Body condition influences sexual signal expression independent of circulating androgens in male red-backed fairy-wrens

Abstract

Androgens play a major role in the regulation of sexual signal expression of male vertebrates. In this study we assessed the prevalent, yet largely untested, assumption that signal honesty is maintained through condition-dependent androgen regulation by experimentally manipulating body condition of male red-backed fairy-wrens (Malurus melanocephalus) through trimming several flight feathers before the prenuptial molt. In their first reproductive season males of this species exhibit androgen-regulated plasticity in plumage coloration, ranging from red/black (high androgens) to brown (low androgens). Red/black plumage is preferred by females and might be constrained by a negative relationship between body condition and androgen levels. We also evaluated whether corticosterone changes to altered conditional state mediate the relationship between condition and androgens. While we predicted that males with trimmed feathers would expend greater energy and thus be in poorer condition at the time of molt, they were counter-intuitively in better condition compared to control birds, likely as a consequence of subtle behavioral changes. These birds in better condition molted a greater proportion of red/black plumage, as predicted, and also molted more heavily. However, experimental and control birds did not differ in their androgen or corticosterone concentrations. Furthermore, analysis of long-term data from the same population revealed no correlation between condition and androgen levels. Collectively, these results challenge the notion that condition-dependent androgen regulation alone is responsible for maintaining the honesty of sexual signals and highlights the necessity of considering alternate explanations.