Aggressive competition and the signals used during aggressive competition have historically been considered characteristics of male animals produced by sexual selection. Nevertheless, female competitive traits are common across a wide variety of taxa. In most cases, these female traits are understudied. A deeper understanding of how these competitive traits function in female lives is necessary to understand how these traits evolve.Here I explore two understudied competitive traits in female house wrens (Troglodytes aedon), female song and intrasexual aggression. These chapters are connected by three unifying themes: (1) these competitive traits are not merely byproducts of selection on males but instead play functional roles in female lives, (2) although these competitive traits have some similarities to the same traits in males, they are best understood by explicitly considering the contexts in which females compete and the resources that affect female fitness, and (3) (in the case of intrasexual aggression) the same female trait can influence multiple components of female fitness. In the first half of this dissertation, I examine the form and function of female song and vocalizations that may play a role in female competition. In Chapter 1, I demonstrate that female songs likely play a functional role when defending against conspecific intruders that may usurp the nest cavity, destroy eggs, or threaten future male assistance. In Chapter 2, I examine how particular vocalizations are used during contests with simulated female intruders. I show that a particular call is a signal of aggressive intent. These results collectively suggest that, not only do these vocalizations have a functional role for female house wrens, but aggressive interactions between rival females may affect female fitness under natural circumstances. In the second half of this dissertation, I focus explicitly on intrasexual aggression. In Chapter 3, I demonstrate that females are capable of adjusting their aggressive responses in ways that are likely adaptive. I show that females likely value continued access to the nesting cavity more highly than they value direct and indirect benefits offered by males when challenged by simulated female intruders during egg laying. I investigate competition for nesting cavities more explicitly in Chapter 4 by experimentally enhancing the number of competitors searching for cavities. I find that more aggressive females are more likely to win contests over cavities and are more successful at protecting their eggs from intruding conspecifics when competition is intense. In Chapter 5, I look beyond the first half of the nesting cycle to examine the reproductive consequences of female intrasexual aggression throughout the season. In addition to benefits accrued during resource competition, I find that more aggressive females have significantly heavier offspring , a common proxy for offspring quality, even if brood size is experimentally enhanced. More aggressive females are also more likely to raise nestlings to fledging. However, aggressive adults may be less likely to return to the study site in the subsequent year. Taken together, these results suggest that female aggression and female song (which is used in aggressive competition) have important functions and may be under selection in house wrens. While certain benefits (e.g. competition for breeding territories) are likely shared between the sexes, the data suggest that other benefits (e.g. enhanced offspring size) are unique to females. This work highlights the importance of explicitly considering the contexts in which females compete and the resources that influence female fitness when studying intrasexual competition among females.
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