Meta-analysis and sexual selection: past studies and future possibilities

The action of sexual selection is highly variable among taxa. This creates challenges when trying to generalize (e.g. determine if a particular relationship exists based on its average strength, or if it varies in response to theoretically relevant factors). Consequently, accounting for moderating factors is likely to be crucial to explain differences in sexual selection among studies. In principle, given measures of key theoretical parameters we can predict the strength of sexual selection on different sexual signals, the benefits of mate choice, the extent of sex differences (e.g. in immune function or survival) and the likely life history trade-offs between investment into different sexual traits (e.g. sperm vs. courtship) or non-sexual traits (e.g. immune function, traits that increase longevity, parental care). How well does empirical data support theoretical expectations? First, we provide a short history of the use of meta-analysis in sexual selection studies. We present a table summarizing 94 meta-analyses that have asked questions about sexual selection or allied topics of interest to those studying sexual selection (e.g. the link between heterozygosity and fitness). Second, we list the main ways that meta-analysis has been used in sexual selection work and provide illustrative examples. Third, we provide practical advice to identify questions that are ripe for meta-analysis. We highlight 11 sexual selection topics where meta-analyses are needed (e.g. there are no meta-analyses testing game theory models of fighting contests). Finally, we discuss some general issues that will arise as the use of meta-analysis in sexual selection studies becomes more sophisticated.