Abstract
BackgroundSexual signals, such as bright plumage coloration in passerine birds, reflect individual quality, and testosterone (T) may play a critical role in maintaining signal honesty. Manipulations of T during molt have yielded mixed effects on passerine plumage color, in most cases delaying molt or leading to production of drab plumage. However, the majority of these studies have been conducted on species that undergo a post-nuptial molt when T is low; the role of T in species that acquire breeding plumage during a pre-nuptial molt remains largely unexplored.Methodology/Principal FindingsWe experimentally tested the effects of increased T on plumage color in second-year male red-backed fairy-wrens (Malurus melanocephalus), a species in which after-second-year males undergo a pre-nuptial molt into red/black (carotenoid and melanin-based) plumage and second-year males either assume red/black or brown breeding plumage. T treatment stimulated a rapid and early onset pre-nuptial molt and resulted in red/black plumage acquisition, bill darkening, and growth of the sperm storage organ, but had no effect on body condition or corticosterone concentrations. Control males molted later and assumed brown plumage. T treated males produced feathers with similar but not identical reflectance parameters to those of unmanipulated after-second-year red/black males; while reflectance spectra of red back and black crown feathers were similar, black breast feathers differed in UV chroma, hue and brightness, indicating a potentially age and plumage patch-dependent response to T for melanin- vs. carotenoid-pigmentation.Conclusions/SignificanceWe show that testosterone is the primary mechanism functioning during the pre-nuptial molt to regulate intrasexually variable plumage color and breeding phenotype in male red-backed fairy-wrens. Our results suggest that the effects of T on plumage coloration may vary with timing of molt (pre- vs. post-nuptial), and that the role of T in mediating plumage signal production may differ across age classes, plumage patches, and between pigment-types.
Highlights
Condition-dependent regulation of sexually selected display traits is a critical assumption of honest advertisement models of sexual selection, as these traits impact the frequency and outcome of social interactions and can determine reproductive fitness [1,2] Testosterone (T) regulates many male reproductive traits and is a potential mechanism for enforcing signal honesty [3] as heightened T can carry concomitant costs (immunity [3], metabolism [5], behavior [6]) that may differentially effect fitness depending on male quality [7,8,9]
We show that testosterone (T) is the primary mechanism functioning during the pre-nuptial molt to regulate variable plumage color and breeding phenotype in male red-backed fairywrens
Alterations induced by T in young (SY) male plumage color and morphology preceded but closely mirrored seasonal changes documented in un-manipulated older (ASY) males
Summary
Condition-dependent regulation of sexually selected display traits is a critical assumption of honest advertisement models of sexual selection, as these traits impact the frequency and outcome of social interactions and can determine reproductive fitness [1,2] Testosterone (T) regulates many male reproductive traits and is a potential mechanism for enforcing signal honesty [3] as heightened T can carry concomitant costs (immunity [3] (but see [4]), metabolism [5], behavior [6]) that may differentially effect fitness depending on male quality [7,8,9]. Some recent studies have focused on the role of T in regulating color patterns of male birds in the order Passeriformes, where the selective advantages and condition dependence of plumage elaboration has received extensive research interest. In these studies, the effect of T on control of intrasexually variable passerine plumage color has found mixed support (supporting: [15,16,17], opposing: [18,19,20,21]). The majority of these studies have been conducted on species that undergo a post-nuptial molt when T is low; the role of T in species that acquire breeding plumage during a pre-nuptial molt remains largely unexplored
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