Effect of immune challenge on aggressive behaviour: how to fight two battles at once

Animals are attacked by pathogens, predators and competitors, sometimes simultaneously. For example, male crickets fight each other for access to females and such challenges may occur when one or both males are also defending themselves against a pathogen. We tested whether males altered their aggressive motivation when faced with both a competitor and a pathogen simultaneously. We found that immune-challenged male crickets (Gryllus texensis) were more likely to defeat male competitors when the reproductive payoff (i.e. a female) was nearby. However, when females were not present, males showed a decrease in their aggressive behaviour, and were less likely to win encounters with control males. This plasticity of aggressive motivation did not occur when males were chronically immune challenged. Chronically immune-challenged males were more likely to be defeated by control males regardless of the presence of a female. These results suggest that the costs associated with a chronic immune challenge may prevent terminal reproductive investment. We show that males can benefit from terminal reproductive investment because acutely immune-challenged males were still capable of mating and females did not appear to discriminate against them. Male crickets appear to engage in a context-dependent trade-off between the ability to defeat pathogens and the ability to defeat competitors. This trade-off is mediated, in part, by changes in male aggressive motivation (i.e. the critical threshold for the decision to flee).