Adaptive significance and contemporary function of female ornamentation in white-shouldered fairywrens
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Description
A comprehensive understanding of sex-differences in the expression of competitive phenotypes depends on insight into both the female and male perspective. Yet, relative to the male perspective, research that explores the evolution of variable competitive phenotypes in female animals remains comparatively limited. For example, females may use visual and/or acoustic signals to preemptively settle would-be agonistic contests, as these traits may reliably indicate her competitive ability. Agonistic encounters are costly endeavors that risks injury or even death to combatants. As such, selection should favor a mechanism to preemptively settle contests without the need to escalate. Moreover, individuals should have the capacity to recognize variation in the severity of threats perceived and adjust accordingly. My dissertation employs field-based observational and experimental techniques to address these understudied themes in a tropical songbird endemic to New Guinea: white-shouldered fairywrens (Malurus alboscapulatus). These songbirds are ideal for exploring female trait evolution, as female, but not male, plumage coloration varies allopatrically, such that females are fully unornamented (i.e., brown, and thus, populations are highly dichromatic) in one population but may be fully ornamented (black-and-white; thus, nearly monochromatic) in another. First, I found evidence that a population of fairywrens with females that display ornamentation are also more aggressive, sing more complex songs, and have shorter tails (with shorter tails being a putative signal used in dominance contests). Thus, the most parsimonious explanation for the evolution of four phenotypes commonly associated with increased competition is elevated social selection. However, I was unable to detect a relationship between behavior and plumage coloration in either natural or experimental plumage analyses. Finally, I found that females modulate their aggression level based upon the nature of threat perceived, such that females appear to flexibly decrease aggression when the threat is less significant and/or when responding alongside her mate. Taken together, my research highlights that female trait variation is in response to female-specific selection. Female plumage coloration is not a competitive signal in female white-shouldered fairywrens, but the competitive environment nevertheless appears variable enough to maintain behavioral flexibility in females as a possible mechanism to reduce risk-taking aggression.