Why do male antler flies (Protopiophila litigata) fight? The role of male combat in the structure of mating aggregations on moose antlers

The antler fly Protopiophila litigata Bonduriansky (Diptera Piophilidae) fo rms large mating/oviposition aggregations on discarded moose (Alces alces) antlers, where the strikingly aggressive males engage in frequent combat. A ccording to theory, costly fighting behaviour will be maintained by selecti on only if winners sire more progeny than losers. Through a field study of individually marked flies, we addressed the question "Why do male antler fl ies fight?" by investigating: what resources males compete for on antlers, whether or not large male body size and resulting advantage in agonistic en counters confers positional advantage in the mating aggregation, and whethe r the successful males experience greater survivorship or greater mating fr equency As expected, most agonistic contests were won by the larger male. L arge males tended to live longer, mate more frequently, and achieve more ma tings over their lifetimes than small males. Males fought and defended terr itories primarily on the upward-facing ('upper') surfaces of antlers. The m ain oviposition site attracted the highest density of single males, and mea n body size of single males was largest in this region. Males mate-searchin g near the main oviposition site achieved the highest mean lifetime mating success on the upper surface. Multiple regression analysis indicated that t he main oviposition site was the only region where mate-searching tended to increase male mating frequency and, on average, males mated nearly twice a s frequently when mate-searching there as they did when mate-searching else where. These results suggest that male antler flies fight for opportunities to search for females in areas where they will mate more frequently and th us achieve more matings over their lifetimes.