Colour-producing β-keratin nanofibres in blue penguin (Eudyptula minor) feathers

The colours of living organisms are produced by the differential absorption of light by pigments (e.g. carotenoids, melanins) and/or by the physical interactions of light with biological nanostructures, referred to as structural colours. Only two fundamental morphologies of non-iridescent nanostructures are known in feathers, and recent work has proposed that they self-assemble by intracellular phase separation processes. Here, we report a new biophotonic nanostructure in the non-iridescent blue feather barbs of blue penguins (Eudyptula minor) composed of parallel β-keratin nanofibres organized into densely packed bundles. Synchrotron small angle X-ray scattering and two-dimensional Fourier analysis of electron micrographs of the barb nanostructure revealed short-range order in the organization of fibres at the appropriate size scale needed to produce the observed colour by coherent scattering. These two-dimensional quasi-ordered penguin nanostructures are convergent with similar arrays of parallel collagen fibres in avian and mammalian skin, but constitute a novel morphology for feathers. The identification of a new class of β-keratin nanostructures adds significantly to the known mechanisms of colour production in birds and suggests additional complexity in their self-assembly.     

Physiological color change in squid iridophores

Evidence is presented that changes in the optical properties of active iridophores in the dermis of the squid Lolliguncula brevis are the result of changes in the ultrastructure of these cells. At least two mechanisms may be involved when active cells change from non-iridescent to iridescent or change iridescent color. One is the reversible change of labile, detergent-resistant proteinaceous material within the iridophore platelets, from a contracted gel state (non-iridescent) to an expanded fluid or sol state when the cells become iridescent. The other is a change in the thickness of the platelets, with platelets becoming significantly thinner as the optical properties of the iridophores change from non-iridescent to iridescent red, and progressively thinner still as the observed iridescent colors become those of shorter wavelengths. Optical change from Rayleigh scattering (non-iridescent) to structural reflection (iridescent) may be due to the viscosity change in the platelet material, with the variations in observed iridescent colors due to changes in the dimensions of the iridophore platelets.     

The original colours of fossil beetles

Structural colours, the most intense, reflective and pure colours in nature, are generated when light is scattered by complex nanostructures. Metallic structural colours are widespread among modern insects and can be preserved in their fossil counterparts, but it is unclear whether the colours have been altered during fossilization, and whether the absence of colours is always real. To resolve these issues, we investigated fossil beetles from five Cenozoic biotas. Metallic colours in these specimens are generated by an epicuticular multi-layer reflector; the fidelity of its preservation correlates with that of other key cuticular ultrastructures. Where these other ultrastructures are well preserved in non-metallic fossil specimens, we can infer that the original cuticle lacked a multi-layer reflector; its absence in the fossil is not a preservational artefact. Reconstructions of the original colours of the fossils based on the structure of the multi-layer reflector show that the preserved colours are offset systematically to longer wavelengths; this probably reflects alteration of the refractive index of the epicuticle during fossilization. These findings will allow the former presence, and original hue, of metallic structural colours to be identified in diverse fossil insects, thus providing critical evidence of the evolution of structural colour in this group.     

The cause of 50 million-year-old colour

Multilayer reflectors cause structural, 'metallic' colours in a diversity of animals today, yet are unknown in extinct species. We identify a multilayer reflector, causing structural colour, in a 50-million-year-old beetle from Messel, Germany. It is proposed that the original material of this reflector has been preserved, although this is not a precondition for determining original colours from ancient multilayer reflectors. Therefore, the potential exists to reveal the original colours of other (particularly arthropod) extinct species.     

A fourier tool for the analysis of coherent light scattering by bio-optical nanostructures

The fundamental dichotomy between incoherent (phase independent) and coherent (phase dependent) light scattering provides the best criterion for a classification of biological structural color production mechanisms. Incoherent scattering includes Rayleigh, Tyndall, and Mie scattering. Coherent scattering encompasses interference, reinforcement, thin-film reflection, and diffraction. There are three main classes of coherently scattering nanostructures-laminar, crystal-like, and quasi-ordered. Laminar and crystal-like nanostructures commonly produce iridescence, which is absent or less conspicuous in quasi-ordered nanostructures. Laminar and crystal-like arrays have been analyzed with methods from thin-film optics and Bragg's Law, respectively, but no traditional methods were available for the analysis of color production by quasi-ordered arrays. We have developed a tool using two-dimensional (2D) Fourier analysis of transmission electron micrographs (TEMs) that analyzes the spatial variation in refractive index (available from the authors). This Fourier tool can examine whether light scatterers are spatially independent, and test whether light scattering can be characterized as predominantly incoherent or coherent. The tool also provides a coherent scattering prediction of the back scattering reflectance spectrum of a biological nanostructure. Our applications of the Fourier tool have falsified the century old hypothesis that the non-iridescent structural colors of avian feather barbs and skin are produced by incoherent Rayleigh or Tyndall scattering. 2D Fourier analysis of these quasi-ordered arrays in bird feathers and skin demonstrate that these non-iridescent colors are produced by coherent scattering. No other previous examples of biological structural color production by incoherent scattering have been tested critically with either analysis of scatterer spatial independence or spectrophotometry. The Fourier tool is applied here for the first time to coherent scattering by a laminar array from iridescent bird feather barbules (Nectarinia) to demonstrate the efficacy of the technique on thin films. Unlike previous physical methods, the Fourier tool provides a single method for the analysis of coherent scattering by a diversity of nanostructural classes. This advance will facilitate the study of the evolution of nanostructural classes from one another and the evolution of nanostructure itself. The article concludes with comments on the emerging role of photonics in research on biological structural colors, and the future directions in development of the tool.     

Moult speed affects structural feather ornaments in the blue tit

Growing evidence suggests that structural feather colours honestly reflect individual quality or body condition but, contrary to pigment‐based colours, it is not clear what mechanism links condition to reflectance in structural feather colours. We experimentally accelerated the moult speed of a group of blue tits (Cyanistes caeruleus) by exposing them to a rapidly decreasing photoperiod and compared the spectral characteristics of their structural feather colours with those of control birds. Blue tits were sexually dimorphic on the UV/blue crown and on the white cheek feathers. Moult speed, however, dramatically reduced brightness and the saturation only on the UV/blue crown feathers, whereas structural white on the cheek feathers was basically unaffected by moult speed. Given that the time available for moulting is usually confined to the period between the end of the breeding season and migration or wintering, UV/blue colours, but not structural white, may convey long‐term information about an individual’s performance during the previous breeding season. The trade‐off between fast moulting and structural colour expression may represent a previously unrecognized selective advantage for early‐breeding birds.     

STRUCTURALLY COLOURED PATTERN MARKS ON THE INNER WEBS OF FLIGHT FEATHERS.

Some Psittacidae and Coraciidae show the unusual feature of blue and green pattern marks restricted to reverse surfaces of flight feathers.
The optical causes of the colours of these marks are identical with, and the morphological configurations similar to, those which typically cause these colours on obverse surfaces of feathers in the same families. The atypical marks may, however, have been evolved independently from the typically located colour.
Colour patterns on flight feathers are discussed in relation to the mechanics of flight and to optical effect.     

Seasonal Changes in Colour: A Comparison of Structural,Melanin- and Carotenoid-Based Plumage Colours

Background

Plumage coloration is important for bird communication, most notably in sexual signalling. Colour is often considered a good quality indicator, and the expression of exaggerated colours may depend on individual condition during moult. After moult, plumage coloration has been deemed fixed due to the fact that feathers are dead structures. Still, many plumage colours change after moult, although whether this affects signalling has not been sufficiently assessed.

Methodology/Principal Findings

We studied changes in coloration after moult in four passerine birds (robin, Erithacus rubecula; blackbird, Turdus merula; blue tit, Cyanistes caeruleus; and great tit, Parus major) displaying various coloration types (melanin-, carotenoid-based and structural). Birds were caught regularly during three years to measure plumage reflectance. We used models of avian colour vision to derive two variables, one describing chromatic and the other achromatic variation over the year that can be compared in magnitude among different colour types. All studied plumage patches but one (yellow breast of the blue tit) showed significant chromatic changes over the year, although these were smaller than for a typical dynamic trait (bill colour). Overall, structural colours showed a reduction in relative reflectance at shorter wavelengths, carotenoid-based colours the opposite pattern, while no general pattern was found for melanin-based colours. Achromatic changes were also common, but there were no consistent patterns of change for the different types of colours.

Conclusions/Significance

Changes of plumage coloration independent of moult are probably widespread; they should be perceivable by birds and have the potential to affect colour signalling.     

Sexual dimorphism in birds: why are there so many different forms of dimorphism?

Variation in the extent of sexual dimorphism among bird species is traditionally attributed to differences in social mating system. However, there are many different forms of dimorphism among birds, and not all of them show an obvious correlation with social mating system. For example, recent work has shown that many highly polygamous species are, in fact, monomorphic, whereas many putatively monogamous species are dimorphic. In this paper we break up sexual dimorphism into subcomponents and then use comparative analyses to examine the pattern of covariation between these subcomponents and various aspects of sexual, social, and parental behaviour. Our first finding is that size dimorphism and plumage-colour dimorphism do not show the same pattern of covariation. Differences in size dimorphism are associated with variation in social mating system and sex differences in parental care, whereas differences in plumage-colour dimorphism are associated with variation in the frequency of extra-bond paternity. These results suggest that size dimorphism is associated with the sort of intrasexual competition described by traditional classifications of social mating system, whereas plumage-colour dimorphism is associated with cryptic female choice. However, when we break up plumage-colour dimorphism according to whether it is due to melanins, carotenoids or structural colours, we find that each category of plumage-colour dimorphism shows a different pattern of covariation. The correlation between overall plumage-colour dimorphism and the rate of extra-bond paternity is due to structural colours, whereas melanin-based dimorphism is associated with sex differences in parental care. The former result is particularly interesting given that new work suggests structural colours are associated with active sexual displays and the reflection of ultraviolet light.     

Evolution of plumage coloration in the crow family (Corvidae) with a focus on the color-producing microstructures in the feathers: a comparison of eight species

Plumage coloration has been the subject for a variety of questions that comprise the center of modern evolutionary biology. Unlike carotenoids that the concentration directly influences the intensity of the color, melanin, in addition to produce brown or black colors, is often involved in producing the structural coloration such as glossiness or iridescence. As the melanin granules can be located in the barbs or the barbules, we aim to (i) discern if the colors observed at macro scale comes from the barbs, the barbules or both in a series of related species and (ii) estimate the evolutionary history of the color-producing mechanisms in the family Corvidae that are known to have melanin-based coloration. From a preliminary comparative analysis on eight representative species, we found three coloration schemes in Corvidae; (1) matte colors of brown or black that were produced in barbs and barbules; (2) non-iridescent structural colors such as blue, bluish gray and white, that were produced in the barbs and (3) iridescent structural colors that were produced only in distal barbules. Comparative character analysis of these coloration schemes suggests that the ancestral state among these species were the colors produced in the barbs and that the color produced in the distal barbules is a derived character. The evolution of iridescence seems tightly linked to the evolution of the colors produced in the distal barbules. Data from more species should be incorporated in order to grasp a full picture on the evolutionary history of plumage coloration in this group of birds.     

Genotypic variation in yellow autumn leaf colours explains aphid load in silver birch

? It has been suggested that autumn-migrating insects drive the evolution of autumn leaf colours. However, evidence of genetic variation in autumn leaf colours in natural tree populations and the link between the genetic variation and herbivore abundances has been lacking. ? Here, we measured the size of the whole aphid community and the development of green-yellow leaf colours in six replicate trees of 19 silver birch (Betula pendula) genotypes at the beginning, in the middle and at the end of autumn colouration. We also calculated the difference between green leaf and leaf litter nitrogen (N) and estimated the changes in phloem sap N loading. ? Autumn leaf colouration had significant genetic variation. During the last survey, genotypes that expressed the strongest leaf reflectance 2-4 wk earlier had an abundance of egg-laying Euceraphis betulae females. Surprisingly, the aphid community size during the first surveys explained N loss by the litter of different birch genotypes. ? Our results are the first evidence at the tree intrapopulation genotypic level that autumn-migrating pests have the potential to drive the evolution of autumn leaf colours. They also stress the importance of recognizing the role of late-season tree-insect interactions in the evolution of herbivory resistance.     

Is the structural and psittacofulvin‐based coloration of wild burrowing parrots Cyanoliseus patagonus condition dependent?

The bright colours of parrots are caused by psittacofulvin pigments, which appear unique to this Order, and by structural colours. We measured red (psittacofulvin), green (mixed) and blue (structural) colours of wild burrowing parrots Cyanoliseus patagonus of northeastern Patagonia, Argentina, and measured nestlings regularly to obtain data on breeding success and nestling growth. As adult feathers are moulted outside the breeding season, adult body condition could not be measured directly during feather growth, and climatic conditions were used as an indirect parameter. The colony of burrowing parrots is surrounded by Monte steppe habitat, the breeding success has been shown to depend strongly on the climatic patterns. The area experienced a drought with very poor breeding success as well as a year of above‐average precipitation during the study period, serving as a natural experiment. We thus analysed the variability of colouration within the population among and within breeding seasons. We observed strong inter‐annual differences in nestling and adult colouration. Nestlings grew blue feathers with lower achromatic brightness during better conditions, and when controlling for year effects, nestlings with higher mass and from more successful families also had blue feathers with lower achromatic brightness. Adult blue feathers showed the same trend, with lower achromatic brightness in the moult following breeding seasons of better conditions. In contrast, during better conditions, adults grew red feathers with higher achromatic brightness and the colour hue was also affected, and the hue of the red region of nestlings varied with the hatching order. The colour of all three regions of nestlings varied between nests, and the colour of the red region of adult males positively correlated with breeding success (clutch size, brood size). In summary, the present data suggest that environmental conditions contribute to variability in both structural and the psittacofulvin‐based colours of wild burrowing parrots.     

蜻蜒运动检测神经元的光谱反应

蜻蜒腹神经束上存在着自运动检测神经元和目标运动检测神经元.我们采用了两种视觉刺激条件来测试自运动检测神经元的光谱反应.当采用控制强度和波长的闪光进行测试时、它们的光谱反应曲线与绿色光感受器的光谱灵敏度曲线极其相似,峰值位于500nm处.然而采用运动的条纹进行测试时,它们的峰值却位于560nm处.当用一种颜色的运动图案作为目标放置在另一种颜色背景的前方测试时,发现存在某个目标的照明强度值能使反应下降到自发放电的水平,这表明自运动检测器无法检测这二种颜色的差别,即它们是色盲的、它主要接受来自绿色光感受器的信号.目标运动检测神经元的光谱反应特性与自运动检测神经元的不同,目标运动检测神经元在以380nm至580nm的范围中有着平坦的光谱反应曲线,有时在紫外频段出现峰有(?)前景与背景颜色不同且固定背景光的颜色与强度而改变前景的光强时,神经元的反应不会下降到自发放电水平,当背景为绿色而目标为另一个颜色.特别是兰色时,神经元反应强烈,但当背景为兰色而目标为绿色时,它们的反应相对较弱.这些结果表明目标运动检测神经元是对颜色敏感的.     

Carotenoids need structural colours to shine

The bright colours of feathers are among the most striking displays in nature and are frequently used as sexual signals. Feathers can be coloured by pigments or by ordered tissue, and these mechanisms have traditionally been treated as distinct modes of display. Here we show that some yellow plumage colour is created both by reflection of light from white structural tissue and absorption of light by carotenoids. Thus, structural components of feathers contribute substantially to yellow 'carotenoid' displays, but the effect of variation in structural components on variation in colour displays is, to our knowledge, unstudied. The presence of structural colour in some carotenoid-based colour displays will have to be considered in studies of colour signalling.     

Pterin pigment granules are responsible for both broadband light scattering and wavelength selective absorption in the wing scales of pierid butterflies

A small but growing literature indicates that many animal colours are produced by combinations of structural and pigmentary mechanisms. We investigated one such complex colour phenotype: the highly chromatic wing colours of pierid butterflies including oranges, yellows and patterns which appear white to the human eye, but strongly absorb the ultraviolet (UV) wavelengths visible to butterflies. Pierids produce these bright colours using wing scales that contain collections of minute granules. However, to date, no work has directly characterized the molecular composition or optical properties of these granules. We present results that indicate these granules contain pterin pigments. We also find that pterin granules increase light reflection from single wing scales, such that wing scales containing denser granule arrays reflect more light than those with less dense granule collections. As male wing scales contain more pterin granules than those of females, the sexual dichromatism found in many pierid species can be explained by differences in wing scale pterin deposition. Additionally, the colour pattern elements produced by these pterins are known to be important during mating interactions in a number of pierid species. Therefore, we discuss the potential relevance of our results within the framework of sexual selection and colour signal evolution.     

Most and Least Preferred Colours Differ According to Object Context: New Insights from an Unrestricted Colour Range

Humans like some colours and dislike others, but which particular colours and why remains to be understood. Empirical studies on colour preferences generally targeted most preferred colours, but rarely least preferred (disliked) colours. In addition, findings are often based on general colour preferences leaving open the question whether results generalise to specific objects. Here, 88 participants selected the colours they preferred most and least for three context conditions (general, interior walls, t-shirt) using a high-precision colour picker. Participants also indicated whether they associated their colour choice to a valenced object or concept. The chosen colours varied widely between individuals and contexts and so did the reasons for their choices. Consistent patterns also emerged, as most preferred colours in general were more chromatic, while for walls they were lighter and for t-shirts they were darker and less chromatic compared to least preferred colours. This meant that general colour preferences could not explain object specific colour preferences. Measures of the selection process further revealed that, compared to most preferred colours, least preferred colours were chosen more quickly and were less often linked to valenced objects or concepts. The high intra- and inter-individual variability in this and previous reports furthers our understanding that colour preferences are determined by subjective experiences and that most and least preferred colours are not processed equally.     

Structural colour: elusive iridescence strategies brought to light

Understanding structural colours in nature requires the right set of optical experiments: this is illustrated by a new study on iridescent bird of paradise feathers, which suggests the potential behavioural importance of dynamic colour changes.     

Synaesthetic colours do not camouflage form in visual search

One of the major issues in synaesthesia research is to identify the level of processing involved in the formation of the subjective colours experienced by synaesthetes: are they perceptual phenomena or are they due to memory and association learning? To address this question, we tested whether the colours reported by a group of grapheme-colour synaesthetes (previously studied in an functional magnetic resonance imaging experiment) influenced them in a visual search task. As well as using a condition where synaesthetic colours should have aided visual search, we introduced a condition where the colours experienced by synaesthetes would be expected to make them worse than controls. We found no evidence for differences between synaesthetes and normal controls, either when colours should have helped them or where they should have hindered. We conclude that the colours reported by our population of synaesthetes are not equivalent to perceptual signals, but arise at a cognitive level where they are unable to affect visual search.     

Communication and camouflage with the same 'bright' colours in reef fishes

Reef fishes present the observer with the most diverse and stunning assemblage of animal colours anywhere on earth. The functions of some of these colours and their combinations are examined using new non-subjective spectrophotometric measurements of the colours of fishes and their habitat. Conclusions reached are as follows: (i) the spectra of colours in high spatial frequency patterns are often well designed to be very conspicuous to a colour vision system at close range but well camouflaged at a distance; (ii) blue and yellow, the most frequently used colours in reef fishes, may be good for camouflage or communication depending on the background they are viewed against; and (iii) reef fishes use a combination of colour and behaviour to regulate their conspicuousness and crypsis.     

"Basic brown", a too little considered iris color]

The eye colour is mainly effected by the two components melanin and structural blue of the iris stroma which generally are present in various mixtures respectively combinations. Are these components lacking the dark pigment epithelium can produce a so-called "basic brown". The exact distinction between "brown" and "basic brown" is without doubt of considerable value for anthropological as well as for genetical examinations of eye colours.