In the ancient Mediterranean, snail farming was a lucrative business.
In the seaside town of Tyre, in what is now southern Lebanon, predatory sea snails from the genus Murex were harvested by the thousands for a purple chemical that they produced. Used for dyeing cloth, the expensive ‘Tyrian purple’ was so famous for its vibrancy and longevity that Charlemagne, the first emperor of the Holy Roman Empire, chose it for his burial shroud.
Jump ahead 1,200 years and we are still using animals to produce colours for industrial and artistic use — except now we are not limited to extracting the colours from animals themselves. By understanding the science behind how animals produce their colours, we can use them as inspiration to create hues of our own. For example, fashion designers have developed a cloth called Morphotex that adopts the colouration mechanism of the blue morpho butterfly to create shimmering colours without the use of fabric dyes.
The human use of animal pigments is just one facet of what makes animal colouration fascinating. Colours play a host of important roles in the animal world, from attracting mates and warding off predators to camouflage and even communication. They have deep ecological influences, and the chemistry, biology, and physics that underlie their development is still inspiring researchers and engineers today — 4,000 years after the first sea snail gave its life for a drop of purple.
What makes colouration complex
In the late 1800s, Victorian-era poet and priest Gerard Manley Hopkins wrote a religious poem about nature that opened with the line, “As kingfishers catch fire, dragonflies draw flame.” It’s an evocative description of colour in the natural world, but it’s also an example of the hidden complexities of animal colouration.
Take the kingfisher. Different kingfishers have different colours, but the common kingfisher is known for its orange breast and the vibrant blue-green plumage spread across its wings and tail. Each colour is produced by a distinct type of feather, and each of those produces its colour by a distinct mechanism.
The orange breast feathers contain tiny pigment granules that give them their colour, while the blue-green feathers do not have any pigment at all. Instead, they produce their colour by a physical process known as ‘iridescence’ — spongy cells inside the back and tail feathers selectively reflect only blue-green light from incoming white light, creating a specific colour.
Dragonflies also use different processes to produce their colouration. Their bodies are pigmented, but they rely on iridescence for their shimmery wings. Layers of tiny wing scales scatter white light into rainbow-coloured beams.
But dragonflies also change colour throughout the course of their lives. Males and females of different dragonfly species have particular colour schemes, known as ‘morphs.’ Male and female dragonflies can mimic a morph from the opposite sex to their advantage. A male-mimicking female avoids being harassed by genuine males looking to mate, while a female-mimicking male avoids fights over territory with other males to find mates more easily.
Moreover, colour polymorphism — the variation of colours within a species or population — also allows dragonflies to recognize members of their own species. This is thought to discourage interspecies aggression and help maintain a healthy population size. The ability to change colours also has evolutionary advantages, as one morph might camouflage better into an environment than another.
So, in just two simple examples — those of the kingfisher and the dragonfly — we can see the intricacies of how animals produce their colours, as well as some of the ecological advantages that their colours give them.
How animals produce colour
In general, animal colouration processes fall into one of two categories: chemical or structural. Chemical processes use pigments within cells to create colour. On the other hand, structural processes have no pigments; instead, they create hues by exploiting the properties of light through tiny physical structures.
Iridescence is one example of a structural colouration process. Perhaps its most famous user is the blue morpho butterfly, known for its vibrant wings that shimmer between blue and black depending on the angle from which it is seen. Layers of wing scales are shingled over each other to produce a pattern that scatters all coloured light except blue light, which is why the wings appear blue.
However, the reflecting light waves can also overlap, cancelling out in some cases and adding up in others. Glancing at the butterfly from a different angle causes the colour to appear more blue if there is more constructive interference and more black if there is more destructive interference.
The more you look at colouration, the more fascinating the patterns that reveal themselves become. For example, blue and green are common colours in the animal world but are nearly always produced by structural processes. Blue-tinted pigments are simply quite rare, and in many multi-coloured animals — like insects, reptiles, and birds — both processes of colouration are present, each producing a different colour. This is the case with the common kingfisher.
But perhaps the most surprising thing about animal colouration is the way in which a huge variety of colours can be grouped into two simple categories. Scales, fur, skin, and carapace are all coloured by either structural or chemical processes, even if the details differ.
Flamingos dye their feathers a bright pink, but only after they extract the pigment from the shrimp and algae they eat. A sea turtle’s spots are also the result of pigments, but they have to combine different colours to produce them. And the green plumage of an Amazonian parrot comes from a yellow pigmented layer that covers a blue-light producing structure within feathers, a combination that yields green-coloured reflected light.
Is being colourful useful?
If an inherited trait has been around long enough in the population of a species, it’s reasonable to assume that there’s an evolutionary advantage in keeping it around. Colour is no exception. There are a number of benefits conferred by different colours and colour schemes, including those we see in dragonflies.
First of all, colours are useful for attracting and repelling other animals. Many male birds perform elaborate, colourful courtship rituals to attract mates. The most famous example is probably the peacock, which fans out its tail feathers in a bold display.
Sexual signalling is not limited to birds either — fireflies, fruit flies, and even some species of fish have all been observed using their colours to find a mate.
Some predators also generate coloured light to attract prey. This is the case with glow worms, which attract insects by emitting a soft blue light while clinging to the roofs of caves, or female anglerfish, which are equipped with a worm-shaped appendage that can glow to attract smaller fish looking for a meal.
However, colours can also be used for the opposite purpose: warding off other animals. For example, in the Amazon, brightly coloured poison dart tree frogs wear their colours as a warning sign to would-be predators. Moths and fish also use similarly bright colours for self-preservation.
When an animal begins to adopt a set of colours as a repellent, other species have been known to catch on too. Venomous coral snakes have bands of red, black, and yellow along their tails, and numerous non-venomous copycats have evolved similar patterns that enable them to trick predators into staying away. However, sometimes these mimics can be just as dangerous as the originals. Bees and wasps may share the same colours, and both have stingers.
And then, there’s camouflage — the use of specific colours to blend into an environment and hide from predators. Camouflage adaptations are observed across the animal world, from mammals like leopards and foxes to caterpillars that blend into the leaves they eat. Chameleons are often described as master camouflagers for their ability to change the colours of their whole body, but they are also theorized to use colour-shifting for territorial displays and aggression, and even to regulate body temperature.
But the true master of camouflage — and one of the most inspiring animals for modern scientists — is the cuttlefish.
How cuttlefish show us a better way to innovate
Cuttlefish are part of a family called cephalopods, which includes undersea creatures like squids and octopuses. They have an incredible ability to blend into almost any environment they encounter. Not only are they able to change the colour on any part of their body within milliseconds, they can also change the texture of their skin. This enables a cuttlefish to blend in just as easily among rocky outcroppings as on a sandy stretch of seabed.
A cuttlefish’s remarkable camouflage doesn’t come easily — it is the result of four different components. First are ‘chromatophores,’ pigment-carrying cells that can vary in size to change the colour of the cuttlefish. These are supplemented by ‘iridophores,’ cells that structurally produce colours that chromatophores cannot. Then there are ‘papillae’, bumpy parts of the skin that can vary in texture. And, finally, there are cells called ‘leucophores’ that reflect light.
Because of this complexity, the cuttlefish has served as inspiration for a number of new technologies. In 2012, researchers at the University of Bristol developed artificial chromatophores that could be used for adaptable camouflage fabrics. In 2015, an American team developed a soft material that could change texture, made with a 3D printer, with a design based off of cuttlefish papillae.
And just last year, a paper in the journal Nature Communications outlined a process for creating human cells that can vary in transparency, modelled off of the leucophores cuttlefish use for the same purpose.
This practice of loosely adapting biological mechanisms for human use is called ‘bio-inspiration’, and it’s been a huge revolution in the way we approach innovation for over 20 years. Many architectural and design improvements have come from studying nature; Japan’s famously speedy bullet train is a good example — its shape and signature silence are the result of copying bird physiology.
Colour innovations have come to form a substantial subset of bio-inspired technologies. Structural colouration processes have been particularly useful in making new types of materials since they produce vivid, long-lasting colours using a relatively short list of base ingredients.
These materials have found their way into fabrics, textiles, cosmetics, and even credit cards. Biologists have even given input for military developments in camouflage and deception.
Bio-inspired materials could even be useful for solving environmental problems. One 2017 study suggested that animals could be studied to improve reflective surfaces for energy-reducing cooling materials. While current cooling surfaces reflect light in a limited range of wavelengths, there are animals who — over millions of years of evolution — have adapted body surfaces that reflect a wide range of wavelengths.
The earliest human uses of animal colouration involved extracting colours directly from the animals themselves for dyes and paints. However, the more we delve into the fascinating, complex science of colour, the more we understand ways to replicate colours without harming animals. With every discovery, it becomes clearer that, even though we’re always surrounded by colours, there’s always more to them than meets the eye.