Sex role alternation in the simultaneously hermaphroditic pond snail Lymnaea stagnalis is determined by the availability of seminal fluid.

Encounters between potential mating partners are usually accompanied by sexual conflict. In the case of internally fertilizing simultaneous hermaphrodites that perform one sexual role at a time, this conflict arises over the sexual roles. If both animals prefer to mate in one role, and simultaneous reciprocal insemination is not possible, the conflict can be resolved by sex role swapping after the first insemination, potentially resulting in a form of sperm trading called conditional reciprocity. Although sequentially reciprocal insemination has been reported in several hermaphroditic species, we investigated whether conditional reciprocity is the only possible explanation for such mating patterns. We studied the pond snail Lymnaea stagnalis in which sexual isolation increases male sexual drive. Sex role alternation occurred only in pairs where both animals had been isolated and may be conditional. Previous studies in freshwater molluscs have interpreted the occurrence of role alternations per se as conditional reciprocity. However, based on our finding and a review of previous studies, we conclude that future experiments need to be designed very carefully to prove the existence of conditional reciprocity. © 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.