Female song and aggression show contrasting relationships to reproductive success when habitat quality differs

Abstract

Though well studied in males, little is known about the factors influencing variation in expression of exaggerated traits such as intense aggression, elaborate ornaments, and lethal weaponry in females. Current research suggests that these traits are important when females compete for access to limited reproductive resources and that greater trait expression leads to higher reproductive success. However, contest theory predicts that differences in resource availability will alter the costs and benefits of competition and contest rules, potentially changing the strength or direction of selection. Female superb fairy-wrens, a common Australian passerine, compete for exclusive breeding territories using song and aggression. A previous study in a population residing in uniform, high-quality habitat found that strong responses to a simulated intruder were associated with improved reproductive success. Here, we determine whether differences in resource availability, i.e., habitat quality, are associated with changes to this relationship by replicating this study in a second population that resides in lower-quality, patchy habitat. We quantified female response (activity and song rates) to a simulated same-sex intruder and examine the relationships with territory quality and annual reproductive success. Contrary to previous research, we found that in low-quality, patchy habitat, stronger responders occupied poorer quality territories and had lower reproductive success. However, basal song rates and responses to an intruder were overall much stronger in low-quality habitat. These results suggest that female–female contest rules and the intensity of competition differ according to resource availability, which may alter how selection acts on female competitive traits. Females appear to use costly social traits, e.g., ornaments, armaments, complex song, and aggression, in the context of female–female competition for limited resources. However, very little is known about how changes in resource availability might alter female–female contest rules or the relationship between trait expression and fitness estimates. Previous research in a population of superb fairy-wren, a songbird, residing in high-quality habitat, found that female song and aggression were positively related to reproductive success. Here, we replicate that study in a population that resides in low-quality, patchy habitat. We found higher levels of aggression and song and that the relationship between behavior and fitness was in the opposite direction. This suggests that resource availability can affect female behavior, dramatically alter the strength and direction of selection, and may change the rules that females observe when engaging in contests.