Correlational selection on personality and social plasticity: morphology and social context determine behavioural effects on mating success

1. Despite a central line of research aimed at quantifying relationships between mating success and sexually dimorphic traits (e. g., ornaments), individual variation in sexually selected traits often explains only a modest portion of the variation in mating success. 2. Another line of research suggests that a significant portion of the variation in mating success observed in animal populations could be explained by correlational selection, where the fitness advantage of a given trait depends on other components of an individual's phenotype and/or its environment. We tested the hypothesis that interactions between multiple traits within an individual (phenotype dependence) or between an individual's phenotype and its social environment (context dependence) can select for individual differences in behaviour (i. e., personality) and social plasticity. 3. To quantify the importance of phenotype-and context-dependent selection on mating success, we repeatedly measured the behaviour, social environment and mating success of about 300 male stream water striders, Aquarius remigis. Rather than explaining individual differences in long-term mating success, we instead quantified how the combination of a male's phenotype interacted with the immediate social context to explain variation in hour-by-hour mating decisions. We suggest that this analysis captures more of the mechanisms leading to differences in mating success. 4. Males differed consistently in activity, aggressiveness and social plasticity. The mating advantage of these behavioural traits depended on male morphology and varied with the number of rival males in the pool, suggesting mechanisms selecting for consistent differences in behaviour and social plasticity. Accounting for phenotype and context dependence improved the amount of variation in male mating success we explained statistically by 30-274%. 5. Our analysis of the determinants of male mating success provides important insights into the evolutionary forces that shape phenotypic variation. In particular, our results suggest that sexual selection is likely to favour individual differences in behaviour, social plasticity (i. e., individuals adjusting their behaviour), niche preference (i. e., individuals dispersing to particular social conditions) or social niche construction (i. e., individuals modifying the social environment). The true effect of sexual traits can only be understood in interaction with the individual's phenotype and environment.