We assessed the effects of different situational or social determinants on the regulation of female-female competition. We carried out a laboratory study to examine aggression and reproductive success of pairs of wild female mice, Mus musculus domesticus spp., as a function of the timing of settlement of females relative to that of males and the genetic relatedness and familiarity between females (sibling versus nonsibling females). After a few days of cohabitation with a male, females were highly aggressive towards, and intolerant of, any intruder female, regardless of relatedness and familiarity. In this condition, monogamy was the resulting mating pattern in approximately 80% of cases. Conversely, pairs of females who made contact With each other at the same time, or prior to cohabitation with a male, showed comparatively little aggression and a high degree of reciprocal tolerance. Only in these latter conditions did genetic relatedness and familiarity between females inf! luence their behavioural interactions and reproductive success. Although nonsibling pairs showed higher frequencies of aggressive interactions than siblings, polygyny resulted in 97% of cases. However, in most sibling groups both the females weaned young and had greater reproductive success than nonsiblings. Nonsibling females appeared to compete for reproduction through the inhibition of reproduction or infanticide. These findings suggest that the timing of male/female settlement in a deme determines the level of female competition, which, in turn, affects the resulting mating pattern. Only when females showed social tolerance did genetic relatedness and familiarity influence reproductive success.
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