Imma Perfetto
Cosmos science journalist
You only have to look out the window to appreciate the stunning colours and patterns that paint Earth’s animals with the shades of the rainbow.
But how and why do animals produce these dazzling displays?
Pigments producing bright hues
Animals use a range of strategies to produce the gorgeous colours that we see, the most well-known being pigments which absorb certain wavelengths of light but not others.
“If we see red, it means everything except the red wavelengths have been absorbed,” says Devi Stuart-Fox, a professor of evolution and ecology at the University of Melbourne.
Stuart-Fox spoke to Cosmos about animal colouration for the latest episode of the podcast Science Detectives.
“[Animals] have to acquire those pigments in different ways. Some pigments they can produce, but some pigments they have to acquire from their diet.”
The pigment melanin is found throughout the animal kingdom, but humans produce it inside specialised cells called melanocytes. We have melanin to thank for giving our skin, hair and eyes their colour and for protecting us against damage from ultraviolet light.
“[Other] animals acquire pigments from their diet and then they have to sequester them into their feathers or skin,” says Stuart-Fox.
For instance, flamingos aren’t born with their fabulous pink-orange plumage. Instead, they sequester a pigment called beta carotene from the high concentrations found in their diet.
Some animals including fish, chameleons, and octopuses use specialised cells called chromatophores to change their colouration at will. These are skin cells that contain pigments, and the colour change happens when that pigment moves towards the surface of the skin or away from it within these cells.
Adding some structure to the theory
But pigments aren’t the only method of producing colours. Animals have evolved another strategy: structural colours.
“I like to think you can have purely structural colours, but you can’t have a purely pigment based colour. It’s always a pigment embedded in a structure or embedded in the material and that material is reflecting the wavelengths that are not absorbed,” says Stuart-Fox.
“What I find really fascinating is that there’s a whole class of colours, perhaps even the majority of colours, that are not produced by pigments, but by nanometre-scale, tiny, tiny structures within feathers, or skin, or within the cuticle of a beetle or butterfly wings.”
These micro- or nanometre-sized structures are colourless themselves, but small enough to interfere with visible light.
“With these structures, there’s usually two materials involved: there’s the chitin or the keratin, and then air – or another material that’s different,” says Stuart-Fox.
“Every time the light essentially reaches an interface between these two materials, it’s reflected and if there are many of these structures, all of the wavelengths that are reflected can add together.”
This is called constructive interference, and it’s why structural colours appear so intense.
“Some wavelengths can cancel each other out, so only some wavelengths are reflected and others not at all,” says Stuart-Fox.
These structures can produce amazing optical effects like pearly and metallic colours, and iridescence – where colours change hues at different angles of view.
Give it some glow with bioluminescence
Lastly, bioluminescent animals – like comb jellies, fireflies, and glow worms – can give off their own light through chemical reactions within their own tissues.
This process involves compounds called luciferins that are catalysed by enzymes known as luciferases to react with oxygen, releasing light. Mixing and matching the luciferins and luciferases involved in this process produces different colours of light.
OK, but why are animals coloured?
So, why go to all the effort to produce these breathtaking displays? Well, the roles colouration plays are as varied as the organisms they appear in.
Camouflage is one tactic used disguise animals’ appearance and mask their location, identity and movement. Camouflage can be incredibly important for predators that rely on stealth to get close to their prey.
“Tiger patterns and cheetahs’ spots really help blend into a complex dappled kind of background. And we think that they’re orange, but a lot of mammals don’t see red,” says Stuart-Fox.
If a prey animal’s camouflage has been broken, it doesn’t have to give in yet! A startle display can kick in, like a moth opening their cryptically coloured wings to reveal bright, conspicuous eye spots.
“That startle can be enough to just make the predator pause for long enough for the prey to get away. And, you know, that’s the difference between life and death,” says Stuart-Fox.
Other prey attempt to distract predators though visual illusions, like stripes.
“Some colour patterns help to trick predators when prey are moving. When you’ve got a stripy pattern and it’s moving, then it’s a lot harder to accurately work out speed and distance and direction,” says Stuart-Fox. This is called motion dazzle, and zebras are a classic example.
Warning colouration can signal to predators that a prey animal is pretty unpalatable.
“If you’re prey and you’re defended, you’re toxic, its easier for predators to learn that association between ‘this is not good to eat’ if there are bright colours involved because they’re really stimulating,” says Stuart-Fox.
Some harmless species even take advantage of this association by mimicking poisonous species to reduce the chance of being attacked by a predator.
“Mimicry is rife in nature. You have all sorts of animals that are masquerading as twigs and leaves and things like that. You have non-toxic animals that are mimicking toxic animals, you have toxic defended animals that are mimicking other toxic defended animals,” says Stuart-Fox.
This can happen when there is one set of widespread colour patterns, like yellow and black, and a predator is more likely to encounter that pattern and learn that association more easily.
“So toxic animals can all, if you like, converge on the same set of warning colours,” says Stuart-Fox.
But signalling through colour and pattern can also be important between animals of the same species as well. This is why the males of many species are often more colourful or ornamented than the females.
“Females usually have got a limited number of eggs and offspring that she can produce. What matters to her is survival and getting enough food to produce all of those offspring,” says Stuart-Fox.
“For males, they can fertilise a very large number of eggs. They’re not so much limited by their access to resources but competing for access to those females. So they have tended to evolve more weapons, more ornaments to help them in that competition for mates.
“So they tend to be the more ornamented sex.”
