Female mice are said to be unable to distinguish own from alien offspring and will indiscriminately nurse each other's young in communal nests. Here, we present results of a split-litter experiment that tested whether offspring growth was affected depending on whether they were nursed by their own or unrelated foster mothers. Pups of reciprocal crosses between C57/B6 and CBA/Ca strains were fostered in mixed litters that consisted half of their natural siblings and half of unrelated littermates of the reciprocal genotype. Analysing the relative growth of the two pup types showed that offspring gained proportionally more weight when nursed by their own mothers than their cross-fostered litter mates during the period from day 15 until weaning, during which maternal provisioning effort contributes to pup weight gain. Before day 15 of the pups' life, however, we found no advantage of being nursed by biological mothers, and we suggest that this may be due to the effects of paternally expressed genes in young that mask their maternal identity, thus favouring indiscriminate nursing of all young in a communal nest. © 2005 Blackwell Verlag.