When you look up into the blue sky or gaze across the seemingly endless expanse of blue ocean, it’s easy to think that the colour blue is common in nature. But among all the shades found in flowers, plants, and rocks; or the skin, scales, fur and feathers of animals, blue is unexpectedly rare.
But why is blue so rare in nature? The answer stems from the physics and chemistry of how colours are formed — and how we see them.
The science behind blue: How do organisms become blue?
When we talk about colours we generally talk about pigments. All fauna and flora have specific pigments that give off their colours. However, there is no actual natural blue pigment.
So how do blue insects, birds, plants, and animals get their colour if there is no actual blue pigment? It has everything to do with how these organisms are built, the structure of their cells and how light reflects off these structures.
This is known as structural colour. Light hits particular structures on the cell, wing or other parts of a plant or animal which causes the wavelengths to refract or “bounce” off them in a certain way, resulting in the colour blue.
For example, the Eyed Pansy butterfly gets its colour due to its wing scales. The scales are shaped in ridges that cause sunlight to bend in such a way that blue light, at just the right wavelength, makes it to our eye. If the scales were shaped differently or if something other than air was filling the gaps between them, the blue would vanish.
In the natural world, there are always exceptions. And in this case, the Obrina Olivewing butterfly is the exception. It is the only known organism in nature to produce a blue pigment, leaving researchers with lots of work on their hands as to how this has come about.
A similar process is seen in plants, where the blue pigment also doesn’t really exist. David Lee, author of Nature’s Palette: The Science of Plant Color, believes that less than 10% of the 280,000 flowering plant species produce blue flowers.
For the world’s flora, blue is created by mixing naturally occurring pigments, like how an artist mixes their colours. Red pigments – called anthocyanins – are used the most, and their appearance can be changed by varying acidity. Together with light reflections, these changes can create some stunning outcomes, for example, the Blue Commelina.
Birds with the colour blue, for example, Blue Waxbills and various Rollers and Kingfishers get their colour through a similar, yet slightly different process. Each feather contains light-scattering, microscopic beads. These beads are spaced in such a way that all other wavelengths of light are cancelled out except for blue light. Any animal with a shade of blue on them (including humans’ blue eyes) has it because of light refraction like this.
Most animals have a slightly different method when it comes to their colouration: most pigments that animals parade on their skin, fur, or feathers (non-blue colouration) is a result of the food they consume. Flamingos are pink because of the colourant they get from eating their favourite food, shrimp. Crested Barbets get their golden yellow colouration from the fruits and nectars that form part of their diet.
For animals, striking blue colours – in anything from birds to frogs to butterflies and beetles – is beneficial for getting attention. This attention is either good, like attracting a mate (for example, the display of a Lilac-breasted Roller for courtship), or bad, like warning off predators (for example, the Blue Poison Dart Frog (Dendrobates azureus) found in the rainforests of Central and South America).
Why bother being blue if it’s so difficult?
Scientists are yet to have a confirmed answer, but a popular theory states that being blue became more valuable for survival – to stand out to warn predators or to attract mates. To become blue was evolutionarily easier for animals to alter their structure and take advantage of the physics of light, rather than manufacture a new blue pigment.
For plants, having different coloured flowers can help attract a unique pollinator. And recent studies have suggested that the colour preferences of pollinating bees have propelled the diverse range of flower colours we see today, including blue.
How the rarity of blue has impacted humans’ view of the colour
Because of blue’s scarcity in nature, the word for blue was a relative latecomer to languages around the world, appearing after the words for black, white, red and yellow. With this, it was a difficult colour to obtain until relatively recently when synthetic dyes allowed any colour to be made. Artists yearned after blue (for example, Picasso’s blue period) and people have viewed it as a high-status colour for thousands of years. In many ancient cultures, only royalty and nobility were allowed to wear the colour or have it painted on their skin because it was so difficult to create – think royal blue.
According to a YouGov poll, almost every country on the planet lists blue as its favourite colour, making it the most popular colour in the world. However, less than 1 in 10 plants have blue flowers and far fewer animals are blue.
Perhaps that is why blue is so popular in the human world – because it is so rare in nature.