Evidence for Strategic Sex Allocation in the Guppy (Poecilia reticulata): Brood Sex Ratio and Size Predict Subsequent Adult Male Mating Success

Sex allocation theory predicts that females should produce more sons when the reproductive success of sons is expected to be high, whereas they should produce more daughters, not daughters when the reproductive success of sons is expected to be low. The guppy (Poecilia reticulata) is a live-bearing fish, and female guppies are known to produce broods with biased sex ratios. In this study, we examined the relationship between brood sex ratio and reproductive success of sons and daughters, to determine whether female guppies benefit from producing broods with biased sex ratios. We found that sons in male-biased broods had greater mating success at maturity than sons in female-biased broods when brood sizes were larger. On the other hand, the reproductive output of daughters was not significantly affected by brood sizes and sex ratios. Our results suggest that female guppies benefit from producing large, male-biased brood when the reproductive success of sons is expected to be high.