Mate choice and mate guarding under the influence of a vertically transmitted, parasitic sex ratio distorter

We investigated the effects of a vertically transmitted, feminizing parasite on mate choice and precopula guarding in its crustacean host Gammarus duebeni. Males guard females in precopula for several days before mating at the time of egg laying. They choose females on the basis of size and time to laying. We investigated two alternative hypotheses. (1) The parasite manipulates host behaviour leading to increased pairing success for infected females. (2) Male G. duebeni avoid the costs of parasitism by mating selectively with uninfected females. We also compared the morphology and activity of uninfected and infected females. We found no evidence for parasite manipulation of host mating success. Pairing success did not differ between uninfected and infected females at the point of collection in the field. There was no difference in the morphology or activity of uninfected and infected females and infected females showed no changes in their response to male mate-guarding attempts. We also found little evidence for selective mating by males with uninfected females. Males showed no preference for uninfected versus infected mates in laboratory choice tests. However, males guarded uninfected females for longer in laboratory conditions, suggesting that males ale willing to invest more in guarding uninfected mates. Our results are in accordance with previous predictions of reduced virulence for vertically transmitted parasites. (C) 2001 The Association for the Study of Animal Behaviour.