From candies to flags to universities, the color blue is ubiquitous in the man-made world. Blue-colored animals and plants, however, are relatively rare.
Unlike other colors, such as brown and black which typically result from melanin, blue pigments are not commonly found in animals. In fact, the obrina olivewing butterfly is one of the few organisms that has blue pigment in its body. The blue bands on its forewings are colored with a pterobilin-based pigment (1, 2).
Beyond pigments, other organisms take advantage of chemistry to achieve a blue color. For example, hydrangea flowers use a class of chemicals called anthocyanins for their bluish hue. Rather than being a consistently blue pigment, anthocyanins are more akin to an indicator. Only when the plant can obtain sufficient aluminum ions from the soil to convert the naturally red anthocyanins to a bluer shade will the hydrangea flowers turn blue. Because aluminum ions move more easily through acidic soils than basic ones, hydrangeas bloom blue in acidic soils and pink in basic soils. Consequently, while the flower can act as a soil pH indicator, it can only maintain a bluish tone in acidic soils (3).
In contrast, some organisms use physical structures instead of pigmentation to achieve their blue coloring—a phenomenon called structural coloration. The berry of the Pollia condensata plant derives its lustrous ultramarine sheen through its shape. Coiled cells in the berry’s skin stack on top of each other in such a way that only light wavelengths within the blue range are reflected back to the viewer’s eyes. The berry shines an intense blue in spite of containing no blue pigments (4).
In the same fashion, the blue morpho butterfly also uses structural coloration to create its brilliant blue wings. Butterfly wings contain two layers of interlocking and overlapping scales. The blue morpho’s wings contain additional ridges in the scales. When sunlight hits these scales, the nano-striations within the butterfly’s wings reflect only blue light, making the blue morpho worthy of its name (5).
In another instance of structural coloration, the blue in a blue jay’s wings is achieved through a physical phenomenon called the Tyndall Effect. The Tyndall Effect describes how shorter, bluer wavelengths of light can scatter through a suspension of small particles more efficiently than longer, redder wavelengths. Consequently, the passing light can give the suspension a blue tinge that intensifies when against a darker background. For the blue jay, the bird incorporates a layer of microscopic air pockets in the barbs of its feathers against a dark melanin background to emphasize the Tyndall Effect (6, 7).
Structural coloration has long existed in animals to create the color blue. When scientists recently discovered a 50 million year old beetle in the earth, the first color to greet them was the blue of the beetle’s carapace (8).
Why would plants and animals have evolved such complex mechanisms to display a single color? One popular hypothesis states that the uniqueness of the blue shade helps organisms stand out among their competitors and obtain a survival advantage. In the case of the Pollia condensata berry, scientists believe that the berry’s remarkably reflective colors lead birds to bring the berry to their nests as decor and consequently spread the berry’s seeds to new locations, thus expanding its geographic range (4).
Other animals use blue-creating mechanisms together with other features to blend into their environment. These animals combine blue structural coloration with yellow chemical pigments to turn themselves green, a massive advantage when trying to blend in among green plants. The cells of the European tree frog, for instance, include layers of yellow chromatophores (cells containing pigment) on top of blue light-reflecting guanophores (cells containing guanine crystals) all against a backdrop of dark melanocytes (cells containing melanin) to color its skin green. Some species of parrots, similarly to the blue jay, have feathers that can reflect blue light. By combining this feature with yellow feathers, they can display feathers that appear green (6).
Researchers are still exploring the extraordinary mechanisms behind the color blue as well as their evolutionary origins. Although no one knows what they’ll find, one thing is certain: the examination of nature’s blues will continue to be a colorful journey.
References:
- Zawischa, Dietrich. "Colours of plants and animals." University of Hanover, https://www.itp.uni-hannover.de/fileadmin/arbeitsgruppen/zawischa/static_html/botzooE.html
- Vane-Wright, Dick. The coloration, identification and phylogeny of Nessaea butterflies (Lepidoptera: Nymphalidae). Bulletin of the British Museum (Natural History), 1979.
- Schreiber, Henry. "Curious Chemistry Guides Hydrangea Colors." American Scientist, 5 Dec. 2014, https://www.americanscientist.org/article/curious-chemistry-guides-hydrangea-colors.
- Joyce, Christopher. “A Berry So Shiny, It's Irresistible (And Inedible).” NPR, 11 Sep. 2012, https://www.npr.org/2012/09/11/160894364/how-to-get-birds-to-pick-blue-berries-they-can-t-eat.
- Debat, Vincent & Berthier, et al. “Why are Morpho Blue?.” 2018, https://doi.org/10.1016/B978-1-78548-277-9.50009-7.
- Zawischa, Dietrich. "Scattering of Light." University of Hanover, https://www.itp.uni-hannover.de/fileadmin/arbeitsgruppen/zawischa/static_html/scattering.html.
- Carpenter, Anita. "What Color Is a Bluejay?" Wisconsin Department of Natural Resources, 2003, https://dnr.wi.gov/wnrmag/html/stories/2003/feb03/jays.htm.
- Parker, Andrew R. and McKenzie, David R.. “The cause of 50 million-year-old colour.” Proceedings. Biological sciences, no. 270, 2003, pp. S151–S153, http://doi.org/10.1098/rsbl.2003.0055.