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. Pr & eacute;dire l'intensit & eacute; et la direction de la s & eacute;lection sexuelle implique de prendre en compte l'environnement abiotique, l'environnement social, et la plasticit & eacute; comportementale. Le Sexe Ratio Op & eacute;rationnel (SRO) joue un r & ocirc;le central dans cette question car (i) en tant que param & egrave;tre de l'environnement social, il affecte l'intensit & eacute; et la direction de la comp & eacute;tition pour la reproduction, et (ii) il peut & ecirc;tre lui-m & ecirc;me affect & eacute; par l'environnement abiotique par l'interm & eacute;diaire d'effets sexe-sp & eacute;cifiques de l'environnement. Pour comprendre les dynamiques de s & eacute;lection sexuelle en populations naturelles, il est donc n & eacute;cessaire d'int & eacute;grer les points (i) et (ii). Nous proposons d'appr & eacute;hender ce probl & egrave;me en termes de normes de r & eacute;actions. L'& eacute;tude inclut six populations du gobie nageur Pomatoschistus flavescens situ & eacute;es le long d'un gradient de latitude, que nous avons suivies durant toute une saison de reproduction. Il s'agit de la premi & egrave;re & eacute;tude comparant les variations temporelles d'environnement social et de comportement de parade nuptiales entre diff & eacute;rentes populations naturelles. Le principe de norme de r & eacute;action, appliqu & eacute;e & agrave; la th & eacute;orie du SRO, montre que ce qui semble & ecirc;tre une grande vari & eacute;t & eacute; de comportements de parades nuptiales, suit des r & egrave;gles simples. Les comportements de parades nuptiales suivent des normes de r & eacute;actions fonction de l'environnement social qui sont les m & ecirc;mes d'une population & agrave; l'autre. En revanche, les fluctuations temporelles de l'environnement social sont diff & eacute;rentes dans chaque population. En observant les changements de comportement au cours du temps, mais aussi & agrave; diff & eacute;rentes latitudes, nous avons r & eacute;v & eacute;l & eacute; les liens complexes qui relient l'environnement abiotique, l'environnement social et les comportements de parades nuptiales, qui contribuent aux dynamiques de s & eacute;lection sexuelle.