Influence of dietary nutrient balance on aggression and signalling in male field crickets

Indicator models of sexual selection predict that sexually selected trait elaboration should covary positively with condition. However, nutrition might influence the expression of multiple traits, where high-quality diets may result in positive trait correlations and low-quality diets in trade-offs. Although previous studies have examined how diet quantity or single nutrients, such as protein, carbohydrate and phosphorus, influence sexual traits, few have examined how dietary nutrient balance affects sexual trait expression. We therefore investigated how dietary protein:carbohydrate ratio and percentage of phosphorus influence the relationship between investment in mate attraction signalling and aggression. We fought groups of six adult male Gryllus veletis crickets, each consuming a unique diet, while recording their prefight and postfight signalling parameters. We found no evidence that diet influenced aggression or prefight signalling, with the exception that high-phosphorus diets had a negative influence on several signalling effort parameters. Body size was an important predictor of aggressive behaviour and most signalling parameters, suggesting that developmental diet may have a greater influence on these sexual traits. Several prefight signalling parameters were weakly related to aggression, suggesting that signalling may advertise competitive abilities. Males consuming high-carbohydrate and equal protein:carbohydrate diets experienced changes in signalling parameters that represented an overall increase in signalling effort following aggressive contests compared to males consuming high-protein diets, suggesting that dietary effects on signalling may only become apparent following periods of highly energetic activity. Changes in signalling following aggressive contests were also related to aggression levels modulated by dietary phosphorus content, such that only males consuming low-phosphorus diets were able to invest heavily in signalling after investing heavily in aggression. Our findings highlight the importance of research on the interplay between multiple sexually selected traits, and how dietary nutrient balance influences these relationships.