Reproductive asynchrony and infanticide in house mice breeding communally

Earlier findings suggest that female house mice, Mus musculus, breeding communally care for each other's offspring indiscriminately in a communal nest. The ultimate explanation for this apparently altruistic behaviour is still not well understood. Communal breeding creates a situation in which deceptive behaviour may be an alternative tactic, possibly coexisting with genuine altruism. To investigate this phenomenon we studied caring behaviour and infanticide as two opposite facets of communal breeding in triplets of unrelated females, and developed a dynamic model to help interpret our results. Of the 142 litters observed, in 30 all pups were killed by adult females and in 37 only some of the litter survived infanticide. Our empirical results are in concordance with our model's prediction and show that asynchrony in reproduction has a strong nonlinear effect on reproductive success: pups of litters born in the middle of the caring period of any female in the group had the lowest expected survival probability. Females that partly or totally lost their litter tended to spend less time caring for pups that were not their own, but they still contributed considerably to the common care. These findings suggest that infanticide is an effective strategy to exploit nestmates. As house mice are unable to discriminate between similarly aged young pups, synchronous breeding (e.g. by oestrus synchronization) may be an effective counterstrategy against infanticidal conspecifics.