Measuring and interpreting sexual selection metrics: evaluation and guidelines

Abstract

Summary 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. 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. 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. We expect this meta‐analytical review to aid future studies in providing less biased and more informative estimates of sexual selection.

Time spent in distinct life-history stages has sex-specific effects on reproductive fitness in wild Atlantic salmon

“…We found a significant positive relationship between mating and reproductive success in both male and female Atlantic salmon indicating that having more mating partners results in more offspring in both sexes similar to reports in North American Atlantic salmon (Garant et al 2001). The relationship between reproductive success and mating success does not differ between the sexes suggesting that the strength of sexual selection is similar between males and females (Anthes et al 2017;Arnold & Duvall 1994;Janicke et al 2016;Jones 2009). This result is surprising, as it is generally thought that sexual selection is stronger among males in Atlantic salmon (Fleming 1998;Fleming 1996;Fleming & Einum 2011).…”

Dietary stress increases the total opportunity for sexual selection and modifies selection on condition‐dependent traits

“…The use of replicate social groups allowed episodes of pre‐ and postcopulatory sexual selection to play out under social conditions that facilitate realistic estimates of relative fitness (Evans & Garcia‐Gonzalez ). Moreover, by monitoring all interactions among fish during mating trials we were able to quantify male mating success (through behavioural observation) and siring success (through paternity assignment), thus avoiding the pitfall of having to estimate mating rates indirectly from parentage data, which potentially underestimates variance in male mating success (Anthes et al ).…”

Sexual selection and its evolutionary consequences in female animals

“…Quantitative evidence includes statistical measurements and other indications of the opportunity, direction, and strength of sexual selection. The most widely used measures are I s (the opportunity for sexual selection, a relative measure of intrasexual variance in mating success; Wade, ; Arnold & Wade, ), β ss (the Bateman gradient, a least‐squares regression of relative reproductive success on relative mating success for each sex, capturing the fitness gain associated with additional matings; Arnold & Duvall, ), and s ′ max (the Jones index, which combines the previous two measures to define the upper limit of sexual selection; Jones, ; Anthes et al, ).…”