Résumé
(1) Routine assessments of overall sexual selection, including comparisons
of its direction and intensity between sexes or species, rely on summary
metrics that capture the essence of sexual selection. Nearly all currently
employed metrics require population-wide estimates of individual mating
success and reproductive success. The resulting sexual selection metrics,
however, can heavily and systematically vary with the chosen approaches in
terms of sampling, measurement, and analysis. (2) Our review illustrates
this variation using the Bateman gradient, a particularly prominent sexual
selection metric. It represents the selection gradient on mating success
and – given the latter's pivotal role in defining sexual selection –
reflects a trait-independent integrative proxy for the maximum strength of
sexual selection. Drawing from a recent meta-analysis, we evaluate
potential biases arising from study design, data collection, and parameter
estimation, and provide suggestions to mitigate such biases in future
studies. (3) With respect to study design, we argue that currently almost
inexistent manipulative studies must complement the dominating correlative
studies to inform us about causality in sexual selection. With respect to
data collection, we outline how different measures of mating and
reproductive success affect the components of sexual (and natural)
selection that are reflected in standard summary metrics. With respect to
parameter estimation, we show the potential impact of decisions about data
inclusion and the chosen quantitative approach on inferences of sexual
selection and its sex difference. (4) We expect this meta-analytical
review to aid future studies in providing less biased and more informative
estimates of sexual selection.