Data from: The effect of colour producing mechanisms on plumage sexual dichromatism in passerines and parrots

1.Sexual dichromatism (SD) often reflects intense sexual selection on males. It has been hypothesised that sexual selection should favour the elaboration of those male colours that honestly signal quality, and that such colours should therefore show higher SD. Costliness of colours is expected to vary according to their production mechanism (pigment type, feather microstructure, and combinations thereof). Carotenoid-based colours, due to their dietary origin and competing functions of carotenoid pigments, are the best documented costly colours, while endogenous pigments and structural colours are expected to be less costly. However, how SD varies with colour producing mechanism has not been systematically addressed.

2.Here we test the link between SD and mechanisms of colour production across nearly all Australian passerines (n=302) and parrots (n=48), using reflectance spectrometry and visual models. We quantified SD (chromatic [hue and saturation] and achromatic [light to dark]) for individual plumage patches as well as for the whole plumage. We then compared whether these SD estimates vary for plumage produced by different mechanisms of colour production.

3.Not unexpectedly, individual plumage patches were most sexually dichromatic when males and females have different mechanisms of colour production. In most cases however, both sexes share the same mechanism. Among these, red carotenoid-based plumage in passerines showed the highest SD, while in passerines as well as parrots, melanin-based and white plumage have the lowest levels of SD.

4.Average SD within species is weakly predicted by the proportion of the plumage produced by different mechanisms. This effect is strongest for red carotenoids, but equally so for males and females. In contrast, effects of structural and pheomelanin-based colours are different between sexes, highlighting the links between high SD and the evolution of different mechanisms of colour production in males and females.

5.While our analysis showed that colours based on red carotenoids have higher levels of SD, the differences were small, and most mechanisms of colour production were able to produce a similar range of SD. These results do not support the hypothesis that sexual selection should preferentially lead to the elaboration of some types of colours over others.

1. 性二色性（sexual dichromatism, SD）常反映雄性受到强烈的性选择。已有假说提出，性选择会青睐那些能够可靠传递个体质量信号的雄性色彩演化，因此这类色彩应表现出更高的性二色性程度。色彩的生产成本会随其产生机制（色素类型、羽毛微观结构及其组合）而有所差异。类胡萝卜素基色彩（carotenoid-based colours）因其膳食来源以及类胡萝卜素色素的竞争性功能，是目前证据最为充分的高成本色彩；而内源性色素与结构色（structural colours）则被认为生产成本较低。不过，性二色性如何随色彩产生机制发生变化，尚未得到系统性的探讨。
2. 本研究针对澳大利亚几乎全部鸣禽（passerines，n=302）与鹦鹉（parrots，n=48），借助反射光谱法（reflectance spectrometry）与视觉模型（visual models），检验性二色性与色彩产生机制之间的关联。我们量化了单个羽毛区域以及整体羽毛的性二色性，包括色度（色相与饱和度）与非色度（明暗程度）。随后我们比较了这些性二色性评估值在不同色彩产生机制生成的羽毛间是否存在差异。
3. 不出所料，当雌雄个体拥有不同的色彩产生机制时，单个羽毛区域的性二色性程度最高。但在多数情况下，两性共享相同的色彩产生机制。在此类情形中，鸣禽的红色类胡萝卜素基羽毛表现出最高的性二色性程度；而鸣禽与鹦鹉中，黑色素基羽毛与白色羽毛的性二色性程度最低。
4. 不同色彩产生机制所生成羽毛的占比，仅能微弱预测物种内部的平均性二色性程度。这一效应在红色类胡萝卜素基色彩中最为显著，且在雌雄两性中表现一致。与之相反，结构色与褐黑素基色彩在两性间的效应存在差异，这凸显了高程度性二色性与两性间不同色彩产生机制演化之间的关联。
5. 尽管我们的分析显示，基于红色类胡萝卜素的色彩具有更高的性二色性程度，但二者间的差异较小，且多数色彩产生机制都能产生范围相近的性二色性程度。这些结果并不支持"性选择会优先促进某些类型色彩的演化增强"这一假说。