Reproductive skew, costs, and benefits of cooperative breeding in female wood mice (Apodemus sylvaticus)

Two current models seek to explain reproduction of subordinates in social groups: incentives given by dominants for peacefully remaining in the group (reproductive skew model) or incomplete control by dominants. These models make different predictions concerning genetic relatedness between individuals for the distribution of reproduction and the stability of cooperative breeding associations. To test these models and to further explore the relationships between reproductive skew, genetic relatedness, and investment of each participant, we performed behavioral observations of female wood mice (Apodemus sylvaticus) raising pups communally. Our results do not support previous models. Differences in lifetime reproductive success were significantly greater within mother-daughter pairs than within pairs of sisters or unrelated females. Subordinate females of either breeding unit did not differ in their direct reproduction. Calculations of inclusive fitness based on our results lead to the following predictions: (1) Communal nests should occur only when ecological circumstances prevent solitary breeding. (2) Subordinate females gain the highest inclusive fitness joining their mothers; they also show the highest nursing investment. (3) Mothers should accept daughters, who have no opportunity for solitary breeding. (4) Dominant sisters and unrelated females should reject subordinate females because cooperative breeding reduces their reproductive success. However, breeding units of dominant sisters and unrelated females nevertheless occur and can be explained by our finding that such females significantly reduce nursing time, which may help them save energy for future breeding cycles. Our data demonstrate that both genetic relatedness and investment skew are important in the complex evolution of reproductive skew in cooperative breeding.