Effect of strange male odour on parental care in lactating female mice

This paper analyses the behaviour of lactating female outbred mice,Mus musculus domesticusin the presence of male conspecific odours. When olfactory cues were left in the environment by a sexually naive adult male, a potentially infanticidal animal, the mother took longer to reach her litter following 30 min of separation. Odours left by the sexual partner, by an unknown male of parental status, or by a young naive male did not modify the mother's behaviour, compared with the control situation (absence of male odour). The number of ultrasonic calls of pups varied according to the characteristics of the male but did not modify the behaviour of the dam. Females took longer to reach pups on day 8 of lactation than on days 4 or 12. We suggest adaptive reasons why females take longer to reach pups when the situation is more risky. We tested the hypothesis that the loss of an 8-day-old litter is more expensive, in term of the mother's future reproductive success, than the loss of younger and older litters. Females conceived a new litter within a few days (the inter-birth interval varied according to the age of the litter previously removed) but, even though no difference in size and weight of litters was recorded, females that had the litter removed on day 8 postpartum (compared with days 4 and 12) suffered from a higher mortality rate in the next litter. We suggest that the time the mother takes to reach her pups in the presence of a potentially infanticidal male could represent a measure of parental investment.