Social hierarchies emerge when animals compete for access to resources such as food, mates or physical space. Wild and laboratory male mice have been shown to develop linear hierarchies, however, less is known regarding whether female mice have sufficient intrasexual competition to establish significant social dominance relationships. In this study, we examined whether groups of outbred CD-1 virgin female mice housed in a large vivaria formed social hierarchies. We show that females use fighting, chasing and mounting behaviors to rapidly establish highly directionally consistent social relationships. Notably, these female hierarchies are less linear, steep and despotic compared to male hierarchies. Female estrus state was not found to have a significant effect on aggressive behavior, though dominant females had elongated estrus cycles (due to increased time in estrus) compared to subordinate females. Plasma estradiol levels were equivalent between dominant and subordinate females. Subordinate females had significantly higher levels of basal corticosterone compared to dominant females. Analyses of gene expression in the ventromedial hypothalamus indicated that subordinate females have elevated ERα, ERβ and OTR mRNA compared to dominant females. This study provides a methodological framework for the study of the neuroendocrine basis of female social aggression and dominance in laboratory mice.

14Social hierarchies emerge when animals compete for access to resources such as food, mates or physical 15 space. Wild and laboratory male mice have been shown to develop linear hierarchies, however, less is 16 known regarding whether female mice have sufficient intrasexual competition to establish significant 17 social dominance relationships. In this study, we examined whether groups of outbred CD-1 virgin female 18 mice housed in a large vivaria formed social hierarchies. We show that females use fighting, chasing and 19 mounting behaviors to rapidly establish highly directionally consistent social relationships. Notably, these 20 female hierarchies are less linear, steep and despotic compared to male hierarchies. Female estrus state 21was not found to have a significant effect on aggressive behavior, though dominant females had elongated 22 estrus cycles (due to increased time in estrus) compared to subordinate females. Plasma estradiol levels 23were equivalent between dominant and subordinate females. Subordinate females had significantly 24 lower levels of basal corticosterone compared to dominant females. Analyses of gene expression in the 25 ventromedial hypothalamus indicated that subordinate females have elevated ERα, ERβ and OTR mRNA 26 compared to dominant females. This study provides a methodological framework for the study of the 27 neuroendocrine basis of female social aggression and dominance in laboratory mice. 28

No abstract

No abstract