Who to include in measures of sexual selection is no trivial matter

Abstract

In many animals acquiring limited reproductive opportunities involves competition for resources, mates and opposite-sex gametes. There is ambiguity in which competitive steps are included in measures of sexual selection: individuals who fail to obtain resources necessary for reproduction are often excluded. We illustrate the implications of variation in who is included in measures of selection. We quantified selection on male length and the opportunity for selection associated with nest acquisition, mate acquisition, and fertility of mates at two levels of density and two levels of nest availability in the sand goby. Both measures varied significantly across the three episodes of selection. Nest and mate acquisition contributed substantially to the overall opportunity for selection and selection on male size. Focusing only on males with nests led to lower estimates of selection. The effects of density and nest availability depended on the selective episodes considered. While there is nothing wrong with focusing on particular episodes of interest, inconsistency in who is included in measures of sexual selection across studies will make it difficult to answer broad research questions.