Dietary protein level affects iridescent coloration in Anna's hummingbirds, Calypte anna

Prediction of individual differences in non-iridescent structural plumage colour from nanostructural periodicity and regularity

Non-iridescent structural plumage reflectance is a sexually selected indicator of individual quality in several bird species. However, the structural basis of individual differences remains unclear. In particular, the dominant periodicity of the quasi-ordered feather barb nanostructure is of key importance in colour generation, but no study has successfully traced back reflectance parameters, and particularly hue, to nanostructural periodicity, although this would be key to deciphering the information content of individual variation. We used matrix small-angle X-ray scattering measurements of intact, stacked feather samples from the blue tit crown to estimate the sex-dependence and individual variation of nanostructure and its effects on light reflectance. Measures of nanostructural periodicity successfully predicted brightness, ultraviolet chroma and also hue, with statistically similar effects in the two sexes. However, we also observed a lack of overall effect of the nanostructural inhomogeneity estimate on reflectance chromaticity, sex-dependent accuracy in hue prediction and strong sex-dependence in position estimation error. We suggest that reflectance attributes are modified by other feather structures in a sex-specific manner, and that within-individual variation in nanostructural parameters exists within or among feathers and this confounds the interpretation of structure-reflectance relationships at the plumage area level.

Complex plumages spur rapid color diversification in kingfishers (Aves: Alcedinidae)

Colorful signals in nature provide some of the most stunning examples of rapid phenotypic evolution. Yet, studying color pattern evolution has been historically difficult owing to differences in perceptual ability of humans and analytical challenges with studying how complex color patterns evolve. Island systems provide a natural laboratory for testing hypotheses about the direction and magnitude of phenotypic change. A recent study found that plumage colors of island species are darker and less complex than continental species. Whether such shifts in plumage complexity are associated with increased rates of color evolution remains unknown. Here, we use geometric morphometric techniques to test the hypothesis that plumage complexity and insularity interact to influence color diversity in a species-rich clade of colorful birds-kingfishers (Aves: Alcedinidae). In particular, we test three predictions: (1) plumage complexity enhances interspecific rates of color evolution, (2) plumage complexity is lower on islands, and (3) rates of plumage color evolution are higher on islands. Our results show that more complex plumages result in more diverse colors among species and that island species have higher rates of color evolution. Importantly, we found that island species did not have more complex plumages than their continental relatives. Thus, complexity may be a key innovation that facilitates evolutionary response of individual color patches to distinct selection pressures on islands, rather than being a direct target of selection itself. This study demonstrates how a truly multivariate treatment of color data can reveal evolutionary patterns that might otherwise go unnoticed.

Animal Coloration in the Anthropocene

Natural habitats are increasingly affected by anthropogenically driven environmental changes resulting from habitat destruction, chemical and light pollution, and climate change. Organisms inhabiting such habitats are faced with novel disturbances that can alter their modes of signaling. Coloration is one such sensory modality whose production, perception and function is being affected by human-induced disturbances. Animals that acquire pigment derivatives through diet are adversely impacted by the introduction of chemical pollutants into their environments as well as by general loss of natural habitat due to urbanization or logging leading to declines in pigment sources. Those species that do manage to produce color-based signals and displays may face disruptions to their signaling medium in the form of light pollution and turbidity. Furthermore, forest fragmentation and the resulting breaks in canopy cover can expose animals to predation due to the influx of light into previously dark environments. Global climate warming has been decreasing snow cover in arctic regions, causing birds and mammals that undergo seasonal molts to appear conspicuous against a snowless background. Ectotherms that rely on color for thermoregulation are under pressure to change their appearances. Rapid changes in habitat type through severe fire events or coral bleaching also challenge animals to match their backgrounds. Through this review, we aim to describe the wide-ranging impacts of anthropogenic environmental changes on visual ecology and suggest directions for the use of coloration both as an indicator of ecological change and as a tool for conservation.

Morphogenesis of iridescent feathers in Anna's hummingbird Calypte anna

Color is a phenotypic trait of utmost importance, particularly in birds, which are known for their diverse color signals and color-producing mechanisms including pigment-based colors, light scattering from nanostructured feather tissues and combinations thereof. Bright iridescent plumage colors of hummingbirds are caused by light scattering by an organized array of flattened, pigment organelles, containing air-filled vesicles, called melanosomes. These hollow platelets are organized in multilayer arrays that contain numerous sharp air/melanin refractive index interfaces, producing brilliant iridescent colors. Despite their ecological significance and potential for inspiration of new optical materials, how platelets form and spatially arrange in nanostructures in growing feathers remains unknown. Here, we tested the hypothesis that melanosome formation and organization occurs mostly through passive self-assembly processes by assembling a developmental time series of growing hummingbird feathers using optical and electron microscopy. We show that hummingbird platelets contain air bubbles or vesicles upon their formation in pigment-producing cells, melanocytes. When melanosomes are transferred to neighboring keratinocytes (the cells shaping barbule structure) they drastically expand in size; and variation in this enlargement appears to be driven by physical constraints caused by the placement of the melanosomes within the barbule plate and their proximity to other melanosomes. As the barbule elongates and narrows, polymerizing feather corneous beta protein (CβP) orients melanosomes unilaterally, forcing them into a stacked configuration. These results reveal potentially novel forces driving the self-assembly of the nanostructures producing some of the brightest colors in nature.<.

Differential effects of early growth conditions on colour-producing nanostructures revealed through small angle X-ray scattering and electron microscopy

The costs associated with the production and maintenance of colour patches is thought to maintain their honesty. Although considerable research on sexual selection has focused on structurally coloured plumage ornaments, the proximate mechanisms of their potential condition dependence, and thus their honesty, is rarely addressed, particularly in an experimental context. Blue tit (Cyanistes caeruleus) nestlings have ultraviolet (UV)-blue structurally coloured tail feathers, providing a unique opportunity for investigation of the causes of variation in their colour. Here, we examined the influence of early growing conditions on the reflectance and structural properties of UV-blue-coloured tail feathers of blue tit nestlings. We applied a two-stage brood size manipulation to determine which stage of development more strongly impacts the quality of tail feather colouration and microstructure. We used small-angle X-ray scattering (SAXS) and electron microscopy to characterise the nanoscale and microscale structure of tail feather barbs. Nestlings from the broods enlarged at a later stage of growth showed a sex-specific rectrix development delay, with males being more sensitive to this manipulation. Contrary to predictions, treatment affected neither the quality of the barbs' nanostructures nor the brightness and UV chroma of feathers. However, at the microscale, barbs' keratin characteristics were impaired in late-enlarged broods. Our results suggest that nanostructure quality, which determines the UV-blue colour in tail feathers, is not sensitive to early rearing conditions. Furthermore, availability of resources during feather growth seems to impact the quality of feather microstructure more than body condition, which is likely to be determined at an earlier stage of nestling growth.

Bioinspired Superhydrophobic Thermochromic Films with Robust Healability

Thermochromic films with intriguing functionalities have great potential in soft actuators, heat storage devices, and interactive interface sensors. Inspired by the unique features of bird feathers (such as Nicobar pigeon, Anna hummingbird, mandarin duck, etc.), a superhydrophobic thermochromic film (STF) with robust healability is proposed for the first time through sandwiching an electric heater between a top thermochromic layer and a bottom poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) substrate. The STF exhibits fast and reversible color conversions of blue-pink-yellow under a low input power and has a superhydrophobic property with a contact angle of 155°. Furthermore, owing to the strong dynamic dipole-dipole interactions between the polar CF₃ groups of flexible PVDF-HFP chains, the STF possesses a robust healing capability of structure and conductivity. By means of the temperature difference generated by the objects contacting (finger, iron, and water) as a stimulus, the STFs achieve tactile imaging and writing record with advantages of transient display, automatic erasure, and excellent reusability. Additionally, the STF-based anti-counterfeiting security labels with superhydrophobicity and three-state color switching simultaneously realize facile distinguishment and difficult forgery. The findings conceivably stand out as a new methodology to fabricate functional thermochromic materials for innovative applications.

Structural coloration predicts the outcome of male contests in the Amazonian damselfly Chalcopteryx scintillans (Odonata: Polythoridae)

Iridescence is an optical effect that produces angle dependent coloration in animals. Recently, studies have attempted to unveil structures behind such elaborated visual signals and associated behaviors in Odonata. Here, we studied males of the Amazonian damselfly Chalcopteryx scintillans, which have hindwings that exhibit pronounced iridescence. This optical feature is used by the damselflies for intra-specific communication during territorial fights and courtship. The main question we addressed was whether male wing structural coloration may predict the outcome of male-male contests. We also studied the wing ultrastructure, in order to reveal the mechanisms that are responsible for wing coloration. Using various microscopal and spectroscopal techniques, we demonstrate that hindwing coloration is derived from two main effects: (1) light interference in the cuticle multilayer and (2) a specific angle dependent light scattering and antireflective properties of the epicuticular wax coverage. The results of our field experiment show that wing pigmentation and the hue of the dorsal surface of the hindwings is correlated with the outcome of territorial contests. This is one of the first studies showing that structural coloration derived from multilayer interference may influence the outcome of intrasexual agonistic interactions. This indicates that multicomponent structural coloration in visually guided insects may be under selective forces of male-male competition for resources and females.

Quantitative characterization of iridescent colours in biological studies: a novel method using optical theory

Iridescent colours are colours that change with viewing or illumination geometry. While they are widespread in many living organisms, most evolutionary studies on iridescence do not take into account their full complexity. Few studies try to precisely characterize what makes iridescent colours special: their angular dependency. Yet, it is likely that this angular dependency has biological functions and is therefore submitted to evolutionary pressures. For this reason, evolutionary biologists need a repeatable method to measure iridescent colours as well as variables to precisely quantify the angular dependency. In this study, we use a theoretical approach to propose five variables that allow one to fully describe iridescent colours at every angle combination. Based on the results, we propose a new measurement protocol and statistical method to reliably characterize iridescence while minimizing the required number of time-consuming measurements. We use hummingbird iridescent feathers and butterfly iridescent wings as test cases to demonstrate the strengths of this new method. We show that our method is precise enough to be potentially used at intraspecific level while being also time-efficient enough to encompass large taxonomic scales.

Iridescent colouration of male Anna's hummingbird (Calypte anna) caused by multilayered barbules

The male Anna’s hummingbird features a brightly reddish-pink reflecting gorget, due to large stacks of melanosomes in the feather barbules, arranged in layers separated by keratin. Direct observations together with detailed scatterometry demonstrated that the barbules reflect incident light in an approximately specular manner. The structural colouration is iridescent, i.e. varies with a changing angle of light incidence. Spectrophotometrical measurements of the barbule reflectance and absorbance can be well interpreted with calculated spectra obtained with a transfer matrix method for optical multilayers, using anatomical data and measured refractive index spectra. The organization of the reflectors as a Venetian blind presumably functions to create a high spectral contrast of the male’s plumage during courtship.

Reflectance variation in the blue tit crown in relation to feather structure

Structural plumage colour is one of the most enigmatic sexually selected traits. The information content of structural colour variation is debated, and the heterogeneity of the findings is hard to explain because the proximate background of within-species colour differences is very scarcely studied. We combined measurements of feather macrostructure and nanostructure to explain within-population variability in blue tit crown reflectance. We found that sexual dichromatism in aspects of crown reflectance was explained only by feather macrostructure, whereas nanostructural predictors accounted for some of the age-related differences in reflectance. Moreover, we found that both mean reflectance and spectral shape traits reflected a combination of quantity and regularity aspects in macrostructure and nanostructure. This rich proximate background provides ample scope for reflectance to convey various types of information on individual quality.

Molecular and iridescent feather reflectance data reveal recent genetic diversification and phenotypic differentiation in a cloud forest hummingbird

The present day distribution and spatial genetic diversity of Mesoamerican biota reflects a long history of responses to habitat change. The hummingbird Lampornis amethystinus is distributed in northern Mesoamerica, with geographically disjunct populations. Based on sampling across the species range using mitochondrial DNA (mtDNA) sequences and nuclear microsatellites jointly analysed with phenotypic and climatic data, we (1) test whether the fragmented distribution is correlated with main evolutionary lineages, (2) assess body size and plumage color differentiation of populations in geographic isolation, and (3) evaluate a set of divergence scenarios and demographic patterns of the hummingbird populations. Analysis of genetic variation revealed four main groups: blue‐throated populations (Sierra Madre del Sur); two groups of amethyst‐throated populations (Trans‐Mexican Volcanic Belt and Sierra Madre Oriental); and populations east of the Isthmus of Tehuantepec (IT) with males showing an amethyst throat. The most basal split is estimated to have originated in the Pleistocene, 2.39–0.57 million years ago (MYA), and corresponded to groups of populations separated by the IT. However, the estimated recent divergence time between blue‐ and amethyst‐throated populations does not correspond to the 2‐MY needed to be in isolation for substantial plumage divergence, likely because structurally iridescent colors are more malleable than others. Results of species distribution modeling and Approximate Bayesian Computation analysis fit a model of lineage divergence west of the Isthmus after the Last Glacial Maximum (LGM), and that the species’ suitable habitat was disjunct during past and current conditions. These results challenge the generality of the contraction/expansion glacial model to cloud forest‐interior species and urges management of cloud forest, a highly vulnerable ecosystem to climate change and currently facing destruction, to prevent further loss of genetic diversity or extinction.

Preservation of iridescent colours in Phorinia Robineau-Desvoidy, 1830 (Diptera: Tachinidae)

BACKGROUND

Iridescent blue-green colours are exhibited by various organisms including several taxa in the Tachinidae (Diptera) with notable examples within the Afrotropical members of the genus Phorinia Robineau-Desvoidy, 1830. The vivid colouration observed in life quickly fades to a dull golden-yellow when a specimen is dried. Although well known, no published explanation has been given for this phenomenon.

NEW INFORMATION

We illustrate the mechanism associated with this colour change. We also test and propose technical alternatives to retain the living colours in dried specimens.

Eye Morphology and Retinal Topography in Hummingbirds (Trochilidae: Aves)

Hummingbirds are a group of small, highly specialized birds that display a range of adaptations to their nectarivorous lifestyle. Vision plays a key role in hummingbird feeding and hovering behaviours, yet very little is known about the visual systems of these birds. In this study, we measured eye morphology in 5 hummingbird species. For 2 of these species, we used stereology and retinal whole mounts to study the topographic distribution of neurons in the ganglion cell layer. Eye morphology (expressed as the ratio of corneal diameter to eye transverse diameter) was similar among all 5 species and was within the range previously documented for diurnal birds. Retinal topography was similar in Amazilia tzacatl and Calypte anna. Both species had 2 specialized retinal regions of high neuron density: a central region located slightly dorso-nasal to the superior pole of the pecten, where densities reached ∼45,000 cells·mm-2, and a temporal area with lower densities (38,000-39,000 cells·mm-2). A weak visual streak bridged the two high-density areas. A retina from Phaethornis superciliosus also had a central high-density area with a similar peak neuron density. Estimates of spatial resolving power for all 3 species were similar, at approximately 5-6 cycles·degree-1. Retinal cross sections confirmed that the central high-density region in C. anna contains a fovea, but not the temporal area. We found no evidence of a second, less well-developed fovea located close to the temporal retina margin. The central and temporal areas of high neuron density allow for increased spatial resolution in the lateral and frontal visual fields, respectively. Increased resolution in the frontal field in particular may be important for mediating feeding behaviors such as aerial docking with flowers and catching small insects.

Melanosomes and ancient coloration re-examined: A response to Vinther 2015 (DOI 10.1002/bies.201500018)

Round to elongate microbodies associated with fossil vertebrate soft tissues were interpreted as microbial traces until 2008, when they were re-described as remnant melanosomes - intracellular, pigment-containing eukaryotic organelles. Since then, multiple claims for melanosome preservation and inferences of organismal color, behavior, and physiology have been advanced, based upon the shape and size of these microstructures. Here, we re-examine evidence for ancient melanosomes in light of information reviewed in Vinther (2015), and literature regarding the preservation potential of microorganisms and their exopolymeric secretions. We: (i) address statements in Vinther's recent (2015) review that are incorrect or which misrepresent published data; (ii) discuss the need for caution in interpreting "voids" and microbodies associated with degraded fossil soft tissues; (iii) present evidence that microorganisms are in many cases an equally parsimonious source for these "voids" as are remnant melanosomes; and (iv) suggest methods/criteria for differentiating melanosomes from microbial traces in the fossil record.

Key ornamental innovations facilitate diversification in an avian radiation

Patterns of biodiversity are often explained by ecological processes, where traits that promote novel ways of interacting with the environment (key innovations) play a fundamental role in promoting diversification. However, sexual selection and social competition can also promote diversification through rapid evolution of ornamental traits. Because selection can operate only on existing variation, the tendency of ornamental traits to constrain or enable the production of novel phenotypes is a crucial but often overlooked aspect of diversification. Starlings are a speciose group characterized by diverse iridescent colors produced by nanometer-scale arrays of melanin-containing organelles (melanosomes) that play a central role in sexual selection and social competition. We show that evolutionary lability of these colors is associated with both morphological and lineage diversification in African starlings. The solid rod-like melanosome morphology has evolved in a directional manner into three more optically complex forms that can produce a broader range of colors than the ancestral form, resulting in (i) faster color evolution, (ii) the occupation of novel, previously unreachable regions of colorspace, and ultimately (iii) accelerated lineage diversification. As in adaptive radiations, key innovations in ornament production can provide high phenotypic trait variability, leading to dramatic effects on the tempo and mode of diversification.

UV-green iridescence predicts male quality during jumping spider contests

Animal colour signals used in intraspecies communications can generally be attributed to a composite effect of structural and pigmentary colours. Notably, the functional role of iridescent coloration that is ‘purely’ structural (i.e., absence of pigments) is poorly understood. Recent studies reveal that iridescent colorations can reliably indicate individual quality, but evidence of iridescence as a pure structural coloration indicative of male quality during contests and relating to an individual’s resource-holding potential (RHP) is lacking. In age- and size-controlled pairwise male-male contests that escalate from visual displays of aggression to more costly physical fights, we demonstrate that the ultraviolet-green iridescence of Cosmophasis umbratica predicts individual persistence and relates to RHP. Contest initiating males exhibited significantly narrower carapace band separation (i.e., relative spectral positions of UV and green hues) than non-initiators. Asymmetries in carapace and abdomen brightness influenced overall contest duration and escalation. As losers retreated upon having reached their own persistence limits in contests that escalated to physical fights, losers with narrower carapace band separation were significantly more persistence. We propose that the carapace UV-green iridescence of C. umbratica predicts individual persistence and is indicative of a male’s RHP. As the observed UV-green hues of C. umbratica are ‘pure’ optical products of a multilayer reflector system, we suggest that intrasexual variations in the optical properties of the scales’ chitin-air-chitin microstructures are responsible for the observed differences in carapace band separations.

A window on the past: male ornamental plumage reveals the quality of their early-life environment

It is well established that the expression of many ornamental traits is dependent on the current condition of the bearer. However, conditions experienced in early life are also known to be important for an individual's subsequent fitness and therefore, directly or indirectly, for the fitness of their mate. Specifically, a recent hypothesis suggests that sexually selected traits might be sensitive to conditions experienced during early-life development and thereby function as honest indicators of developmental history. Whether this applies to colourful male plumage, however, is largely unknown. We tested this idea with a field experiment by manipulating neonatal nutrition in a sexually dichromatic passerine, the hihi (Notymystis cincta). We found that carotenoid supplementation increased nestling plasma carotenoid concentration, which was in turn correlated with increased yellow saturation in male breeding plumage after moulting. We also found that the post-moult luminance (lightness) of the white ear-tufts tended to be reduced in males that had received an all-round nutritional supplement as nestlings. Black breeding plumage was not affected by neonatal nutritional treatment. Although the mechanisms that generate colourful plumage are evidently diverse, our results show that at least some parts of this display are accurate indicators of environmental conditions during development.