Twilight fighting in the evening brown butterfly, Melanitis leda (L.) (Nymphalidae): Age and residency effects

Butterflies are conspicuous among animals that fight for mating opportunities because it is not clear how contest costs could accrue. Nevertheless, the bulk of research in this group suggests that contests are settled on the basis of asymmetries in fighting ability (the superior competitor hypothesis). There is also a consistent effect on contest escalation due to apparent 'confusion' over residency, which could result from resident butterflies playing different tactics than non-residents, or from a causal link between residency and fighting ability. I addressed these hypotheses by studying contest behavior in the crepuscular species Melanitis leda (L.) (Nymphalidae) over a 5-month period in tropical Australia. Males competed via conspicuous two-stage maneuvers of a form unique among butterflies. Prior residents won 77% of all contests. Non-resident males arrived and perched in occupied sites until challenged by incumbent residents, and the aggressiveness of these interlopers increased as a function of the time before they were detected. Contest winners also tended to be younger than losers, and contest escalation was negatively related to both the age of the losing male and the magnitude of the between-combatant age asymmetry. These results are consistent with the superior competitor hypothesis based upon age as a determinant (or correlate) of fighting ability, but also suggest residency has a unique influence on contest behavior. Further research is clearly required to unravel the effects of age, residency and other potential biophysical determinants of fighting ability in this species.