Gender contests in a simultaneous hermaphrodite snail: a size-advantage model for behaviour

Abstract

Adaptive gender expression in simultaneous hermaphrodites will reflect the relative costs and benefits of each sexual function. The net gain through either or both functions may vary with body size. Shell length measurements of Physa gyrinaand P. heterostropha in the laboratory and P. heterostropha in the field revealed that successful mating generally involved smaller sperm donors and larger recipients. In contrast, attempts by larger snails to donate sperm were contested by smaller snails. These data suggest substantial plasticity in gender expression in accordance with a game-theoretic extension of Ghiselin’s (1969, Q. Rev. Biol., 44,189–208, 1974; The Economy of Nature and the Evolution of Sex) size-advantage model. Specifically, the larger of two interacting snails is more fecund, so the smaller snail has more to gain by donating sperm. This asymmetry creates a contest in which the smaller snail can afford to escalate the contest more and still profit, so long as escalation costs are not greater for the smaller snail. In the present study, the usual form of contest escalation was a visually obvious shell-swinging display, which attracts predators. Thus, the smaller of two contestants can use the risk of predation as a weapon in contest escalations (i.e. a game of chicken), rather than using a purely physical contest that might favour the larger contestant.