Host Sex Modulates the Energetics of Pathogen Proliferation and Its Dependence on Environmental Resources

AbstractSex differences in immunity are predicted to underlie much of the frequently observed sex differences in the prevalence or severity of infection. We propose the additional hypothesis that differences in the ability of males and females to acquire and use resources will also affect how readily a pathogen can convert host energy into transmission stages, thereby contributing to sex differences in infection dynamics. To test this we manipulated the resource environment of male and female Daphnia magna by altering the availability of food and then exposed hosts to a bacterial pathogen. We measured the production of transmission spores and virulence via the reduction in life span, together with feeding rates and changes in mass-independent metabolic rate, as a measure of the intake and expenditure of energy during infection. When raised in the presence of high resource levels, females more readily allowed for resources in the environment to be translated to pathogen exploitation, as represented by increased spore production, greater virulence, and higher energy use. In contrast, the traits of infected males were robust to changes in resource availability. High food availability thus exaggerated the degree of sexual dimorphism observed between the sexes. It also modified the relationship between host energy use, virulence, and pathogen spore production for each sex. These results suggest that a host's resource environment can affect how a male or female is exploited by a pathogen and may thus be an additional factor driving sex-specific patterns of disease susceptibility or severity.