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, 6 (10), e26067

Sexually Selected Male Plumage Color Is Testosterone Dependent in a Tropical Passerine Bird, the Red-Backed Fairy-Wren (Malurus Melanocephalus)

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Sexually Selected Male Plumage Color Is Testosterone Dependent in a Tropical Passerine Bird, the Red-Backed Fairy-Wren (Malurus Melanocephalus)

Willow R Lindsay et al. PLoS One.

Abstract

Background: Sexual 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 findings: We 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/significance: We 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.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Treatment effects on androgen metabolite concentrations (A), cloacal protuberance volume (B), and bill color (C).
Changes in mean response ± SE for second-year testosterone (T) and control implanted males, and unmanipulated after-second year (ASY) males at implantation, the mid-treatment recapture, implant removal, and the post-treatment recapture.
Figure 2
Figure 2. Treatment effects on molt score (A) and extent of nuptial plumage coloration (% red/black; B).
Changes in mean response ± SE for second-year testosterone (T) and control implanted males, and unmanipulated after-second year (ASY) males at implantation, the mid-treatment recapture, implant removal, and the post-treatment recapture.
Figure 3
Figure 3. Reflectance spectra of back (A), breast (B), and crown (C) feathers.
Lines indicate mean reflectance from testosterone (T, N = 6), control (N = 6), and after-second year (ASY, N = 4) males from whom feathers were collected at implant removal.

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