This led me down a Wikipedia hole where I learned about John B. Calhoun’s experiments with mice - https://en.m.wikipedia.org/wiki/John_B._Calhoun#Mouse_experi... Very interesting - once the “mouse universe” reached a high population density, it descended into violence and chaos, which was succeeded by massive behavioral changes across the entire population - male mice became introverted, ceased fighting and attempting to reproduce, and simply ate, drank, slept and groomed themselves until the population collapsed.