Female house mice (Mus domesticus) are known to perform communal breeding more often with
kin than with non-kin partners. When mice are grouped in semi-natural enclosures, related females develop prereproductive alliances more frequently than unrelated ones. However, little is known about the behavioural
mechanisms and the factors promoting kin preferential cooperative associations in female mice. Here we evaluate the relative importance of familiarity and age disparity on the pre-reproductive development of agonistic behaviour and spatial associations within groups of three related females, freely interacting in semi-natural indoor enclosures. We found that familiarity clearly promoted female alliance formation by reducing aggression and enhancing spatial cohabitation, while genetic relatedness per se did not alleviate the effects of unfamiliarity on female grouping. Older sisters clearly dominated younger ones, even if they had lower body weights, or if they had to confront two allied littermate sisters. Hence, our findings corroborate previous findings on familiarity as a proxy for kin-preferential alliance formation in female mice. Moreover, we observed a strong impact of age stratification on female aggression-mediated dominance development. We
suggest that this age effect could be expected from a queuing-for-reproduction view of younger house mouse females.