A test of operational sex ratio theory across latitudes reveals temporal variation in sex-specific behavioural reaction norms.

Predicting the strength and direction of sexual selection is a challenge, as the effects of ecological factors, social environment and behavioural plasticity all need to be considered. The operational sex ratio (OSR) is a key variable, which has been shown to (i) affect the strength and direction of mating competition, as a social environment cue, and (ii) be affected itself by ecological conditions through sex-specific environmental effects. Gaining a global view of (i) and (ii) in wild populations represents a necessary step for our understanding of sexual selection dynamics in the wild. Here, we address this challenge within the reaction norm framework. We conducted an extensive field study on the two-spotted goby Pomatoschistus flavescens, monitoring six populations along a latitudinal gradient during an entire breeding season. We compared the temporal trajectories in social environment and sexual displays across populations, which is unprecedented. Using a reaction norm framework based on OSR theory, we show that what appears to be great variation in sexual displays across populations and sampling times, follows consistent rules. Sexual display behaviour followed behavioural reaction norms in response to the social environment that were consistent across populations, but social environment fluctuations were specific to each population. Recording behaviour not only over time, but also along a latitudinal gradient where ecological conditions vary and in turn affect OSR, was necessary to reveal the relationship between social environment and sexual displays, which in turn contributes to sexual selection dynamics.