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

The effect of diet quality and wing morph on male and female reproductive investment in a nuptial feeding ground cricket

A common approach in the study of life-history trade-off evolution is to manipulate the nutrient content of diets during the life of an individual in order observe how the acquisition of resources influences the relationship between reproduction, lifespan and other life-history parameters such as dispersal. Here, we manipulate the quality of diet that replicate laboratory populations received as a thorough test of how diet quality influences the life-history trade-offs associated with reproductive investment in a nuptial feeding Australian ground cricket (Pteronemobius sp.). In this species, both males and females make significant contributions to the production of offspring, as males provide a nuptial gift by allowing females to chew on a modified tibial spur during copulation and feed directing on their haemolymph. Individuals also have two distinct wing morphs, a short-winged flightless morph and a long-winged morph that has the ability to disperse. By manipulating the quality of diet over seven generations, we found that the reproductive investment of males and females were affected differently by the diet quality treatment and wing morph of the individual. We discuss the broader implications of these findings including the differences in how males and females balance current and future reproductive effort in nuptial feeding insects, the changing nature of sexual selection when diets vary, and how the life-history trade-offs associated with the ability to disperse are expected to differ among populations.

Authors: Matthew D. Hall, Luc F. Bussière & Robert Brooks