On Calhoun's "Mouse Utopia" studies on overcrowding:

"Dominant males became aggressive, some moving in groups,

attacking females and the young. Mating behaviors were disrupted.

Some became exclusively homosexual. Others became pansexual and

hypersexual, attempting to mount any rat they encountered. Mothers

neglected their infants, first failing to construct proper nests, and then

carelessly abandoning and even attacking their pups. In certain sections of the pens, infant mortality rose as high as 96%, the dead cannibalized by adults. Subordinate animals withdrew psychologically, surviving in a physical sense but at an immense psychological cost. They were the majority in the late phases of growth, existing as a vacant, huddled mass in the centre of the pens. Unable to breed, the population plummeted and did not recover. The crowded rodents had lost the ability to co-exist harmoniously, even after the population numbers once again fell to low levels. At a certain density, they had ceased to act like rats and mice, and the change was permanent."

http://eprints.lse.ac.uk/22514/1/2308Ramadams.pdf

( 59pg download )

I recommend becoming familiar with Calhoun's work.

We are in the fight against our own "behavioral sink."