See also: The Calhoun Mouse Experiments.
https://cosmosmagazine.com/science/mathematics/calhoun-roden...
Faced with population pressures, a lot of rats just gave up or altered their behavior.
> Among the aberrations in behavior were the following: expulsion of young before weaning was complete, wounding of young, increase in homosexual behavior, inability of dominant males to maintain the defense of their territory and females, aggressive behavior of females, passivity of non-dominant males with increased attacks on each other which were not defended against
> After day 600, the social breakdown continued and the population declined toward extinction. During this period females ceased to reproduce. Their male counterparts withdrew completely, never engaging in courtship or fighting and only engaging in tasks that were essential to their health. They ate, drank, slept, and groomed themselves – all solitary pursuits. Sleek, healthy coats and an absence of scars characterized these males. They were dubbed "the beautiful ones". Breeding never resumed and behavior patterns were permanently changed.
https://en.wikipedia.org/wiki/John_B._Calhoun
This was also just a social structure; those rats might not have been exposed to high levels endocrine disrupting plastics, lead, obesity, etc., that modern humans are. But that mouse behavior looks an awful lot like what we're seeing these days in online incel, or trans, communities, et al.