Sperm trading by volume in a hermaphroditic flatworm with mutual penis intromission

When sperm donation is costly, insemination is reciprocal and ejaculate size is a good predictor of an individual's reproductive state, internally fertilizing hermaphrodites are expected to donate more sperm when they receive more, resulting in sperm trading by volume. We compared the amounts of sperm exchanged reciprocally between partners of the planarian flatworm Dugesia gonocephala. In the field, mating individuals had more self sperm, but were less allosperm depleted than nonmating individuals. Larger individuals also had more self sperm and transferred more sperm. Although individuals mated assortatively by size, self sperm reserves were not correlated within pairs. Nevertheless, mating partners exchanged similar amounts of sperm, even after correcting for self sperm availability. Hence, individuals give more sperm not only when they have more, but also when they receive more. This is the first indication that sperm trading by volume occurs in a simultaneous hermaphrodite with mutual penis intromission. It appears to be achieved by both partners donating sperm until the partner with the lowest self sperm reserves finishes ejaculation and completes spermatophore transfer. In response, the other partner ends sperm donation as well, and transfers its spermatophore with only a short delay. The evolution of conditional sperm exchange in this species may be explained by the fact that male mating rate is limited owing to the time needed to replenish self sperm reserves. Copyright 1998 The Association for the Study of Animal Behaviour