Gender trading in a hermaphrodite

Although males and females may face severe sexual conflicts [1], the decision over who donates and who receives sperm is fixed. This is not necessarily the case in simultaneous hermaphrodites, where each mate may insist on copulating as ‘male,’ ‘female’ or both. The resulting conflicts over gender provide an opportunity to study conflict-solving strategies. By directly manipulating mating interests in the sea slug Chelidonura hirundinina, we present the first unequivocal evidence for ‘sperm trading’ [2], a mating strategy where sperm donation is conditional on sperm receipt. Trading enforces reciprocity, offering a simple solution for the gender conflict