My piece on the hellish "rodent universes" of John Calhoun is now online at Cabinet magazine. Researching overcrowding, Calhoun built elaborate "utopias" for rats and mice, in which they enjoyed unlimited resources and freedom from disease or predation - but limited space. Once population density passed a certain point, the mouse heaven always became mouse hell, rife with violence, rape and asocial freakery. This research formed a cornerstone of the overpopulation and anti-urban doom-mongering of the 1970s - and informed fictions from Stand on Zanzibar to Soylent Green.
Here's Calhoun in one of his universes, via Wikimedia: