Male eastern bluebirds trade future ornamentation for current reproductive investment

Life-history theory proposes that organisms must trade-off investment in current and future reproduction. Production of ornamental display is an important component of reproductive effort that has rarely been considered in tests of allocation trade-offs. Male eastern bluebirds (Sialia sialis) display brilliant ultraviolet-blue plumage that is correlated with mate acquisition and male competitive ability. To investigate trade-offs between current reproductive effort and the future expression of a sexually selected ornament, we manipulated the parental effort of males by changing their brood sizes. We found that parents provisioned experimentally enlarged broods more often than reduced broods. As predicted by life-history theory, the change in parental effort had a significant effect on the relative plumage ornamentation of males in the subsequent year: males with reduced broods significantly increased in plumage brightness. Moreover, this change in plumage coloration had a direct effect on the timing of breeding in the following season: males that displayed brighter plumage in the year following the manipulation mated with females that initiated egg laying earlier in the season. These data indicate that male bluebirds must trade-off conserving energy for production of future ornamentation versus expending energy for current reproduction

A synergistic combination of structural and pigmentary colour produces non-spectral colour in the purple-breasted cotinga, Cotinga cotinga (Passeriformes: Cotingidae)

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

Beyond colour: consistent variation in near infrared and solar reflectivity in sunbirds (Nectariniidae)

The visible spectrum represents a fraction of the sun’s radiation, a large portion of which is within the near infrared (NIR). However, wavelengths outside of the visible spectrum that are reflected by coloured tissues have rarely been considered, despite their potential significance to thermal effects. Here, we report the reflectivity from 300 to 2100 nm of differently coloured feathers. We measured reflectivity across the UV-Vis-NIR spectra of different (a) body parts, (b) colour-producing mechanisms and (c) sexes for 252 individuals of 68 sunbird (family: Nectariniidae) species. Breast plumage was the most reflective and cap plumage the least. Female plumage had greater reflectivity than males. Carotenoid-based colours had the greatest reflectivity, followed by non-iridescent and iridescent melanin-based colours. As ordered arrays of melanin-filled organelles (melanosomes) produce iridescent colours, this suggests that nanostructuring may affect reflectance across the spectrum. Our results indicate that differently coloured feathers consistently vary in their thermal, as well as obvious visual, properties

Elucidation of the hierarchical structure of natural eumelanins

Eumelanin is one of the most ubiquitous pigments in living organisms and plays an important role in coloration and UV protection. Because eumelanin is highly cross-linked and insoluble in solvents, the chemical structure is still not completely known. In this study, we used atomic force microscopy, X-ray photoelectron spectroscopy and solid-state nuclear magnetic resonance (NMR) to compare intact eumelanosomes (pigment granules mostly made of eumelanin) from four phylogentically distant species: cuttlefish (Sepia officinalis) inks, black fish crow (Corvus ossifragus) feathers, iridescent wild turkey (Melleagris gallopavo) feathers and black human hair. We found that eumelanosomes from all four species are composed of subunit nanoparticles with a length of 10–60 nm, consistent with earlier observations in eumelanosomes from the sepia ink and human hair. The solid-state NMR results indicate the presence of quinone methide tautomers in all four eumelanins. We also found clear differences in the UV absorbance, the ratio of 5,6-dihydroxyindole-2-carboxylic acid/5,6-dihydroxyindole and protonated aryl carbon ratios in sepia eumelanin relative to the other three. This comparison of natural eumelanin across a phylogenetically broad group of organisms provides insights into the change in the eumelanin structure over the evolutionary history and enables the production of synthetic eumelanin with properties that are similar to natural eumelanin

Iridescence: a functional perspective

In animals, iridescence is generated by the interaction of light with biological tissues that are nanostructured to produce thin films or diffraction gratings. Uniquely among animal visual signals, the study of iridescent coloration contributes to biological and physical sciences by enhancing our understanding of the evolution of communication strategies, and by providing insights into physical optics and inspiring biomimetic technologies useful to humans. Iridescent colours are found in a broad diversity of animal taxa ranging from diminutive marine copepods to terrestrial insects and birds. Iridescent coloration has received a surge of research interest of late, and studies have focused on both characterizing the nanostructures responsible for producing iridescence and identifying the behavioural functions of iridescent colours. In this paper, we begin with a brief description of colour production mechanisms in animals and provide a general overview of the taxonomic distribution of iridescent colours. We then highlight unique properties of iridescent signals and review the proposed functions of iridescent coloration, focusing, in particular, on the ways in which iridescent colours allow animals to communicate with conspecifics and avoid predators. We conclude with a brief overview of non-communicative functions of iridescence in animals. Despite the vast amount of recent work on animal iridescence, our review reveals that many proposed functions of iridescent coloration remain virtually unexplored, and this area is clearly ripe for future research