There is a very interesting and famous rat experiment performed by
Dr. Calhoun in 1958 on overcrowding theory.
In a barn in Rockville, Maryland - he designed a series of experiments in which rat populations could build up freely under conditions that would permit detailed observations without humans influencing the behavior of the rats relating to each other. Calhoun built four pens, each with all the accoutrements for normal rat life and divided by electrified partitions. Initially, eight infant rats were placed in each pen, and when they reached maturity, Calhoun installed ramps between each pen. At that point, the experiment took its own course in terms of the
effects of population growth in a limited area.
Normally, rats have a fairly simple form of social organization, characterized by groups of 10-12 hierarchically ranked rats defending their common territory. There is usually one male dominating the group, and status is indicated by the amount of territory open to an individual. As Calhoun's rat population grew from the original 32 to 60, one dominant male took over each of the two end pens and established harems of 8-10 females. The remaining rats were congregated in the two middle pens, where problems developed over congestion at the feeding hoppers. As the population grew from 60 to 80, behavior patterns developed into what Calhoun called a "behavioral sink" - gross distortions of behavior resulting from animal crowding. Behavior remained fairly normal in the two end pens, where each dominant male defended his territory by sleeping at the end of the ramp, but in the two middle pens there were severe changes in sexual, nesting, and territorial behavior. Some of the males became sexually passive; others became sexually hyperactive, chasing females mercilessly; and still another group of males was observed mounting other males, as well as females. Females became disorganized in their nesting habits, building very poor nests, getting litters mixed up, and losing track of their young. Infant mortality rose significantly. Finally, males appeared to alter their concept of territoriality, and three times a day, the males fought at the eating bin.
Conclusion - Calhoun's study can be summarized by noting that, among his rats, crowding (an increase in the number of rats within a fixed amount of space) led to the disruption of important social functions and to social disornigazation. Releated to these changes in social behavior were signs of physiological stress, such as changes in their hormonal systems that made it difficult for females to bring pregnancies to term and care for their young.
There are 3 more pages talking about overcrowding and behavioral change. Very fascinating.... This was my Urban Study class. The textbook's called
Population - An Introduction to Concepts and Issues - EXCELLENT book to read. I have read cover-to-cover twice and the professor for this class is very brilliant.
