ALTERNATIVE HYPOTHESES LINKING THE IMMUNE-SYSTEM AND MATE CHOICE FOR GOOD GENES

Why do males often have extravagant morphological and behavioural trai ts, and why do females prefer to mate with such males? The answers hav e been the focus of considerable debate since Darwin's The descent of man, and selection in relation to sex appeared in 1871. Recently, the broadening of investigation to include fields outside evolutionary bio logy has shed new light on mate choice and sexual selection. Here, we focus on a specific set of hypotheses relating the biology of resistin g disease-causing organisms with the production of condition-dependent sexual signals (advertisements). We present a framework that distingu ishes three different hypotheses about trade-offs within the immune sy stem that affect general condition. The original Hamilton & Zuk hypoth esis suggests that hosts fight off disease via resistance to particula r pathogens, which lowers resistance to other pathogens. Changes in pa thogens over evolutionary time in turn favours changes in which genes confer the best resistance. Alternatively, the immunocompetence hypoth eses suggest that the energetic costs of mounting a response to any pa thogen compete for resources with other things, such as producing or m aintaining advertisements. Finally, improving resistance to pathogens could increase the negative impacts of the immune system on the host, via immunopathologies such as allergies or autoimmune diseases. If bot h disease and immunopathology affect condition, then sexual advertisem ents could signal a balance between the two. Studies of hypothesized l inks between genes, condition, the immune system and advertisements wi ll require careful consideration of which hypothesis is being consider ed, and may necessitate different measures of immune system responses and different experimental protocols.