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Mating with close relatives often causes a decline in offspring fitness, termed inbreeding depression. Consequently, pre- and postcopulatory inbreeding avoidance mechanisms are likely to evolve, especially in females due to their relatively high levels of reproductive investment. If precopulatory inbreeding avoidance mechanisms are absent or unreliable, females may exploit postcopulatory sexually selected inbreeding avoidance mechanisms by mating with multiple males (polyandry). Despite this expectation, few studies have tested whether females alter their propensity to mate multiply in an inbreeding avoidance context. Here, we tested whether the perceived risk of inbreeding promotes polyandrous behaviour in guppies, Poecilia reticulata, a species susceptible to inbreeding depression. Virgin females were assigned to either a sibling or nonsibling mating and subsequently presented with three unrelated males simultaneously. Consistent with previous studies, we found that females did not discriminate between initially encountered males based on relatedness. However, females increased their propensity to mate multiply after mating with a brother. Subsequent paternity tests were performed to determine whether polyandry minimizes inbreeding through a dilution effect (i.e. greater number of sires in the related treatment) or through postcopulatory biasing mechanisms (i.e. greater paternity skew towards unrelated males). Surprisingly, our results supported neither scenario; we detected no significant difference in paternity skew or the number of sires between treatments. Overall, our study provides a rare example of females responding to an increased risk of inbreeding by increasing their propensity to remate, but the postcopulatory processes arising from this behaviour, along with the mechanisms underlying kin recognition, await further investigation.
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