Interactions among performance capacities predict male combat outcomes in the field cricket

1. There is a growing appreciation that male fighting ability is influenced by multiple traits. In particular, studies of male combat have identified a variety of performance capacities that predict victory in conflicts between conspecifics in several animal species. However, while multiple aspects of male fighting ability are often examined with a single study, the interactions among traits are rarely considered.
2. We conducted fight trials between size-matched males in the black field crickets, Teleogryllus commodus, with the aim of testing how two whole-organism performance traits, jumping ability and bite force, together with weight at eclosion, interact to determine combat outcomes.  We then examined the relationships between these traits and the outcome of a fight using: a univariate framework, where we examined the difference between winners and losers for each trait separately; and, a multivariate framework known as response surface analysis, where we examined how the interactions amongst traits predict the outcome of a fight.
3. We show that males that won fights tended to exert significantly more force when biting, but were neither better at jumping nor heavier at eclosion than their rivals. However, when we examined the effects of these three traits while statistically controlling for one another in a multiple regression, we showed that overall fighting ability depends not only on contributions of individual traits to contest resolution, but also on the interactions amongst traits and their relationship to the capabilities of the rival male. We found that the probability of winning was highest for the male with the greatest positive difference from his rival in the combination of bite force, jump power and weight at eclosion.
4. These findings highlight how studying multiple performance capabilities in isolation from each other may obscure the importance of some variables. Our multivariate analyses revealed that the interactions between multiple dimensions of performance are important in male combat. We discuss the broader implications of these findings, including the role of whole-organism performance in determining male quality.

Authors: Matthew D. Hall, Luke McLaren, Robert C. Brooks & Simon P. Lailvaux

Sex-specific fitness effects of nutrient intake on reproduction and lifespan

Diet affects both lifespan and reproduction, leading to the prediction that the contrasting reproductive strategies of the sexes should result in sex-specific effects of nutrition on fitness and longevity and favor different patterns of nutrient intake in males and females. However, males and females share most of their genome and intralocus sexual conflict may prevent sex-specific diet optimization. We show that both male and female longevity were maximized on a high-carbohydrate low-protein diet in field crickets Teleogryllus commodus, but male and female lifetime reproductive performances were maximized in markedly different parts of the nutrient intake landscape. Given a choice, crickets exhibited sex-specific dietary preference in the direction that increases reproductive performance, but this sexual dimorphism in preference was incomplete, with both sexes displaced from the optimum diet for lifetime reproduction. Sexes are, therefore, constrained in their ability to reach their sex-specific dietary optima by the shared biology of diet choice. Our data suggest that sex-specific selection has thus far failed fully to resolve intralocus sexual conflict over diet optimization. Such conflict may be an important factor linking nutrition and reproduction to lifespan and aging.

Experimental evidence that sexual conflict influences the opportunity, form and intensity of sexual selection

Sexual interactions are often rife with conflict. Conflict between members of the same sex over opportunities to mate has long been understood to effect evolution via sexual selection. Although conflict between males and females is now understood to be widespread, such conflict is seldom considered in the same light as a general agent of sexual selection. Any interaction between males or females that generates variation in fitness, whether due to conflict, competition or mate choice, can potentially influence sexual selection acting on a range of male traits. Here we seek to address a lack of direct experimental evidence for how sexual conflict influences sexual selection more broadly. We manipulate a major source of sexual conflict in the black field cricket, Teleogryllus commodus, and quantify the resulting changes in the nature of sexual selection using formal selection analysis to statistically compare multivariate fitness surfaces. In T. commodus, sexual conflict occurs over the attachment time of an external spermatophore. By experimentally manipulating the ability of males and females to influence spermatophore attachment, we found that sexual conflict significantly influences the opportunity, form and intensity of sexual selection on male courtship call and body size. When males were able to harass females, the opportunity for selection was smaller, the form of selection changed, and sexual selection was weaker. We discuss the broader evolutionary implications of these findings, including the contributions of sexual conflict to fluctuating sexual selection and the maintenance of additive genetic variation.

Authors: Matthew D. Hall, Luc F. Bussière, John Hunt and Robert C. Brooks