The colours out of (nano)space

This blog is part of our colourful countdown to the holiday season where we’re celebrating the diversity and beauty of the natural world. Click here to read the rest of the colour countdown series.

Natalia Cristina García from the Museo Argentino de Ciencias Naturales takes us on a journey into space and into the centre of a feather to explain how ultra-reflective colours are created by nature.

The title I chose for this blog is a word play with one of my favourite stories by H. P. Lovecraft, The colour out of space, originally published in 1927. I love finding small connections between the topics I research and anything pop-culture related, and when I first approached the study of colour evolution in birds, Lovecraft’s story about a new, unknown colour that arrived from outer space immediately came to my mind. Due to certain properties of birds’ eyes, their colour perception is quite different from ours, including the fact that many bird species can see ultraviolet light, thus perceiving colours no human has ever seen.

Another fact that fascinates me is the very different mechanisms that can be involved in the elaboration of plumage colouration, allowing birds to produce an incredible variety of hues. Blue plumages, for example, result from the interaction of rays of light with nanostructures inside the feather barbs. These barbs contain a spongy matrix, consisting of air bubbles immersed in a keratin matrix. Given the way these bubbles are spatially organised, light rays of short and very short wavelength are preferentially reflected by the feather surface, while other wavelengths are cancelled or absorbed by a layer of melanin pigments located below the spongy matrix.

Figure caption: General structure of a feather showing it main parts, and zoom into a cross section of a barb of a back feather of male Tersina viridis, showing its internal spongy matrix. Diagram based on figure by Shyamal (https://commons.wikimedia.org/wiki/File:Feather_zipping_microstructure.svg) and SEM (scanning electron microscope) image obtained at the Universidad de Buenos Aires.

I loved the idea of colours originating in the nano-space within the feather so much that I helped my colleague and friend Ana Barreira to study the colouration of the Swallow Tanager (Tersina viridis) for her PhD thesis, as a side project while I was doing my own PhD. This beautiful passerine can be found perching high in the trees of the Atlantic rainforest in South America. Dorsally, males of this species look either blue or light greenish-blue depending on the relative position of the observer and the light source.

More recently, we became curious about the white feathers in the ventral side of male swallow tanagers. White feathers in many species have a matrix of keratin and air within their barbs, but this matrix lacks any structural organisation, leading to a wavelength-independent scattering of light. This could have been the case of the white feathers of the swallow tanager, but we predicted we would find something different. More than a decade ago, colleagues from another lab compared a blue feather of a normal individual of Stellar’s Jay with a white feather from an amelanotic individual (an individual that had normal coloured plumage and then molted into an entire white plumage) of the same species. They found that the barbs of both feathers contained the expected normal, well-formed spongy structure that would produce a blue colouration. However, the lack of melanin was the key to the white colouration of the white individual. In an analogous way, we imagined the white feathers of the swallow tanager would lack melanin but would not differ dramatically in nanostructure from blue feathers.

With colleagues from the Museo Argentino de Ciencias Naturales and the Physics Department of the Universidad de Buenos Aires, we explored how feather nanostructure is combined with other elements (pigments and barb shape) in differently coloured plumage patches of the Swallow Tanager. We looked into the morphology of the greenish-blue feathers from the bird’s back and the white feathers from the belly, and measured light reflectance from both types of feathers. We first noticed that belly feathers are not uniformly white, but have a hint of light greenish blue colouration in their tips. Interestingly, we found that both plumage patches have a light reflectance peak around 550 nm (perceived as a greenish blue hue by humans), but this is much less intense in the belly. And, as we expected, we found that the barbs of both blue and white feathers have similar spongy matrices at their tips, consistent with their similar reflectance spectra. Why do they look so different then? The main differences between blue and white feathers was not the organization itself of the spongy matrix but its distribution (uniform across the barbs of blue feathers, reduced towards the rachis or central stem of the feather in the white feathers) and the lack of melanin pigments in the white feathers.

From a developmental point of view, this is interesting because we still do not know much about how birds manage to produce differently coloured feathers in their bodies. Our results contribute to the idea that drastic colour differences such as blue versus white do not require drastic changes in the internal nanostructures from which light is reflected, but instead these may be achieved through the regulation of pigment deposition and/or of the proportion of spongy matrix present. From an evolutionary perspective, we are still trying to understand if and how birds communicate information about their individual quality to competitors and potential mates. A widespread idea is that bright, conspicuous colours may be costly to produce. However, I think certain lines of evidence challenge this idea. For instance, we know that black feathers in some fairy-wren subspecies contain high quantities of melanin masking a nano-structure that would produce the same blue coloration exhibited by other populations of the same species. This makes me think, if feather nano-structure is costly to produce, why “waste” it in feathers that would end up looking white or black due to a lack or an excess of melanin? Our results, together with other studies, point at melanin content as rapidly changing between populations, species and plumage patches, suggesting that these pigments are key to understanding the diversity of colours observed in birds, including those whose production is attributed to nanostructures.

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