Female ornamentation does not predict aggression in a tropical songbird

Abstract

Although conflict is often adaptive and necessary to secure limited resources, it is also frequently a costly endeavor. Signals that reliably communicate competitive ability are commonly employed by animals to reduce conflict costs. Both male and female signals have the capacity to serve as honest indicators of competitive ability, but the extent to which this occurs in females has received relatively limited attention. We studied how natural feather coloration and experimentally manipulated plumage ornamentation in female white-shouldered fairywrens (Malurus alboscapulatus moretoni) relates to their aggressive behavior. These fairywrens are a useful study system in that female, but not male coloration varies throughout New Guinea. We tested behavioral responses to simulated rivals both prior to and after plumage manipulation via two distinct behavioral assays: simulated territorial intrusions (where both sexes jointly respond to rival intruders) and mirror image simulation (where females are isolated from their mate). Plumage manipulation treatments had no measurable impact on female aggression during mirror image simulation tests, though aggression did decrease over the course of multiple assays. Similarly, using simulated territorial intrusion assays, we found no difference in female aggression with respect to both natural coloration and manipulation treatment. Finally, our correlative analysis of natural feather color also revealed no detectable relationship. These findings suggest that female white scapular coloration is unlikely to function as a signal of competitive ability in this tropical species with derived female ornamentation and we discuss possible alternative explanations.

Significance statement

Like their showy male counterparts, female animals may use plumage-based signals to mediate aggressive encounters. Determining if and how signals used in female-female competition may mitigate the costs of agonistic encounters is particularly important, as it is thought to be a key driver of female signal evolution. We used a repeated-measures design and two distinct behavioral assays to explore how both manipulated and natural feather coloration relate to aggression in female white-shouldered fairywrens of Papua New Guinea – a species with female, but not male, plumage polymorphisms. We were unable to detect any differences in aggression with respect to female plumage coloration despite two distinct behavioral assays that mimic distinct social contexts, suggesting that female plumage coloration may function in other contexts or lack a modern signaling function altogether in this system.