Sexual conflict over copulation duration in a cannibalistic spider

The evolutionary significance of sexual cannibalism depends on the timing of male death. Both female and male spiders possess paired genital organs and, while females can mate multiply, males of some species have limited mating rates because they can use each copulatory organ only once. By attacking a male after he has used one of his pedipalps but before he has used the other one, females may severely reduce male reproductive success. In the orb-web spider Argiope bruennichi, females attack every male during his first copulation and only 20% of virgin males survive such an attack. To measure costs and benefits associated with single versus double insertions for males, we created two double-mating treatments: we experimentally manipulated first males to use one or both of their pedipalps with a single female and then assessed paternity success of a second male (P2). Double insertions reduced P2 by increasing copulation duration rather than by increasing the number of insertions per se. When we separated the consumption of males experimentally from the act of killing, consumption had no effect on paternity. By counting sperm in used and unused pedipalps of males, we determined that sperm transfer is directly related to copula duration and that males are able to transfer 50% of their sperm load in each palp within 10 s of copulation. Our experiments revealed that sexual cannibalism after the first insertion is costly for males and we propose that these costs arise because females benefit from polyandry.