Gender choice and gender conflict in a non-reciprocally mating simultaneous hermaphrodite, the freshwater snail, Physa

Like freshwater pulmonate snails in general, mature Physa heterostropha pomilia (Say) are hermaphroditic but cannot reciprocally inseminate each other simultaneously. One member of a pair assumes the male role and the other the female, after which swapping may occur. The tendency for a pair of prospective copulants to elect the same sexual role was defined as gender conflict, and distinguished from sexual conflict, the more widely noted tendency for females to reject low-quality males. Experiments assessed whether previous reproductive history affected the likelihood of mating and the sexual role assumed and evaluated the strength of gender and sexual conflicts. Snails were selected from isofemale lines fixed for albinism at two complementary loci. Ten pairings were made involving all combinations of four categories of reproductive history: previously selfing virgin, not previously selfing virgin, previously selfing mated, and not previously selfing mated. Encounter times, evasive behaviours, and polarity of mating were recorded. Results suggested that the probability of initiating copulation as male in any encounter increased with autosperm store, and that snails mounted as female may decrease the overall probability of mating by rejective behaviour when their reserves of allosperm are high. There was no evidence that snails ever use any mechanism to increase their likelihood of being mounted as female. Evasive behaviours observed in encounters between virgin snails were best attributed to male-male gender conflict, but such behaviour patterns seemed to derive from sexual conflict when virgin snails encountered previously mated snails. Male-male gender conflict did not, however, detectably lengthen contact times, lead to bouts of sperm trading or ultimately lower mating success. (C) 1996 The Association for the Study of Animal Behaviour.