In the late 1960s, John B. Calhoun placed four breeding pairs of mice into a Mouse Utopia, free of predators and full of food and water.
The population grew exponentially — for a while. For the first 620 days, the population doubled every 55 days. Then, the doubling slowed down. Then, growth stopped, and the dysfunction began.
In the original paper, Death Squared: The Explosive Growth and Demise of a Mouse Population, Calhoun draws these conclusions:
The results obtained in this study should be obtained when customary causes of mortality become markedly reduced in any species of mammal whose members form social groups. Reduction of bodily death (i.e. ‘the second death’) culminates in survival of an excessive number of individuals that have developed the potentiality for occupying the social roles characteristic of the species. Within a few generations all such roles in all physical space available to the species are filled. AT this time, the continuing high survival of many individuals to sexual and behavioural maturity culminates in the presence of many young adults capable of involvement in appropriate species-specific activities. However, there are few opportunities for fulfilling theses potentialities. In seeking such fulfilment they compete for social role occupancy with the older established members of the community. This competition is so severe that it simultaneously leads to the nearly total breakdown of all normal behaviour by both the contestors and the established adults of both sexes. Normal social organization (i.e. ‘the establishment’) breaks down, it ‘dies’.
So far, that sounds almost like Peter Turchin’s notion of elite overproduction.
Young born during such social dissolution are rejected by their mothers and other adult associates. This early failure of social bonding becomes compounded by interruption of action cycles due to the mechanical interference resulting from the high contact rate among individuals living in a high density population. High contact rate further fragments behaviour as a result of the stochastics of social interactions which demand that, in order to maximize gratification from social interaction, intensity and duration of social interaction must be reduced in proportion to the degree that the group size exceeds the optimum. Autistic-like creatures, capable only of the most simple behaviours compatible with physiological survival, emerge out of this process. Their spirit has died (‘the first death’). They are no longer capable of executing the more complex behaviours compatible with species survival. The species in such settings die.
You may be wondering about his references to the first death and the second death. They’re Biblical references — with which he opens his scientific paper — in the Proceedings of the Royal Society of Medicine in 1973:
I shall largely speak of mice, but my thoughts are on man, on healing, on life and its evolution. Threatening life and evolution are the two deaths, death of the spirit and death of the body. Evolution, in terms of ancient wisdom, is the acquisition of access to the tree of life. This takes us back to the white first horse of the Apocalypse which with its rider set out to conquer the forces that threaten the spirit with death. Further in Revelation (ii.7) we note: ‘To him who conquers I will grant to eat the tree of life, which is in the paradise of God’ and further on (Rev. xxii.2): ‘The leaves of the tree were for the healing of nations.’
Anyway, the Mouse Utopia experiment is usually interpreted in terms of social stresses related to overcrowding, but, Bruce Charlton points out, there’s another explanation:
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 crowing, 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.