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Abstract
Whereas several studies have shown that experimentally increased levels of the androgenic steroid testosterone can affect female behavior, fewer studies have focused on the activational effects of exogenous testosterone on female morphology. With respect to colorful displays in birds, almost exclusively the effects of testosterone manipulation on female carotenoid-based colorations have been studied. Other color types such as structural colors (i.e. UV, blue and violet colors that result from differential light reflection in the nanostructures of the tissue) remain largely unstudied. Here, we investigated the short- and long-term effects of exogenous testosterone on the expression of structural bare-part coloration in female budgerigars, Melopsittacus undulatus. In this parrot species, bare-part coloration is expressed in the cere, a structure over the beak which is brown in females and structural blue in males. We experimentally increased plasma testosterone levels in testosterone-treated females (T-females) compared to controls (C-females) and we performed weekly spectrophotometric measurements of the cere for five weeks after implantation and one measurement after ten weeks. We also estimated the extent to which testosterone masculinized female cere color by comparing the experimental females with untreated males. We found significant effects of testosterone on cere color from week four after implantation onwards. T-females expressed significantly bluer ceres than C-females with higher values for brightness and UV reflectance. T-female cere color, however, remained significantly less blue than in males, while values for brightness and UV reflectance were significantly higher in T-females than in males. Our quantitative results show that exogenous testosterone induces the expression of structural blue color in females but does not strongly masculinize female cere coloration. We provide several potential pathways for the action of testosterone on structural color.
Highlights
The effects of the androgenic hormone testosterone (T) on male traits such as ornamentation, courtship and mating behaviors have been well documented, in birds (e.g. [1,2,3])
We quantitatively investigated the activational effects of exogenous T on the expression of structural bare-part coloration in females of the budgerigar
Females from either experimental group did not differ in body weight (t15 = 20.32, P = 0.75) or in blue chroma of the cere (t15 = 20.90, P = 0.38), indicating that females were randomly distributed between both treatments
Summary
The effects of the androgenic hormone testosterone (T) on male traits such as ornamentation, courtship and mating behaviors have been well documented, in birds (e.g. [1,2,3]). Implantation studies have shown that exogenous T can enhance the expression of maletypical traits in females of many species (reviewed in [4]). In contrast to plumage coloration, bare-part colors can change rapidly in response to the current physiological condition of its bearer [15,16,17]. Being dynamic, such colorations can be used for example to continuously estimate mate quality, before as well as after pair formation [17]. Experimental studies have shown that bare-part color in males can change in response to a variety of cues, including increased T-levels, over short periods of two to four weeks In most studies the timing of Tinduced color changes have not been monitored in detail (but see [9] for an exception)
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