Sex-biased transmission of a complex life-cycle parasite: Why males matter

Male-bias in parasite infection exists in a variety of host-parasite systems, but the epidemiological importance of males and, specifically, whether males are responsible for producing a disproportionate amount of onward transmission events (male-biased transmission) has seldom been tested. The primary goal of our study was to experimentally test for male-biased transmission in a system with no sex-biased prevalence. We performed a longitudinal field experiment and continuously removed intestinal nematode parasites from either male or female white-footed mice and recorded the subsequent transmission among the untreated sex. We predicted males are responsible for the majority of transmission and female mice would have lower infection prevalence under the male-anthelmintic treatment than controls and that male mice would experience little or no change in infection prevalence under female-anthelmintic treatment compared to controls. Our second goal was to evaluate physiological hypotheses relating to the mechanisms that could generate the observed transmission pattern. To that end, we examined a cross-sectional sample of hosts to explicitly test for differences in parasite intensity, parasite egg shedding rate and reproductive output per parasite between male and female hosts. Removing parasites from male mice resulted in lower infection rates among female mice but, in contrast, there was no effect of female-deworming on infection rates among male mice; providing evidence that males provide disproportionately greater numbers of transmission events than females. We found no difference in prevalence, intensity, or fecundity of parasites between sexes in the cross-sectional sample of mice and rejected the mechanistic hypotheses. Without male-biased prevalence, intensity, or parasite fecundity, we concluded that male-biased transmission is unlikely to be created via physiological differences and the parsimonious explanation is that male behavior spreads infective stages in a more successful manner. We demonstrate that transmission heterogeneities can exist in the absence of individual heterogeneities in infection.