Been going through some articles and Youtube videos on the Mouse Utopia Experiment by John Calhorn. Unfortunately the paper is behind a paywall, but there's a decent article.

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People mistake the experiments for being solely about "overpopulation", but Calhorn explains in an interview that the problem isn't necessarily overpopulation and the issue is far more complicated than that. There are a lot of possible outcomes of human "overpopulation" and this experiment isn't necessarily reflective of humans (I'm not interested in the topic of overpopulation.)

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The idea behind the experiment was to give mice everything they needed (food, water and clean nesting space) in a clean and disease free environment. There were enough food feeders to support >9000, enough water dispensers for >6000 and enough nesting space for >3800.

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The population peeked at 2200 after two years and went into decline until all the mice died four years later. During the final months of the decline there 22 male mice and 100 female mice. The male mice simply refused to breed. These special male mice that spent all their time eating, sleeping, drinking and grooming were deemed the "Beautiful ones", because their perfect physical appearance.

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Brief summary of the experiment (1968 - 1974)

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Considering the time required to eat and drink, access to food would not have been a limiting variable until a population of 9500 was reached, or 6144 for water. Considering that there were 256 nest retreat sites in the 16 cells one would not expect shelter to be a limiting factor until the population exceeded 3840. Due to the tendency of many animals to choose to crowd together in numbers in excess of 15 per nest site, at the peak population size of 2200 mice, 20% of all nest sites were usually unoccupied. Thus there was always opportunity for females to select an unoccupied space for rearing young if they so chose.

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Beginning at Day 315 after colonization and continuing for 245 more days, the population grew at a much slower rate, doubling only every 145 days rather than each 55 days as in Phase B.

Population increase abruptly ceased on Day 560 after colonization. A few mice born up until Day 600 survived past weaning. Between these times deaths just slightly exceeded births. Beyond the time of the last surviving birth on Day 600 the incidence of pregnancies declined very rapidly with no young surviving. Last conception was about Day 920.

Male counterparts to these non-reproducing females we soon dubbed the 'beautiful ones'. They never engaged in sexual approaches toward females, and they never engaged in fighting, and so they had no wound or scar tissue. Thus their pelage remained in excellent condition. Their behavioural repertoire became largely confined to eating, drinking, sleeping and grooming, none of which carried any social implications beyond that represented by contiguity of bodies.

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Someone commented on a Youtube video with

Mice Going Their Own Way

It's hilarious because he was right. The "alpha" mice were reproducing (more in the article and videos), while the "beta" mice were pushed out of territories and began assaulting each other. The "beautiful ones" checked out. They were mice groomed like cats, checked out, while everything around them was collapsing. Beautiful.