Sex 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.

The resource acquisition of male and female Daphnia magna was manipulated by altering the availability of food, and then hosts were exposed to the bacterial pathogen Pasteuria ramosa. Via infection trials, traits related to pathogen fitness, such as the production of mature transmission spores and virulence, together with infection-induced changes in host metabolic rate as a measure of the way a host might use and processes energy during infection were measured. In a follow-up experiment, we quantified how differences in energy intake, as estimated via feeding trials, might help explain the observed patterns of pathogen spore production, virulence, and energy expenditure.

Pathogen proliferation and infection induced changes in host traits

The influence of host sex and resource availability on: A) the production of transmission spores (Transmission_spores); B) virulence as estimated by the reduction in host lifespan caused by infection (Reduction_lifespan); and relative to controls, C) the change in host body size caused by infection (Change_bodysize); and, D) the increase in mass-independent metabolic rate of the host following infection (Rel_residual_mr).  

infection-induced-changes-data-dryad.csv

Response surface data

Data describing how the standardised reduction in host lifespan caused by the pathogen (Zvirulence) and the standardised increase in mass-independent host metabolic rate following infection (Zrel_resid_mr) predict relative pathogen spore production (Wspores) for both males and females in high and low resource environments.  

response-surface-data-dryad.csv

Feeding rate data

Data describing how the influence of host sex, infection treatment, and resource availability on feeding rates. 

feeding-data-dryad.csv