they did build them. In the general filth and confusion they would not put themselves out to save offspring they were momentarily separated from. Frantic, even sadistic competitionn was going on all round them and rendering their lives chaotic.

from The Mid-Atlantic Man by Tom Wolfe pp 297 - 300
(condensed somewhat by me.)

Get the book and read the rest yourself.