Sperm trading in a hermaphroditic flatworm: reluctant fathers and sexy mothers.

When matings are frequent and received sperm are digested, hermaphrodites should trade sperm when mating. We investigated sperm trading in the flatworm Schmidtea (Dugesia) polychroa and manipulated mating interests to investigate its possible causes. In 106 mating pairs consisting of nonisolated individuals, no sperm donation in either direction (35%) and reciprocal exchange (38%) were more common than expected by chance, whereas unilateral transfer (27%) was less frequent, confirming sperm trading. The amount of sperm donated depended on the availability of self-sperm, not on the amount received. Animals with more allosperm from previous matings had more self-sperm and consequently donated more. This suggests that sperm digestion boosts sperm production. In a second experiment, 'mixed-interest' pairs consisting of a nonisolated (N) and an isolated individual (I), NxI, were compared with IxI and NxN pairs. Whereas IxI pairs were eager and NxN reluctant to mate, NxI pairs showed an intermediate likelihood of mating. Whereas NxN pairs traded sperm, the other two groups did not. The change in behaviour in N individuals in the NxI treatment suggests precopulatory assessment and mating in relation to phenotypic mate quality. Isolated individuals are attractive, presumably because they donate large sperm clumps unconditionally and contain fewer allosperm, implying reduced sperm competition. The reduced reluctance in N individuals to mate with, and to inseminate, previously isolated partners suggests that female quality is an important factor in male sperm donation decisions. Hence, S. polychroa may be choosier than previously assumed. Copyright 2000 The Association for the Study of Animal Behaviour.