Hormonal regulation of alternative reproductive strategies

Abstract

Variation in reproductive behavior permits individuals to optimize fitness given their genetics, physiological condition, social status, age and current environmental conditions. For species with alternative reproductive strategies, behavioral differences are driven by natural and sexual selection and further mediated by the endocrine system. In this study, I explore endocrinological mechanisms associated with alternative reproductive strategies in the ruff (Philomachus pugnax). Ruffs breed on leks, where males use one of three genetically determined alternative reproductive strategies or phenotypes. Resident males rely on male-male aggression, colorful plumage, large body size and intensive display activity to establish courts on a lek and attract females. Satellite males operate outside the dominance hierarchy of the lek, relying on semi-cooperative displays with residents, lighter display plumage coloration and agility to access receptive females. Males using the third male strategy, considered 'sneakers' or 'female mimics', completely forgo typical male breeding plumage, territoriality and obvious display behaviors to obtain clandestine access to females. The differences in level of aggression and display behaviors among phenotypes suggest predictable differences in steroid hormone concentrations, which play a critical role in breeding behavior in birds. I therefore examined seasonal hormone profiles of male ruffs and applied predictions from the challenge hypothesis, which posits circulating androgen concentration will vary in relation to intensity of male-male competition during the breeding season. Consistent with the challenge hypothesis, androgen levels were higher during periods of lek instability in territorial resident ruffs. In contrast, when housed in large flocks, androstenedione was elevated during the breeding season in satellite ruffs and followed the pattern typically observed for testosterone in polygamous birds. In response to distinctly different seasonal androgen and androstenedione profiles, I further examined the activational role of androstenedione on satellite male breeding behavior. Androstenedione stimulated both resident and satellite typical behaviors consistent with genetic morph, and thus did not specifically activate the satellite phenotype.