Prum Richard O, Torres Rodolfo
Department of Ecology and Evolutionary Biology, and Natural History Museum, Dyche Hall, University of Kansas, Lawrence, KS 66045-7561, USA.
J Exp Biol. 2003 Jul;206(Pt 14):2409-29. doi: 10.1242/jeb.00431.
Structural colours of avian skin have long been hypothesized to be produced by incoherent (Rayleigh/Tyndall) scattering. We investigated the colour, anatomy, nanostructure and biophysics of structurally coloured skin, ramphotheca and podotheca from 31 species of birds from 17 families in 10 orders from across Aves. Integumentary structural colours of birds include ultraviolet, dark blue, light blue, green and yellow hues. The discrete peaks in reflectance spectra do not conform to the inverse fourth power relationship predicted by Rayleigh scattering. The dermis of structurally coloured skin consists of a thick (100-500 micro m) layer of collagen that is usually underlain by a layer of melanin granules. Transmission electron micrographs (TEMs) of this colour-producing dermal collagen layer revealed quasi-ordered arrays of parallel collagen fibres. Two-dimensional (2-D) Fourier analysis of TEMs of the collagen arrays revealed a ring of peak spatial frequencies in the spatial variation in refractive index that are the appropriate size to make the observed ultraviolet-yellow colours by coherent scattering alone. One species, Philepitta castanea (Eurylaimidae), has exceptionally ordered, hexagonal arrays of collagen fibres that produce a hexagonal pattern of spatial frequency peaks in the power spectra. Ultraviolet, blue, green and yellow structural colours of avian skin are produced by coherent scattering (i.e. constructive interference) by arrays of collagen fibres in the dermis. Some yellow and orange skin colours are produced with a combination of structural and pigmentary mechanisms. These combined colours can have reflectance spectra with discrete peaks that are more saturated than hues produced by carotenoid pigments alone. Bluish facial skin from two species of Neotropical antbirds (Thamnophilidae) are nanostructurally too small to produce visible light by coherent scattering, and the colour production mechanism in these species remains unknown. Based on the phylogenetic distribution of structurally coloured skin in Aves, this mechanism of colour production has evolved convergently more than 50 independent times within extant birds.
长期以来，人们一直推测鸟类皮肤的结构色是由非相干（瑞利/廷德尔）散射产生的。我们研究了来自鸟类10个目17个科的31种鸟类的结构色皮肤、喙和跗跖的颜色、解剖结构、纳米结构和生物物理学。鸟类的体表结构色包括紫外线、深蓝色、浅蓝色、绿色和黄色色调。反射光谱中的离散峰不符合瑞利散射预测的四次方反比关系。具有结构色的皮肤真皮由一层厚厚的（100 - 500微米）胶原蛋白组成，其下方通常有一层黑色素颗粒。对这种产生颜色的真皮胶原层的透射电子显微镜（TEM）图像显示，胶原纤维呈准有序排列。对胶原纤维阵列的TEM图像进行二维（2 - D）傅里叶分析，发现在折射率的空间变化中存在一个峰值空间频率环，其大小恰好能仅通过相干散射产生观察到的紫外线 - 黄色颜色。有一种鸟类，栗腹肉垂鸟（阔嘴鸟科），具有异常有序的六边形胶原纤维阵列，在功率谱中产生六边形的空间频率峰值图案。鸟类皮肤的紫外线、蓝色、绿色和黄色结构色是由真皮中胶原纤维阵列的相干散射（即相长干涉）产生的。一些黄色和橙色的皮肤颜色是由结构和色素机制共同作用产生的。这些组合颜色的反射光谱可能具有离散峰，比仅由类胡萝卜素色素产生的色调更饱和。两种新热带蚁鸟（蚁鸟科）的蓝色面部皮肤在纳米结构上太小，无法通过相干散射产生可见光，这些物种的颜色产生机制仍然未知。基于鸟类中具有结构色皮肤的系统发育分布，这种颜色产生机制在现存鸟类中已经独立进化了50多次。
