Why does lifelong conventional housing reduce the sociability of female mice?

Abstract

Compared to peers raised in well-resourced, 'enriched' environments (‘EE’), female laboratory mice from conventional barren cages are more aggressive to their cage-mates, and less sociable with familiar non-cage-mates (especially if these too are from conventional housing, ‘CH’). But how do such effects occur? Using Social Approach Tests, and middle-aged mice whose lifelong behavioural time budgets were known, we re-tested the hypotheses that CH C57/BL6 (‘C57′) subjects paired with non-cage-mate ‘stimulus mice’ would, compared to EE C57 subjects, be less sociable, less socially attractive, and more agonistic to unfamiliar conspecifics. We also assessed whether any effects reflected that subject mice had, over their lifespans, been involved in more agonistic interactions with cage-mates, and/or spent much of their time inactive but awake (‘IBA’ – a depressive-like behaviour) or performing stereotypic behaviour. Results replicated previous findings: CH C57 subjects were less sociable with non cage-mates than were EE C57s (especially when inferred from sniffing conspecifics versus novel objects). Results also revealed that CH subjects’ reduced sociability was best explained by their elevated agonism in their home cages, as well as by how much they were aggressed themselves by their cage-mates. Additional, albeit less robust, roles were also played by displaying lifelong high levels of IBA and SB. Furthermore, although CH subjects were no more aggressive than EE subjects towards stimulus mice, BALB/c (‘BALB’) stimulus mice were much more agonistic to CH subjects than to EE subjects. As additional evidence that mice can discriminate between CH and EE conspecifics, housing also affected BALBs’ social attractiveness: EE BALBs tended to be preferred over CH BALBs by subject mice. We could not ascertain how mice distinguished between differentially housed peers, but these results suggest hitherto unsuspected social abilities for BALBs (typically characterised as ‘anti-social’ when CH): if EE, or faced with EE companions, mice of this strain se mice seem attractive and affiliative. Overall, lifelong poor welfare in barren housing can thus negatively impact social behaviour in middle-aged female mice. Results broadly suggest that the individuals most adversely affected by CH conditions also show the most reduced sociability. ‘Enriched’ cages may therefore enhance welfare, not only by physically enabling natural behaviours, but also by promoting positive social interactions. How mice discriminate between EE and CH conspecifics now needs investigating.