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 parasit e on mate choice and precopula guarding in its crustacean host Gammarus due beni. Males guard females in precopula for several days before mating at th e time of egg laying. They choose females on the basis of size and time to laying. We investigated two alternative hypotheses. (1) The parasite manipu lates host behaviour leading to increased pairing success for infected fema les. (2) Male G. duebeni avoid the costs of parasitism by mating selectivel y with uninfected females. We also compared the morphology and activity of uninfected and infected females. We found no evidence for parasite manipula tion of host mating success. Pairing success did not differ between uninfec ted and infected females at the point of collection in the field. There was no difference in the morphology or activity of uninfected and infected fem ales and infected females showed no changes in their response to male mate- guarding attempts. We also found little evidence for selective mating by ma les with uninfected females. Males showed no preference for uninfected vers us infected mates in laboratory choice tests. However, males guarded uninfe cted females for longer in laboratory conditions, suggesting that males ale willing to invest more in guarding uninfected mates. Our results are in ac cordance with previous predictions of reduced virulence for vertically tran smitted parasites. (C) 2001 The Association for the Study of Animal Behavio ur.