NatureZen: Blue Monday
words and photos by Melissa McMasters
A few decades ago, a British travel agency began a marketing campaign around the idea that the third Monday in January was the most depressing day of the year. They called it “Blue Monday,” with the idea that the end of the holiday season, grim weather, and already-defunct New Year’s resolutions combined to make everyone miserable. The solution, of course, was to book a vacation, which they’d be happy to do for you!
Here in Memphis, we spend the third Monday of January honoring the legacy of Dr. Martin Luther King, Jr. with service projects all over town. (A huge thanks to everyone who braved today’s frigid temperatures to help us with trail repairs!) So the concept of Blue Monday has always taken a backseat to this spirit of working toward a better city. But if you’re holed up inside today and feeling low, I’m going to turn Blue Monday’s frown upside-down and offer a little cheer by spotlighting blue wildlife.
Relatively speaking, blue is an unusual color to find in nature. Fewer than 10% of plants have blue flowers, and among animals the percentage that appear blue is even smaller.
The main reason that blue is so scarce is that nature doesn’t produce a lot of true blue pigments. Think about the leaves of plants: they contain the chemical compound chlorophyll, which is a green pigment. In addition to trapping light that allows the plant to produce energy, chlorophyll also gives the leaves their green color. Carotenoid pigments, which are yellow, orange, and red, are found in fruits, vegetables, and seeds–they give color to carrots and berries, for example. There’s no corresponding true-blue in the plant world, and since many animals derive their colors from the pigments in the foods they eat, it takes a special effort for animals to appear blue.
If plants aren’t producing blue pigment, how do blue flowers exist? Channel your inner Bob Ross and imagine mixing colors on your palette to come up with something entirely different; plants can mix pigments. They can also change color based on the pH of the soil. Home gardeners may know that they can alter the appearance of a hydrangea from pink (expressing natural red pigments) to blue by increasing the acidity of the soil. This is because the molecules of the flower’s red pigments, called anthocyanins, appear pink when stacked far apart from each other and blue when closer together. Acidic soil is heavier in aluminum, which when absorbed by hydrangeas allows anthocyanin molecules to squeeze closer together.
A flower’s petals have to absorb all the colors of light they don’t reflect, and for blue flowers, this means absorbing red light. This is a resource-intensive process, much more costly to a plant than reflecting red light (which is why there are so many more flowers with “warm” colors). So why would any plant expend extra energy to appear blue? It’s likely to do with attracting pollinators. Bees in particular can’t see the color red, but they have excellent green, blue, and ultraviolet vision and are especially attracted to those colors.
Flowers can mix pigments to appear blue, but animals are doing something else entirely. Birds and insects use structural color to bounce light in a certain way. When light hits structures on the wing cells of this dragonfly, it’s refracting (or bending) light at a short wavelength, causing the wings to appear blue. Get this same bug into some shade and the color looks a muddy brown, because the blue isn’t a pigment but a dance between light and cells.
Blue jays have melanin in their feathers, which is a pigment that appears brown or black. But we see these birds as blue because their feathers have microscopic pockets made of air and keratin that absorb every wavelength of light other than blue. This light scattering, where only blue is refracted back to our eyes, is the same optical illusion that makes indigo buntings and bluebirds look blue.
This structural refraction of melanin is the same reason hummingbirds’ throats sparkle at certain angles but look black at others. In hummingbirds, there’s an added dimension of iridescence, which happens because the melanin granules are stacked in neat, flat rows. As the hummingbird turns its head and brings light onto the feathers, some light is reflected back while the rest moves down to the next layer, and so on up to 15 layers. The effect this produces is a brilliant shimmering. Reminiscent of the pigment molecules in hydrangeas, which appear blue when stacked more closely together, hummingbirds’ feathers appear blue when their melanin layers are thinner and more densely packed.
Iridescence, as well as blue coloration, works similarly in butterflies. Butterfly wings are actually transparent, but they’re covered in tiny dust-like scales. Many of the bold colors we see in butterflies are derived from pigments contained in the food eaten by caterpillars–this accounts for red, yellow, and black wing colors, as well as orange, cream, and green. But as we know, there’s not a blue pigment for caterpillars to eat, so the color is coming from the structure of the scales. Tiny crystal-like shapes help bend the light to create a blue appearance.
This structural blue comes with an evolutionary advantage for some butterflies. Because female butterflies have evolved away from the pheromone glands that moths use to find partners, they base their choice of mate on visual cues. Male common blue butterflies use the shining blue on the dorsal sides of their wings to communicate with females, and studies have shown this color fades substantially less than the pigment-based colors on its ventral wing surfaces when the butterfly is exposed to stress.
Structural blue also functions as a sign of a healthy mate in some birds, like wild turkeys, that have facial skin or wattles. This skin gets its blue appearance from the way arrays of parallel collagen fibers scatter the light.
In strawberry poison dart frogs, significant differences between color and pattern have evolved over thousands of years, with up to 30 different color morphs seen across Central America. Males use color to signal to each other (back off–this is my territory!), to females (I’m worthy of your attention), and to predators (don’t eat me, I’m toxic!). The “blue jeans” morph is common throughout Costa Rica and Panama.
I’ve barely scratched the surface of the complex ways blue is expressed throughout nature, and the reasons organisms have evolved to expend the effort to display it. Learning about what it takes for wildlife to look blue has given me a greater appreciation for what a gift it is to run across a shiny blue beetle or the feather of a blue jay. It’s not easy being blue, but I’m happy there are creatures giving it a shot!