Effect of diet and rearing density on contest outcome and settlement in a field cricket

Abstract

The ability to win fights is expected to be influenced by the rearing and nutritional conditions experienced by individuals. We hypothesized that crowded rearing conditions would foreshadow intense competition for resources in adult male Gryllus firmus (Scudder) field crickets and thus favour greater investment in mandibular weaponry. In line with prediction, males developed relatively larger weapons in crowded rearing conditions but this investment did not translate into greater success in contests against size-matched males that were reared singly. Rather, we found that low-density males were more likely to defeat high-density males under some experimental conditions, possibly because low-density males are more aggressive. Overall, our strongest observed effect was that males with larger weaponry were more likely to win contests, independent of rearing density. We also manipulated the adult diet of males to test the hypothesis that greater fat loads, and thus potentially greater stamina, would improve success in contests. Contrary to prediction, increased fat load did not increase a male’s probability of winning a contest possibly because fights in field crickets are too short to invoke stamina. Broadly speaking, animals decide whether to give up in a fight by assessing either their own (self-assessment) resource-holding potential (RHP) or by assessing a combination of their own and their opponent’s RHP (mutual assessment). Our study lends some support to the hypothesis that male G. firmus crickets decide to persist in a fight by assessing their own RHP rather than via mutual assessment. Animals fight each other for access to, and control of, resources that they require for survival and reproduction (e.g. food, shelter, and mates). Although considerable empirical research has shown that individuals having a larger body size or weapons generally possess greater fighting ability—resource-holding potential (RHP)—and are thus typically more successful in combat, the factors contributing to the development of an individual’s RHP have received less attention. One potential environmental factor dictating RHP is population density; however, the effect of density on RHP is difficult to predict. Developing under high density could cause individuals to invest more in RHP-related traits because these conditions might signal intense competition for mates in the near future. However, individuals reared under high density are often less aggressive than their low-density counterparts. The food available to an adult could also be important because the consumption of fat could improve an individual’s stamina, an RHP-related trait important in combat. Our study testing the impact of these two factors on male fighting ability in the sand field cricket (Gryllus firmus) found limited support for the hypothesis that rearing density and diet significantly influence a male’s ability to win a fight against another male. Instead we found stronger support for the notion that, all else being equal, males with larger mandibles, or weapons, than their opponent were more likely to win a fight.