LEONARD, J.L.; Univ. of California, Santa Cruz: Bateman�s principle and simultaneous hermaphrodites: a paradox

Bateman�s principle states that reproductive success is limited a) in females by the resources available for egg production; and b) in males, only by access to females and/or eggs. Therefore, variance in reproductive success should be higher in males than in females and the eagerness of males in mating encounters can be explained by the greater �upside potential� available to males. For simultaneous hermaphrodites, Bateman�s principle predicts that the male role will also be preferred due to its greater �upside potential�. However, this creates a clear paradox. Gillespie�s principle, supported by probability theory, predicts that where two strategies have equal mean fitness, which must be the case for male and female function, the strategy with the lower variance in reproductive success will have higher fitness. Therefore, Bateman�s principle predicts that hermaphrodites should prefer the sexual strategy with potentially lower fitness for an individual. Application of Modern Portfolio Theory, along with Gillespie�s principle, predicts that hermaphrodites will prefer the role with lower variance in reproductive success. All else being equal, that will be the female role. In gonochores, male �eagerness� may represent �making the best of a bad job�. Both principles predict sexual conflict between mating hermaphrodites due to a preference for one sexual role. The Hermaphrodite�s Dilemma model predicts that this conflict will result in mating systems based on reciprocity. Empirical data suggest that reciprocity in many forms occurs among hermaphrodites and that the sexual role preferred varies among taxa. The factors which may determine the preferred role include control of fertilization, breeding sex ratio, effects of gamete production on future growth.