Assortative pairing by parasitic prevalence in Gammarus insensibilis (Amphipoda): patterns and processes

Abstract Field data and experiments on the presence of parasites and mate choice in Gammarus insensibilis are presented in an attempt to identify the processes responsible for assortative pairing by parasitic prevalence. Males and females captured in precopula in the field were matched for parasitic prevalence, confirming previous observations. Experiments in the laboratory indicated that assortative pairing by prevalence was largely due to the vertical segregation of infected and uninfected individuals in the water column, resulting from the behavioural alterations induced by the parasite Microphallus papillorobustus (Trematoda, Microphallidae) on its gammarid host. Assortative pairing by parasitic prevalence was not observed when water depth was experimentally reduced to a few centimetres. However, assortative pairing by size, a common pattern of pairing in gammarids, was observed whatever the water depth. Choice experiments, in the absence of vertical segregation, showed that males were choosing females on the basis of time left to moult, but were also capable of discriminating between infected and uninfected females. In addition, infected males had a longer latency to precopula during choice tests. The results are discussed in relation to current ideas on how parasites can affect mate choice.