The importance of elaborate secondary sex characteristics in female animals is often underestimated, and evolutionary mechanisms of these traits are not as well understood as they are in males. Mating competition is thought to be the main selective pressure acting on elaborate traits in males, but in most females, fertilizations are not limited. A more likely pressure acting on female reproductive output related to trait elaboration is same-sex competition for reproductive resources or opportunity. This body of work was guided by the main question: In species where both sexes exhibit the same elaborate secondary sex characteristic and display them at similar intensities, are the selective mechanisms the same or different between the sexes? Because the ability to answer this question is hampered by our lack of knowledge on in females, I also ask: What selects for elaborate traits in females? I studied birdsong, an elaborate secondary sex characteristic well-known in male birds. Here I examine singing behavior, physiological adaptation and reproductive output in stripe-headed sparrows, Peucaea r. ruficauda, a songbird in which both sexes sing. 1) I found evidence for strong selection on female song relative to males, when measured by repertoire complexity, size of song control regions in the brain and song output. Females also showed repertoires specialized for territory defense, and males showed repertoires specialized for mate attraction. 2) In a playback experiment, females defended territories more aggressively than males, and did so against the same sex. 3) Observational study showed that males sang more when they lacked a mate, while females sang more when they became territorial. 4) During same-sex territorial intrusion, older females sang more aggressively than younger females. Older females were also more likely to have access to good quality territories and were more likely to produce young. I conclude that in stripe-headed sparrows, selective mechanisms for the evolution of song differ between the sexes, largely because females gain reproductive advantage primarily by singing to compete with other females for reproductive resources or opportunity, while males likely gain reproductive advantage not only by singing to defend territories, but also by singing to attract mates.
|Advisor:||Beecher, Michael D.|
|School:||University of Washington|
|School Location:||United States -- Washington|
|Source:||DAI-B 73/07(E), Dissertation Abstracts International|
|Subjects:||Neurosciences, Evolution and Development, Behavioral Sciences|
|Keywords:||Birdsong, Peucaea r. ruficauda, Sexual selection|
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