Male and female competitive strategies of wild house mice pairs (Mus musculus domesticus) confronted with intruders of different sex and age in artificial territories

Male and female aggression at different reproductive stages was investigated in pairs of wild mice. Fourteen pairs of laboratory-outbred wild mice were established, each pair living in a multiple set of cages, connected by runaways. Intruder tests were carried out at different stages of the reproductive cycle i.e. 48 h after introduction, during pregnancy and lactation. In these stages, a female, a male and two pups were consecutively introduced in each territory (24 hrs separating each intrusion). Male residents were highly aggressive towards (and always intolerant of) male but not female intruders. Conversely, resident females preferentially attacked same sex intruders after colony establishment and during pregnancy, but they attacked either sex of intruder when nursing young. Seven out of 14 female intruders were tolerated 48 h after introduction of residents but tolerance of females decreased during pregnancy and lactation. Male and female residents were essentially responsible for the intolerance of same-sex intruders. Both males and females exhibited infanticide, but sex differences in the timing of attack on alien pups were observed. In the 7 colonies where the intruder female was tolerated (since that two females were present) only one female reproduced successfully. This suggests that, as in males, females of this stock compete for the opportunity to reproduce; they can be exclusively territorial or form a dominance hierarchy which probably determines reproductive success. While male competitive aggression appears to be mostly directed to other males, females seem largely responsible of the regulation of the reproductive potential of a deme unit throughout intrasexual aggression (intolerance towards other females), and possibly also inhibition of subordinate reproduction and killing of unrelated pups.