PROXIMATE CAUSES AND ULTIMATE CONSEQUENCES OF PHENOTYPIC VARIATION IN MALE RED-BACKED FAIRY-WRENS

Douglas Garr Barron

Sexual signals are often plastic, and should therefore be adjusted to match an individual's phenotype and optimize lifetime fitness. However, little is known of physiological mechanisms permitting such phenotypic integration, or of evolutionary mechanisms maintaining sexual preferences for plastic traits. We attempted to fill these knowledge gaps through experimental and correlative investigations of the red-backed fairy-wren (Malurus melanocephalus), an Australian songbird in which males exhibit discrete breeding phenotypes varying in attractiveness, androgen concentrations, and fitness (high to low): red/black breeder, brown breeder, or non-breeding brown natal auxiliary. We first experimentally manipulated male condition to test whether honesty of red/black plumage is maintained through condition-dependent androgen regulation. Experimental birds in better condition acquired more red/black plumage, as predicted. However, experimental and control birds did not differ in androgen concentrations, and long-term data from the same population revealed no correlation between condition and androgen levels. We then investigated whether morphological and behavioral traits are correlated, and if so, whether this phenotypic integration could arise from pleiotropic actions of androgens. Red/black breeders invested more in mating behaviors and less in parental behaviors than brown breeders and auxiliaries. However, injection with GnRH failed to expose phenotype-specific constraints on androgen production, and individual behaviors were unrelated to baseline or GnRH-induced androgen levels. Finally, we examined the fitness benefits underlying female preferences for and investment in red/black males. Females paired with red/black males began breeding earlier, fed nestlings more, and produced more offspring than those paired with brown males. Despite similar pre-breeding energy stores and markedly higher reproductive investment, females with red/black males finished breeding with greater energy stores, survived at higher rates, and had higher lifetime fitness. Moreover, females breeding earlier with red/black males had more grandchildren because their sons were more likely to display attractive plumage and sire offspring. Collectively these results demonstrate that male sexual phenotype is honestly maintained through condition-dependent signaling, though circulating androgens likely do not mediate this link or directly underlie phenotypic differences in behavior. It is clear that females receive direct reproductive, survival, and inclusive fitness benefits by pairing with and investing heavily in red/black males.

PROXIMATE CAUSES AND ULTIMATE CONSEQUENCES OF PHENOTYPIC VARIATION IN MALE RED-BACKED FAIRY-WRENS

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