Sexual selection is potentially stronger than natural selection when the variance in male reproductive fitness exceeds all other components of fitness variance combined. However, measuring the variance in male reproductive fitness is difficult when nonmating males are absent, inconspicuous, or otherwise difficult to find. Omitting the nonmating males inflates estimates of average male reproductive success and diminishes the variance, leading to underestimates of the potential strength of sexual

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... selection. We show that, in theory, the proportion of the total variance in male fitness owing to sexual selection is approximately equal to H, the mean harem size, as long as H is large and females are randomly distributed across mating males (i.e., ). In this case, V p H harem mean harem size not only provides an easy way to estimate the potential strength of sexual selection but also equals the opportunity for sexual selection, I mates . In nature, however, females may be overdispersed with or more aggregated than random V ! H harem with . We show that is a good measure of V 1 H H ϩ (k Ϫ 1) harem the opportunity for sexual selection, where k is the ratio . V /H harem A review of mating system data reveals that in nature the median ratio for is 1.04, but as H increases, females tend to become V /H harem more aggregated across mating males with V harem two to three times larger than H.