Beyond carrying capacity: A system dynamics reinterpretation of Calhoun’s Universe 25 experiment and its relevance to managed animal populations

Calhoun’s Universe 25 experiment remains a significant and contested case study of population growth, crowding, social disruption and reproductive decline in a protected mouse population. Although commonly interpreted as evidence of physical overpopulation, its central implication may instead lie in the distinction between physical and social carrying capacity. This paper develops a System Dynamics reinterpretation of Universe 25 to formalise the feedback mechanisms linking population growth, social-role saturation, behavioural disruption, maternal failure, reduced births, ageing and population decline. The model represents the population through age- and behaviour-structured stocks, including pups, immature mice, competent adults, displaced adults, withdrawn adults and senescent adults, together with a stock of social integrity. Flows describe births, weaning, maturation, displacement, withdrawal, ageing, mortality and the deterioration of social organisation. The model is presented as a formal dynamic hypothesis that enables exploratory sensitivity analysis of mechanisms implicit in the original experiment, including social niche capacity, contact pressure, maternal-care degradation, reproductive suppression, reduced mortality and the absence of dispersal. By separating resource adequacy from social organisation, the model offers a framework for assessing which mechanisms are sufficient to reproduce the observed transition from rapid growth to stagnation, reproductive decline and extinction. The analysis also clarifies the limits of direct extrapolation from Universe 25 while identifying its potential relevance for managed animal populations, including livestock systems, captive breeding, conservation enclosures and wildlife populations. The paper argues that Universe 25 can be interpreted as a possible case of endogenous social regulation failure under abundant resources, restricted dispersal and cumulative behavioural disruption.