| In rodents, chemical signals provide reliable indicators of a male's current infection status, and females prefer the odor of healthy, uninfected males. For example, I have recently found that wild male mice ( Mus musculus domesticus) increase the conspicuousness and attractiveness of their scent marks when they perceive mating opportunities, but reduce their scent marking displays when their immune responses are experimentally activated. Furthermore, I found that females are better able to distinguish between the scent marks of experimentally infected versus healthy, uninfected males when males are displaying conspicuously. These results suggest that exaggerated scent marking is a costly sexual display that attracts females because it honestly signals an individual's health (the immunocompetence handicap hypothesis or IHH). The IHH assumes that there is a fitness tradeoff between immune resistance, on one hand, and exaggerated sexual displays on the other so that only the most disease-resistant males can afford the costs of displaying. This assumption, however, has never been tested. The research in my thesis outlines several specific objectives designed to experimentally test the IHH, and to determine how and why a male's odor honestly signals health. First, I tested whether the chemical signals of male house mice provide honest indicators of their 'medical history,' or their genetic resistance to infectious diseases. Second, I tested the tradeoff assumption by manipulating a male's investment into mating effort and scent marking displays, and then measured the effect on immunocompetence. Third, I determined whether testosterone plays a role in modulating a male's chemosensory displays and immunocompetence. Finally, I examined the ecological relevance of these findings and tested whether females actively prefer to mate with healthy versus infected males. These experiments have important implications for the immunocompetence handicap hypothesis, a proposed mechanism to explain how extravagant secondary sexual traits honestly reflect a male's resistance to infectious diseases. |
