Butterfly contests and flight physiology: Why do older males fight harder?

The males of many butterfly species compete for territories via conspicuous aerial wars of attrition, in which the determinants of persistence ability are largely unclear. Flight performance features, such as stamina, acceleration, and maneuverability, are often assumed to be important in this context, yet there is no direct evidence by which to evaluate these possibilities. Recent research has indicated that competitive ability increases with age in a notably territorial species, Hypolimnas bolina, which could arise from lifetime morphological or physiological changes that directly affect flight performance. I evaluated this hypothesis by investigating how size-independent variance in body composition, energy stores, flight muscle ratio (FMR), and wing condition change with age in this species. Males in five age categories (spanning the functional life span of territorial individuals) were sampled from encounter sites in tropical Australia. Analysis of body composition with respect to an estimate of eclosion mass (forewing length) indicated that total body mass, abdomen mass, and wing area decrease throughout an individual's lifetime, but thorax mass remains unchanged. Wing loading (the ratio of wing area to body mass) is lowest in intermediately aged individuals, but FMR and energetic status remain largely similar regardless of age. On average, therefore, the energetic cost of sustained flight should first decrease, then increase, with age in a male H. bolina (of standardized body size), while available energy reserves decline slightly. Acceleration and maneuverability should remain relatively constant. These results, coupled with the fact that body size is unrelated to contest success in this territorial butterfly, fail to support the idea that age-related competitive ability is mediated simply by energetics or flight performance.