Data and code from: Living in good company: Male-biased sex ratios reduce infection prevalence in females.

Males and female differ in how they move, aggregate, and interact with one another, generating sex ratios that vary across space and time. While an increasing body of both empirical and theoretical work has been framed around the idea of a “sicker sex,” with one sex bearing a higher burden of disease, rarely have we considered what it means for pathogens to encounter host populations where sex ratios are in constant flux. Using a suite of foraging models, we first explored how changes in sex ratios rapidly restructure the availability of resources versus pathogens in the pool hosts collectively draw from, and the consequences this has for altering infection risk and pathogen burden. We then tested these predictions using the filter-feeding host Daphnia magna and its bacterial pathogen Pasteuria ramosa, in experimentally manipulated populations. We find that effects of dynamic sex ratios on disease dynamics can be felt immediately, via competition and ecological feedback between the sexes, over resources. Females experienced lower infection rates at the cost of higher spore yields in male biased populations. Males showed little change in the opposite direction, leading to rapid changes in both the expression of sexual dimorphism in disease related traits as well as which sex fares better or worse. Overall, our results show how the identity of the “sicker sex” can emerge dynamically, as sex ratios shift and ecological feedback arise, highlighting the importance of considering the impact each sex has on others in a population.