The so-called Mouse Utopia experiment was conducted from 1968 by John B Calhoun
The idea was that four breeding pairs of mice were allowed to reproduce freely in a 'utopian' environment with ample food and water; no predators; no disease; comfortable temperature, conditions and space. What happened is described by the author:
Phase A - 104 days - establishment of the mice in their new environment, then the first litters were born.
Phase B - up to day 315 - exponential population growth doubling every 55 days.
Phase C - from day 315-560 population growth abruptly slowed to a doubling time of 145 days.
Phase D - days 560-920; population stagnant with births just matching deaths. Emergence of many pathological behaviours.
Terminal Phase - population declining to zero. The last conception was about day 920, after which there were no more births, all females were menopausal, the colony aged and all of them died.
The Mouse Utopia experiment is usually interpreted in terms of social stresses related to 'over-population' crowding - generating pathological behaviours and a loss of the will to live.
But Michael A Woodley suggests that what might be going on is mutation accumulation, and deleterious genes generating a wide range of maladaptive pathologies, incrementally accumulating with each generation; and rapidly overwhelming and destroying the population before any beneficial mutations could emerge to 'save; the colony from extinction.
So the bizarre behaviours seen especially in Phase D - such as the male 'beautiful ones' who appeared to be healthy and spent all their time self grooming, but were actually inert, unresponsive, unintelligent, uninterested in reproduction - are not adaptations to crowding, but maladaptive outcomes of a population sinking under the weight of mutations.
The reason why mouse utopia might produce so rapid and extreme a mutation accumulation is that wild mice naturally suffer very high mortality rates from predation.
Therefore, because wild mice are so short-lived, mice are not 'built to last' and have the reputation of being unusually-prone to produce new deleterious mutations (and are therefore extremely prone to cancer, and susceptible to carcinogens - which is why mice are used to test for carcinogens).
Thus mutation selection balance is in operation among wild mice, with very high mortality rates continually weeding-out the new mutations (especially among males) - with typically only a small and relatively mutation-free proportion of the (large numbers of)
offspring surviving to reproduce; and a minority of the most active and healthy (mutation free) males
siring the bulk of each generation.
However, in Mouse Utopia, there is no predation and all the other causes
of mortality are reduced to a minimum - so the frequent mutations just accumulate,
generation upon generation - randomly producing all sorts of pathological (maladaptive) behaviours.
To test whether mutation accumulation is the real explanation for the demise of Mouse Utopia, the original experiment should be repeated but with genetic controls. Woodley is hoping to do this himself.
Also, a variant experiment could perhaps be conducted, which maintained utopian conditions but without allowing overcrowding (e.g. by continually splitting-up the growing community, and creating more and more small colonies - or (see comment below) by random culling).
In other words, the social conditions of Utopian mice would be held constant, while mortality rates would be kept low for multiple generations.
My prediction would be that the Mouse Utopians would go through phases A, B, C, D and terminal to become extinct even without increased population density/ overcrowding, and due purely to cumulative genetic damage.