Infection by a fungal pathogen and mating behavior in Pacific treefrogs: a test of the Hamilton-Zuk hypothesis

Understanding why females prefer exaggerated male mating displays, when males offer little more than sperm in lek mating systems, has been an important challenge in behavioral ecology and evolutionary biology. The Hamilton-Zuk hypothesis proposes that the expression of mating displays is limited by parasitism and females choose parasite-resistant males that can produce costly, exaggerated displays. We used Pacific treefrogs ( Pseudacris regilla ) and natural infections by the fungal pathogen Batrachochytrium dendrobatidis ( Bd ), to test predictions of the Hamilton-Zuk hypothesis. Specifically, we predicted that (i) call properties, especially those that are more likely under selection by female choice, correlate with parasitic infection, and (ii) females discriminate against highly infected males based on the properties of their mating call. We found that energetically costly call properties that are likely under selection by female choice, like call rate and call effort, were not influenced by Bd infection. However, males with high levels of Bd infection produced calls with faster pulse rates than males with lower levels of infection. In two-choice tests, females did not show a preference between calls that resemble males with either high or low levels of Bd infection. Our results provide little support for the Hamilton-Zuk hypothesis and suggest that Bd infection may have little effect on female choice in natural settings. We discuss our results in terms of the effects of Bd and other parasites in anuran mating behavior. Significance statement This study shows that infection by a fungal pathogen responsible for world-wide amphibian loss may influence the expression of male mating displays in Pacific treefrogs. Importantly, we also show that females do not discriminate against mating calls typical of highly infected males. These results suggest that asymptomatic levels of infection may have little effect on mating behavior in natural settings. Furthermore, our study offers little support for the influential hypothesis that parasitism limits the expression of mating displays and females choose parasite-resistant males that can produce exaggerated displays.