Male-Biased Susceptibility to Helminth Infection: An Experimental Test with a Copepod

Abstract

Recent reviews on sex biases in parasitism found that males generally suffer from a slightly higher susceptibility to helminth infection than females. Sex hormones like testosterone have been suggested as the mechanism that causes the bias in parasite susceptibility. Many of the original studies used data that stem from free-ranging host populations which were naturally infected. Since gender-related behaviours could result in differential exposure to parasites, it is necessary to experimentally disentangle differences in exposure from differences in susceptibility. We tested whether we could find a sex bias in susceptibility under experimental conditions, i.e. by controlling for gender related differences in exposure. Furthermore, we used an invertebrate host to test whether the pattern observed in many vertebrates can be extended to a host species that lacks testosterone. As models we used the cestode Schistocephalus solidus and its first intermediate host, the copepod Macrocyclops albidus. We found that male copepods were more often infected than females. Since infected males had on average also more parasite larvae than infected females, the chance of a S. solidus larvae to become established in its first intermediate host was more than twice as high in male than in female copepods. This shows that, under experimental conditions, male-biased susceptibility to helminth infection can be very strong. Moreover, the pattern seen in many vertebrates can be extended to an invertebrate host that lacks testosterone.