MATERNAL INVESTMENT OF COMMUNALLY NURSING FEMALE HOUSE MICE (MUS-MUSCULUS-DOMESTICUS)

Female house mice (Mus musculus domesticus) belonging to the same polygynous breeding unit rear their litters communally. This paper analyzes the consequences of communal nursing on the number and weight of offspring produced. The reproductive behaviour of monogamous females was compared to that of females living in polygynous groups, consisting of either two familiar sisters or of two genetically unrelated females (unfamiliar until the age of 7 weeks). The lifetime reproductive success of a female was measured as the total number of offspring weaned during a lifespan of 6 months (standardized as 120 days after mating at the age of 8 weeks). The number of litters produced and average litter size per female did not differ significantly between the 3 groups of females. However, females communally nursing with a sister weaned significantly more young than a monogamously paired female. Those sharing a nest with an unrelated female weaned an intermediate number of offspring. Sharing a communal nest with a sister significantly increased the total mass of offspring weaned in comparison with both monogamous females and unrelated females. Furthermore, communal nests improved the birth weight of the young when compared with young in solitary nests. By cooperative brood care and communal nursing, females improved the weight of the young produced. By having a sister for cooperation, a female further improved the number of offspring weaned when compared with a female rearing litters in solitary nests, as well as the combined weight of offspring weaned compared with all other females studied.