Can live with ‘em, can live without ‘em: Pair housed male C57BL/6J mice show low aggression and increasing sociopositive interactions with age, but can adapt to single housing if separated

Abstract

The basic question as to whether male laboratory mice should be singly or group housed represents a major animal welfare concern within current laboratory animal legislation and husbandry. To better understand the behavioural and physiological mechanisms underlying this issue, we conducted two longitudinal experiments using C57BL/6J mice. In the first experiment (N = 32), we explored social behaviour of pair housed males from weaning to adulthood. We took weekly measures of agonistic, socio-exploratory and affiliative behaviours within two different contexts, i.e. in the undisturbed home cage and immediately after cage cleaning. In the second experiment (N = 36), we investigated whether separation of male pairs into single housing at different ages (35, 56 or 77 days of age) affected welfare-related measures such as faecal corticosterone metabolites (FCMs) and anxiety-like behaviours. In the first experiment we found that levels of agonistic behaviour were higher after cage cleaning than in the undisturbed cage as expected, but did not significantly change with age in either context. Instead, affiliative behaviour increased with age in the undisturbed home cage. In the second experiment, social separation did not affect levels of FCMs or anxiety-like behaviours at any age point. Taken together, this study shows that pair housed male mice can maintain low levels of aggression across a long period of their life and perform increasing levels of sociopositive behaviours which may serve to promote stable social relations. At the same time, our results suggest that male mice can quickly adapt to separation into single housing at different ages, from adolescence to adulthood. These findings are in line with the behavioural ecology of wild male mice, which suggests that both solitary and group living represent two alternative strategies.