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Abstract

Females can compete for mates and resources, resulting in differential reproductive success, and thus, experience sexual selection pressures. To examine the influence of sexual selection on female morphology, physiology, and behavior, I investigated spotted sandpipers (Actitis macularius), a bird species in which the typical ‘sex roles’ are reversed: females face greater competition for mates than males and males predominantly provide parental care. Specifically, I examined whether females have evolved ornamentation that signals individual quality to potential mates or female competitors. To do so, I considered degree of plumage ornamentation (i.e. ventral melanized spots present only during the breeding season), individual condition, reproductive success, and testosterone (a steroid hormone known to regulate male aggression in vertebrates). I determined that females were more ornamented than males in terms of spot size and percent plumage cover. Compared to less ornamented females, those with more ornamentation had fewer ectoparasites, were heavier, had higher hematocrit levels, and were more reproductively successful. Other than ectoparasite load, these patterns were not observed across males. Although 18% - 33% of females in the population were polyandrous, males and females were genetically monogamous to their current mate, perhaps due to the chance of nest abandonment by the male. Considering testosterone as a possible proximate mechanism of female aggression, males had more testosterone than females, but only during courtship, after which male testosterone levels dropped. Female testosterone levels, however, remained constant from courtship to incubation, potentially indicative of the continued competition that females experience throughout the breeding season. Furthermore, females challenged with a simulated territorial intrusion had higher testosterone levels than unchallenged females. In summary, female plumage pattern is an honest signal, likely communicating condition and reproductive ability to conspecifics during courtship or competitive female-female interactions. Additionally, testosterone appears to play a functional role in regulating female aggression.

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