Mating success in fruit flies: courtship interference versus female choice

The outcomes of sexual selection often differ when mating success is determined by male contest rather than female choice. Many studies, however, inferred sexual selection driven by female choice without carefully assessing the role of subtle male aggression. Relying on close-up video analyses, we documented novel courtship interference between male fruit flies, a key model system in research on sexual selection, sexual conflict and speciation. In experiments comparing male mating success under choice (2 males + 1 female) and no-choice (1 male + 1 female) conditions, we found that, in some cases, courtship interference altered male mating success. Both choice and no-choice protocols have known weaknesses. Choice protocols do not control for male–male interactions while no-choice protocols do not allow females to compare and choose between males. To overcome these weaknesses, we developed a new protocol (true-choice), which allows females to freely visit and assess each of two males while preventing direct male–male interactions. Results from the true-choice protocol suggest that traits enhancing male aggression have a greater role in determining mating success in fruit flies. Furthermore, it is possible that the mating system of scramble competition observed in many species should be reclassified as subtle male contest, which can drive sexual selection for aggressive male features.