Vulvar spine and copulation duration: unravelling sexual conflict in Ischnura damselflies

Sexual conflict occurs when the fitness interests of the two sexes do not align. Some traits shared by males and females (like mating rate) have sex-specific fitness optima that cannot be achieved simultaneously, and this conflict can favour the evolution of adaptations that benefit one sex but harm the other, like copulatory wounding. Prolonged copulation in damselflies can be seen as a clear example of sexual conflict: while it allows males to guard their partners, preventing them from remating, females cannot feed during copulation and may be more exposed to predators. Ischnura graellsii is one of the species of damselflies in the Coenagrionidae family characterized by females having a conspicuous vulvar spine on the sternum of the eighth abdominal segment, which makes contact with the male seminal vesicle during copulation. To investigate the role of the vulvar spine in sexual conflict, we evaluated whether there is a difference in copulation duration between females with the spine removed and those with an intact spine (control females) and studied the allometry between spine length and body length. We found that the vulvar spine had a significant effect on copulation duration, since males mated on average for 200 min with control females but increased copulation to 227 min with spineless females. The spine was allometric with wing size, but the slope depended on the regression method used. The male seminal vesicle has a depression exactly where the spine is in contact with it during copulation. There was no evidence of physical damage, and consequently, males mated to control and spineless females had similar longevity. We conclude that this spine allows females to reduce mating duration.