Do I Sound Sick? Condition-Dependent Advertisement Signals in Naturally Infected Frogs

Abstract

Males of many frog and toad species advertise in leks. In these systems, female choice is based on male advertisement calls, which have been selected to convey information about an individual's quality. As such, calling behavior is an important aspect of reproductive fitness. Factors such as disease and infection can affect calling behavior, yet the direction and strength of these effects, as well as their underlying mechanisms, remain unclear. Calls are typically multicomponent displays, and traits within these displays can vary independently from one another both among and within individuals. It is important to understand the proximate infection-imposed effects on signal production, as it allows us to make inferences about the downstream evolutionary consequences of such signaling. We studied the effects of Batrachochytrium dendrobatidis (Bd) infection on spring peeper (Pseudacris crucifer) advertisement calling behavior. We predicted that information about infection status would be present in dynamic traits (i.e., those that have high within-individual variation) such that males with higher Bd infection loads would exhibit call traits less attractive to females. Overall, infection had little effect on male calling. There was no main effect of infection on call rate or dominant frequency. However, Bd infection did have body condition-dependent effects on call duration, a trait with intermediate levels of within-individual variation. As infection loads increased, males in better-than-average condition tended to have longer call durations, while males in worse-than-average conditions tended to have shorter call durations. Our results suggested that some males are more behaviorally tolerant than others when infected, meaning they invest more energy into current reproductive efforts (calling behavior) compared to future ones (fighting off disease). This may have interesting implications for the potential evolution of signals within these populations, as sexual selection for more attractive calls may also be selecting for more infected males.