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

When matings are frequent and received sperm are digested, hermaphrodites s hould trade sperm when mating. We investigated sperm trading in the flatwor m Schmidtea (Dugesia) polychroa and manipulated mating interests to investi gate its possible causes. In 106 mating pairs consisting of nonisolated ind ividuals, no sperm donation in either direction (35%) and reciprocal exchan ge (38%) were more common than expected by chance, whereas unilateral trans fer (27%) was less frequent, confirming sperm trading. The amount of sperm donated depended on the availability of self-sperm, not on the amount recei ved. Animals with more allosperm from previous matings had more self-sperm and consequently donated more. This suggests that sperm digestion boosts sp erm production. In a second experiment, 'mixed-interest' pairs consisting o f a nonisolated (N) and an isolated individual (I), N x I, were compared wi th I x I and N x N pairs. Whereas I x I pairs were eager and N x N reluctan t to mate, N x I pairs showed an intermediate likelihood of mating. Whereas N x N pairs traded sperm, the other two groups did not. The change in beha viour in N individuals in the N x I treatment suggests precopulatory assess ment and mating in relation to phenotypic mate quality. Isolated individual s-are attractive, presumably because they donate large sperm clumps uncondi tionally and contain fewer allosperm, implying reduced sperm competition. T he reduced reluctance in N individuals to mate with, and to inseminate, pre viously isolated partners suggests that female quality is an important fact or in male sperm donation decisions. Hence, S. polychroa may be choosier th an previously assumed. (C) 2000 The Association for the Study of Animal Beh aviour.