Fig 1 - uploaded by Geoffrey E. Hill
Content may be subject to copyright.
Photograph of the amelanotic Steller's jay in Boulder County, CO, USA from which the feathers in this study were taken. Photograph by Bill Schmoker.
Source publication
Non-iridescent structural plumage color is typically produced by coherent scattering of light within a matrix of keratin and air (a ;spongy layer') in feather barbs. It remains unclear what role, if any, the basal melanin layer underlying this spongy layer plays in the production of coloration. Amelanism in birds with structural color is a ;natural...
Similar publications
The tail feathers of magpies are iridescent, with hues ranging from navy to violet and green. It has been previously shown that the hexagonal arrangement of melanosomes in the distal barbules is responsible for these colors, but previous simulation models have relied on average values for the parameters associated with this arrangement (e.g., perio...
Citations
... Cephalopods, for example, produce bright, chromatically tuned color signals through the joint actions of light absorption by pigment-packed chromatophores and light scattering by underlying structures including iridophores and leucophores (Mäthger et al., 2006;DeMartini et al., 2013). Likewise, some birds produce vividly colored feathers through a combination of light absorption by melanin pigment and light scattering by keratin nanostructures (Shawkey et al., 2003;Shawkey and Hill, 2006). To understand the function and evolution of biological color displays, we must consider how light-absorbing pigments and light-scattering nanostructures are used synergistically to enhance the appearance of these signals for potential viewers. ...
Biological visual signals are often produced by complex interactions between light-absorbing and light-scattering structures, but for many signals potential interactions between different light-interacting components have yet to be tested. Butterfly wings, for example, are thin enough that their two sides may not be optically isolated. We tested if ventral wing scales of the Mormon fritillary Speyeria mormonia impact the appearances of dorsal orange patches, which are thought to be involved in sexual signaling. Using reflectance spectroscopy, we found ventral scales, either silvered or non-silvered, make dorsal orange patches significantly brighter, with the silvered scales having the greater effect. Computational modeling indicates both types of ventral scales enhance the chromatic perceptual signal of dorsal orange patches, with only the silvered scales also enhancing their achromatic perceptual signal. A lack of optical independence between the two sides of the wings of S. mormonia implies the wing surfaces of butterflies have intertwined signaling functions and evolutionary histories.
... Melanophores are the deepest cell type, containing the black pigment melanin. Besides producing black coloration, melanin contributes to structural color by absorbing incoherently scattered light (Shawkey and Hill 2006). Importantly, due to the broad overlap in terms of colors produced by each mechanism, this means that similar colors in different species can be produced by various combinations of carotenoids, pterins, and iridophores, or by each mechanism alone (Teyssier et al. 2015;Twomey et al. 2020b;Stuart-Fox et al. 2021a). ...
Convergent evolution is defined as the independent evolution of similar phenotypes in different lineages. Its existence underscores the importance of external selection pressures in evolutionary history, revealing how functionally similar adaptations can evolve in response to persistent ecological challenges through a diversity of evolutionary routes. However, many examples of convergence, particularly among closely related species, involve parallel changes in the same genes or developmental pathways, raising the possibility that homology at deeper mechanistic levels is an important facilitator of phenotypic convergence. Using the genus Ranitomeya, a young, color-diverse radiation of Neotropical poison frogs, we set out to (1) provide a phylogenetic framework for this group, (2) leverage this framework to determine if color phenotypes are convergent, and (3) to characterize the underlying coloration mechanisms to test whether color convergence occurred through the same or different physical mechanisms. We generated a phylogeny for Ranitomeya using ultraconserved elements and investigated the physical mechanisms underlying bright coloration, focusing on skin pigments. Using phylogenetic comparative methods, we identified several instances of color convergence, involving several gains and losses of carotenoid and pterin pigments. We also found a compelling example of nonparallel convergence, where, in one lineage, red coloration evolved through the red pterin pigment drosopterin, and in another lineage through red ketocarotenoids. Additionally, in another lineage, "reddish" coloration evolved predominantly through structural color mechanisms. Our study demonstrates that, even within a radiation of closely related species, convergent evolution can occur through both parallel and nonparallel mechanisms, challenging the assumption that similar phenotypes among close relatives evolve through the same mechanisms.
... The isolated tissue, when affixed to black carbon tape, exhibited a similar blue to that of the intact animal, whereas on a white substrate, the color was much paler and the tissue was almost transparent (Fig. 2i). Previous studies on biological and artificial optical materials have shown that black backgrounds can serve to absorb longer and incoherently scattered wavelengths and help in the saturation of blue color (32,33). In our study, while the paler color of ribbontail ray tissue on white backgrounds suggests a superimposition of all colors in the spectrum, the brighter blue color on carbon tape indicates that the tissue's natural black backing acts as a broadband absorber of longer wavelengths and is therefore an important component of the blue coloration. ...
Blue structural colors, produced by diverse tissue nanostructures, are known from all
major vertebrate clades except cartilaginous fishes (e.g. sharks, rays). We describe a
bright angle-independent structural blue from ribbontail stingray skin, arising from a
novel cell type with unique quasi-ordered arrays of nano-vesicles enclosing guanine
nanoplatelets. This natural architecture —an intracellular photonic glass— coherently
scatters blue, while broadband absorption from closely-associated melanophores
obviates the low color-saturation typical for photonic glasses. This first demonstration
of structural color in elasmobranchs (the oldest extant clade of jawed vertebrates)
illustrates that the capacity for guanine-based colors likely arose extremely early in
vertebrate evolution. The structure-function mechanisms underlying ribbontail stingray
coloration point to selective pressures driving elasmobranch visual ecology and
communication, but also strategies for biomimetic color production.
... The isolated tissue, when a xed to black carbon tape, exhibited a similar blue to that of the intact animal, whereas on a white substrate, the color was much paler and the tissue was almost transparent (Fig. 2i). Previous studies on biological and arti cial optical materials have shown that black backgrounds can serve to absorb longer and incoherently scattered wavelengths and help in the saturation of blue color (32,33). In our study, while the paler color of ribbontail ray tissue on white backgrounds suggests a superimposition of all colors in the spectrum, the brighter blue color on carbon tape indicates that the tissue's natural black backing acts as a broadband absorber of longer wavelengths and is therefore an important component of the blue coloration. ...
Blue structural colors, produced by diverse tissue nanostructures, are known from all major vertebrate clades except cartilaginous fishes (e.g. sharks, rays). We describe a bright angle-independent structural blue from ribbontail stingray skin, arising from a novel cell type with unique quasi-ordered arrays of nano-vesicles enclosing guanine nanoplatelets. This natural architecture —an intracellular photonic glass— coherently scatters blue, while broadband absorption from closely-associated melanophores obviates the low color-saturation typical for photonic glasses. This first demonstration of structural color in elasmobranchs (the oldest extant clade of jawed vertebrates) illustrates that the capacity for guanine-based colors likely arose extremely early in vertebrate evolution. The structure-function mechanisms underlying ribbontail stingray coloration point to selective pressures driving elasmobranch visual ecology and communication, but also strategies for biomimetic color production.
... [55] In nature, evolution has mitigated this problem by the use of broadband absorbing materials, which are typically present in the form of melanin or other dark pigments. [56] Typical examples are the Steller's jay (Cyanocitta Stelleri) bird that displays bright blue and black feathers stemming from a combination of disordered nanostructures and a basal melanin layer, [57] whereas the Pollia condensata fruit presents a metallic blue coloration arising from the combination of layers in thick-walled cells and cell-cores containing dense brown tannin pigments. [58] Several researchers have applied this concept to artificial photonic materials, where synthetic melanin colloidal particles have been used as absorbing building blocks [59][60][61] or carbon black has been added to nanostructured films [61][62][63] to enhance the intensity contrast between band-gap peaks and the background. ...
Creating color through the self-assembly of specific building blocks to fabricate photonic morphologies is a promising and intriguing approach to reproducing the flamboyant visual effects and dynamic properties observed in the natural world. However, the complexity and lack of robustness in the manufacture of these nanostructured materials hinder their technical exploitation on a large scale. To overcome such limitations, here we present a novel methodology to create bioinspired photonic pigments as dispersed and micrometer-scale particles based on highly ordered concentric lamellar microspheres made of block copolymers. First, we introduce the fabrication protocol and the advantages of this approach compared to the traditional colloidal self-assembly. Then, we discuss some possible future research directions focused on developing hybrid organic-inorganic photonic pigments with enhanced dielectric contrast, reduced scattering, and specific functionalities. Finally, we speculate on possible applications for these structures that go beyond their use as simple photonic pigments.
... Complexity in the effects of heavy metals on structural plumage is perhaps unsurprising, given the multiple variables that interact to generate the overall coloration of structural plumage, including amount and type of melanin in the basal layer (Shawkey & Hill 2006), barbule melanization (Shawkey et al. 2005) and spongy layer nanostructure (Shawkey et al. 2005). Each of these variables may in turn be affected by a variety of factors, including hormone levels, diet quality, feather maintenance, and even interactions between the immune system and feather-degrading bacteria (see Chatelain et al. 2017, Lifshitz & St Clair 2019. ...
Mining is present in all major biomes worldwide and overlaps strongly with areas of importance for bird conservation. Mining causes multiple types of environmental disturbance, including habitat alteration, increased human presence, and persistent heavy metal pollution. Feather coloration and fluctuating asymmetry (random deviation from perfect symmetry between the two sides of the body), especially of elaborate or ornamental characters, have been proposed as potentially useful biomarkers of anthropogenic stress in birds. However, there has been virtually no research on their responses to mining disturbance. Here, we use colour and morphometric measurements and collection location data from museum specimens of the Russet‐crowned Motmot Momotus mexicanus to evaluate how their proximity to mining concessions in south‐central Mexico (Morelos, Guerrero, and Oaxaca states) relate to: 1) structural and melanin‐based plumage coloration, and 2) size and fluctuating asymmetry of their distinctive racket‐shaped tail. As a resident, philopatric bird that has been observed nesting directly in contaminated mining waste, motmots may be particularly well‐suited for addressing this question. In birds collected closer to metal mines, the violet under‐eye spot had decreased UV‐blue chroma and the black eye streak was paler, and there was weaker colour contrast between these adjacent conspicuous facial plumage patches. Body, tail, and crown colour and tail length and asymmetry were unaffected by metal mine distance, and no variable was affected by distance to mines extracting non‐metals. Overall, our findings show that aspects of motmots’ coloration are affected by mining disturbance, probably more from heavy metal contamination rather than other aspects of mine‐related anthropization. Further research is needed to determine whether these effects are due to direct toxicity or to ecological effects like decreased food availability and to discern whether altered colour of conspicuous patches affects social processes like mate choice and territorial interactions in populations exposed to mining.
... While generally thought to be the result of structural coloration, melanin pigmentation serves an important function in blue. For example, blue colour in both birds and reptiles depends to differing degrees on (hyper)melanization (Bagnara et al., 2007;Morrison et al., 1995;Shawkey and Hill, 2006;Sunn & Hews, 2003) that absorbs the incoherently scattered light that would otherwise produce white. ...
... Two distinct types of quasi-ordered spongy layers are responsible for the structural colour produced in feather barbs: a bicontinuous channel-type structure and a sphere-type structure (Dyck, 1971). Any remaining incoherently backscattered white light is absorbed by a basal layer of melanosomes found beneath the spongy keratin layer, crucial for production of a saturated non-iridescent colour (Shawkey & Hill, 2006). Although pigmentary and structural colours can exist independently of each other within a feather, the two are sometimes combined to create unique colours that would otherwise be impossible to produce using either mechanism of coloration independently (Shawkey & D'Alba, 2017). ...
... We hypothesize that the structural colour of the barbs is not readily visible without a dark background because these feathers lack the basal melanin layer typical of most structurally coloured feathers. However, the purple pigment cotingin, and perhaps the pigmented feathers themselves, fulfil a similar role to that of a basal melanin layer by absorbing back-scattered light when the feathers are attached to the bird (Shawkey & Hill, 2006). Additionally, given that cotingin has a narrower absorption spectrum than melanin and absorbs light only from ~500 to 650 nm (Völker, 1952), shorter wavelengths of light can still reach and be scattered by the spongy layer beneath. ...
Most studies of animal coloration focus on spectral colours, which are colours evoked by single peaks within the wavelengths of visible light. It is poorly understood how non-spectral colours (those produced by a combination of reflectance peaks) are produced, despite their potential significance to both animal communication and biomimicry. Moreover, although both pigmentary and structural colour production mechanisms have been well characterized in feathers independently, their interactions have received considerably less attention, despite their potential to broaden the available colour spectrum. Here, we investigate the colour production mechanisms of the purple feathers of the purple-breasted cotinga (Cotinga cotinga). The purple feather colour results from both the coherent scattering of light by a sphere-type nanomatrix of β-keratin and air (spongy layer) in the barbs, which produces a blue–green colour, and the selective absorption of light in the centre of the bird-visible spectrum by the methoxy-carotenoid, cotingin. This unusual combination of carotenoid and nanostructure with a central air vacuole, in the absence of melanin, is a blueprint of a synergistic way to produce a non-spectral colour that would be difficult to achieve with only a single colour production mechanism.
... Melanin acts as an absorbing material of broadband visible wavelengths for coloration. For example, amorphous melanosomes in the blue barb in Steller's jay indirectly affect non-iridescent colors as shown in Fig. 4(a) (Shawkey and Hill 2006). Amorphous melanosomes in the barb can absorb incoherent scattering from β-keratin spongy layers and improve the color saturation. ...
Melanin has been a widely researched pigment by scientists for decades as it is undoubtedly the most ubiquitous and ancient pigment found in nature. Melanin plays very significant roles in structural plumage colors in birds: it has visible light-absorbing capabilities, and nanoscale structures can be formed by self-assembling melanin granules. Herein, we review recent progress on melanin-based structural coloration research. We hope that this review will provide current understanding of melanin’s structural and optical properties, natural coloration mechanisms, and biomimetic methods to implement artificial melanin-based structural colors.
... Pigments can also be deposited beneath the spongy layer ( Figure 2B). For example, most non-iridescent structural colors contain a layer of melanin below the spongy layer which absorbs incoherently scattered light; without this pigment layer, the color of the structural color would be washed out (Prum 2006;Shawkey and Hill 2006;D'Alba et al. 2012;Shawkey and D'Alba 2017). Shifts between pheomelanin and eumelanin in the basal melanin layer can alter the appearance of structural colors from purple to blue (Peters et al. 2011;Fan et al. 2019). ...
The colorful phenotypes of birds have long provided rich source material for evolutionary biologists. Avian plumage, beaks, skin, and eggs—which exhibit a stunning range of cryptic and conspicuous forms—inspired early work on adaptive coloration. More recently, avian color has fueled discoveries on the physiological, developmental, and—increasingly—genetic mechanisms responsible for phenotypic variation. The relative ease with which avian color traits can be quantified has made birds an attractive system for uncovering links between phenotype and genotype. Accordingly, the field of avian coloration genetics is burgeoning. In this review, we highlight recent advances and emerging questions associated with the genetic underpinnings of bird color. We start by describing breakthroughs related to 2 pigment classes: carotenoids that produce red, yellow, and orange in most birds and psittacofulvins that produce similar colors in parrots. We then discuss structural colors, which are produced by the interaction of light with nanoscale materials and greatly extend the plumage palette. Structural color genetics remain understudied—but this paradigm is changing. We next explore how colors that arise from interactions among pigmentary and structural mechanisms may be controlled by genes that are co-expressed, co-expressed or co-regulated. We also identify opportunities to investigate genes mediating within-feather micropatterning and the coloration of bare parts and eggs. We conclude by spotlighting 2 research areas—mechanistic links between color vision and color production, and speciation—that have been invigorated by genetic insights, a trend likely to continue as new genomic approaches are applied to non-model species.
