Fat and sassy: factors underlying male mating success in a butterfly

Theory predicts that male mating success depends on resource holding potential ( RHP ), which is reflected by proxies of condition, such as body mass, fat content, strength, or weaponry. In species lacking any physical means to inflict injuries upon combatants, such as butterflies, the factors determining mating success are less clear. Against this background, we explored the determinants of male mating success in the tropical butterfly B icyclus anynana Butler ( L epidoptera: N ymphalidae: S atyrinae), by comparing physiological, immunological, and morphological traits between winning and losing males. Our results showed that successful males are characterized by a better flight performance, evidenced by having longer wings, a heavier thorax, a lighter abdomen, a higher fat content, and higher phenoloxidase expression levels than their unsuccessful counterparts, when being compared after their first mating. Males that won three consecutive trials against the same combatant were also characterized by a better flight performance, having larger forewings, a higher body mass, and a higher fat content. Thus, successful males were larger and in better condition than unsuccessful ones. Strikingly, many differences found indicated an enhanced flight performance for the former, which we suggest ultimately plays the key role for male mating success in B . anynana . As fat is the main energy source for flying insects, being crucial to flight endurance and in turn presumably to male mating success, it may represent a key determinant at the proximate level.