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) forms large mating/oviposition aggregations on discarded moose (Alces alces) antlers, where the strikingly aggressive males engage in frequent combat. According to theory, costly fighting behaviour will be maintained by selection only if winners sire more progeny than losers. Through a field study of individually marked flies, we addressed the question “Why do male antler flies fight?” by investigating what resources males compete for on antlers, whether or not large male body size and resulting advantage in agonistic encounters confers positional advantage in the mating aggregation, and whether the successful males experience greater survivorship or greater mating frequency. As expected, most agonistic contests were won by the larger male. Large males tended to live longer, mate more frequently, and achieve more matings over their lifetimes than small males. Males fought and defended territories primarily on the upward-facing (‘upper’) surfaces of antlers. The main oviposition site attracted the highest density of single males, and mean body size of single males was largest in this region. Males mate-searching near the main oviposition site achieved the highest mean lifetime mating success on the upper surface. Multiple regression analysis indicated that the main oviposition site was the only region where matesearching tended to increase male mating frequency and, on average, males mated nearly twice as frequently when mate-searching there as they did when mate-searching elsewhere. These results suggest that male antler flies fight for opportunities to search for females in areas where they will mate more frequently and thus achieve more matings over their lifetimes.