Little is known about the behavioural mechanisms facilitating kin-preferential communal breeding in wild house mice (Mus domesticus). We evaluated the effect of kinship and male availability on aggression,
social structure and reproductive skew in groups of female mice freely interacting and reproducing in seminatural indoor enclosures. Triplets of either sisters or nonsisters were established in enclosures provided with either one or three littermate males, which were unrelated and unfamiliar to the females. Sisters were more spatially associated and less aggressive than non-sisters, leading to
higher incidences of communal breeding and reproduction. This is in agreement with theoretical considerations on kin selection in house mice. Reproductive success was highly skewed in favour of dominant females due to subordinate infertility or complete loss of first litters, which might have been caused by dominant females. In spite of this, subordinates only rarely dispersed from the
enclosures, suggesting that perceived dispersal risk generally outweighed relatively reduced reproductive potentials.
Aggression levels among females were significantly higher when one male was available, compared to when three males were available. We suggest that this might result from higher female-female competition for mates, due to the risk of missing fertilisation when synchronously oestrous females encounter limited numbers of males in a deme. Our results indicate that, first, communal nursing in house mice might have evolved to ‘make the best out of a bad job’ rather than to enhance offspring fitness; and, second, that female-female matecompetition might play an important role in shaping female social structure in this polygynous mammal.