Because of the differences between male and female physiology, males and females can have different reactions to the same parasite. These variable responses can have important consequences for parasite-host interactions and the overall transmission of the parasite. Take, for example, a relatively recent study of great tits (Parus major, a bird).
Dunn et al. (2011) explored the effect of malarial infection on the behavior of male and female great tits. While the differences between males and females are obvious in many species, sex-specific differences in behavior due to infection are not often investigated. Anyway, in their study, the authors captured wild birds, and compared the behavior of infected and uninfected birds, as well as a number of physiological measurements. A quick summary of the results: Infected males were more likely to problem solve (in this case pulling a lever to release food) than uninfected males, but infected females were less likely to problem solve than their uninfected counterparts. Infected females were more exploratory (exploration of a novel room) than uninfected females, but infection did not influence male exploratory behavior. Infected males were more risk averse (retreating from a disturbance – noise caused by stick movement) than uninfected males, but infection did not affect females.
What does all this mean, exactly? Well, one of two things: One, males who get infected differ from females who get infected. Or two, infection changes the behavior of males and females differently. The first scenario argues that infection does not change behavior, that certain behaviors instead make individuals more or less likely to become infected. According to Dunn et al. (2011), for example, problem solving males should be more adept at using different food resources, and exposure to those different food resources should expose the birds to more parasites than males who are less capable at problem solving and therefore use fewer food resource types. The second scenario asserts that infection itself changes behavior, either as a host response to the parasite or direct manipulation by the parasite. While one can generally use information about the biological system to hypothesize whether behavior causes infection or vice versa, the truly definitive way to test this question would be to do controlled infections and monitor behavior before and after infection.
Such distinctions, as well as male vs. female behavioral differences, can be important for a number of reasons. They might, for instance, affect species management programs. If one had a species infected with a parasite, and one thought that bold individuals were more likely to acquire and spread that parasite, one might imagine a culling or vaccination program that focused on bold individuals to decrease future parasite transmission. However, if infection makes individuals bold, this program could be relatively pointless. If one of the two sexes, moreover, is predominantly responsible for transmission of the parasite, one might focus isolation, vaccination, or culling efforts on that sex. One could probably list many other examples in which sex differences and the direction of causality in behavior-infection connections matter.
Or, one could end the blog. To finish, then: Sex matters.
Dunn, J.C., Cole, E.F. & Quinn, J.L. (2011). Personality and parasites: sex-dependent associations between avian malaria infection and multiple behavioural traits. Behavioral Ecology and Sociobiology, 65, 1459-1471.