Just been reading about Universe 25 and John Calhoun, who conducted the experiment. A 2.6×2.6m cage was constructed, and 34-56 mice placed in it; unlimited food and water was available to the mice, they were protected from all predators, and the cage was divided into sleeping/breeding boxes, feeding, and general sections. The population initially increased exponentially, peaking at around 2200 mice, although the cage was designed to handle up to 4000. At this level of crowding, the mice began to exhibit signs of abnormal, often destructive, behaviours, including refusal to court, and females abandoning their young. Whilst still physically able to reproduce, by the 600th day the mice had lost the social skills needed to form mating pairs. Ultimately the entire mouse population died.

Calhoun published his results in Death Squared: The Explosive Growth and Demise of a Mouse Population, Proc R Soc Med
. 1973 Jan;66(1 Pt 2):80–88. doi: 10.1177/00359157730661P202