Fat and Body Mass Predict Residency Status in Two Tropical Satyrine Butterflies

Among insect species that compete for ownership of mating sites via aerial interactions, there is little consensus about how morphological and physiological traits functionally translate into costs during a fight. In this study, we evaluated the hypothesis that in territorial disputes without physical contact, traits that maximize endurance will be important determinants of winning. To evaluate the occurrence of physical contact, we used high-speed video recording (240 frames per second) in two previously unstudied satyrine butterflies: Hermeuptychia fallax and Moneuptychia soter. Additionally, we performed removal experiments to assess whether wing wear, body mass, fat content, and flight muscle ratio are important determinants of male residency status. We filmed a total of 23 fights in H. fallax and 10 in M. soter. Neither species employed physical contact to settle contests. In H. fallax, younger males with greater fat content accumulated in the resident role (n = 26 pairs), indicating that males of this species may compete via 'endurance contests'. On the other hand, resident males of M. soter were heavier than replacements (n = 11 pairs), but did not differ in any other measured trait. Because disputes occurred without physical contact, it is difficult to imagine how mass or size may functionally affect the chances of victory. Body mass may be related to other unmeasured traits such as condition, parasitic load, or even specific aerodynamic designs related to flight speed or maneuverability.