The sicker sex is plastic: Thermal plasticity determines sex biases in pathogen transmission

Sex differences are predicted to play an important role in the spread and evolution of pathogens. However, attempts to generalize the “sicker” sex have been challenged by intraspecific variability of sex biases across the infection process. Sex-specific plasticity provides a framework to resolve this by elucidating how infection is shaped at the sex-pathogen-environment interface. Using the Daphnia magna and Pasteuria ramosa system, we measure infection outcomes for males and females across three temperatures and seven pathogen densities to quantify how sex-specific plasticity shapes susceptibility, pathogen loads, and ultimately transmission. We find unique forms of plasticity at each stage of infection – including equivalent, sex-specific, and divergent plasticity. Integrating these into a single estimate of transmission reveals a clear pattern—male-biased transmission at cold temperatures, and female-biased transmission at warm temperatures. Sex-specific thermal plasticity can thus determine the “sicker” sex, with implications for pathogen spread and evolution in a warming world.