Text version: http://www.todayifoundout.com/index.php/2018/12/that-time-a-guy-tried-to-build-a-utopia-for-mice-and-it-all-went-to-hell/
"In 1968, an expert on animal behaviour and population control called John B. Calhoun built what was essentially a utopia for mice that was purpose built to satisfy their every need. Despite going out of his way to ensure the inhabitants of his perfect mouse society never wanted for anything, within 2 years virtually the entire population was dead. So what happened?"
Possible explanation: After a certain point of population growth the mice cannot keep track of all the individuals in their group (i.e. in the enclosure) and in nature this would usually result in a non-violent split of the population into two groups that go their own way. But with nowhere to go the mice created smaller subgroups that couldn't go anywhere yet still tried to maintain their own territory, mates etc. which led to conflict.
This would be analogous to humans having the mental capacity to keep up relations with something like 150 to 300 people. If primitive hunter gatherers grew beyond that limit it would result in the exodus of some part of that population into a new group that went elsewhere, probably mainly to retain group cohesion but in part also because the resources needed for survival at any one place were limited. The main reasons why we have not descended into intra-tribe violence once we started to live in permanent settlements and larger villages are probably social innovations as well as recognizing and sharing the benefits of larger scale village structures. Both these things seem to be beyond mouse brains.
Just brainstorming here.
It doesn't really explain what happened though?
It kind of does, the mice stopped breeding and started attacking each other randomly.
No one really knows. Calhoun did a bad job writing it up. When I went looking for details years back, I think the most in-depth primary source I found was like... 2 pages long.
I've wondered if what actually happened was just a contagious infection (sterility is a consequence of many infectious diseases), which given the population density should be near-inevitable. Even if he had checked for infections (it's unclear if he did) it would be easy to miss a lot of organisms, which is why we still regularly run into new evidence of infectious contributions to various problems.
He also ran a number of mouse utopias, IIRC, and usually you hear only about the one which gave the 'collapse' narrative.... At this point, I assign it to the mental bucket of 'wrong 1960s blankslatism like Rosenthal or Pygmalion effect or the Stanford Prison Experiment or Robber's Cave but which will live on forever in pop science because their message is too appealing'.
I've summarized my problems with Mouse Utopia: https://www.gwern.net/Questions#mouse-utopia