Maternal and offspring effects influence provisioning to mixed litters of own and alien young in mice

In communally nesting species such as mice, nursing females may be confronted with alien and own young in mixed litters, and offspring may be fostered by either own or alien mothers. Because of the costly nature of maternal investment and offspring solicitation behaviour, maternal provisioning is expected to favour own young. While previous studies have investigated kin recognition between mother and pups with inconclusive results, it remains unexplored whether any such ability yields a fitness advantage for own pups when nursed in a mixed litter. Moreover, nursing females may have evolved strategies to effect a reduction in fitness costs associated with nursing alien pups, for example, reducing their maternal effort. In this study, we show that young mice procure an advantage by gaining relatively more weight than alien pups when nursed together in a mixed litter by their own mothers. However, we did not detect an effect of offspring or maternal genotype on this difference in weight gain. Furthermore, we show that mothers provide overall fewer resources to mixed litters than to unmixed litters and that two mouse strains differ in the magnitude of this reduction. We suggest that a combination of sibling competition and maternal strategies to reduce costs of nursing unrelated young may be the underlying causes of these findings. © 2007 The Association for the Study of Animal Behaviour.