Today I’m going to talk to you about a bird, I’m also going to talk about a beetle, and also a rock. All of these have one incredible thing in common, and it is nature’s greatest colour trick.
In past blogs, I’ve taken a deep dive into some mind-blowing ways that animals see colour and also how nature makes colour. But I think the colour trick we explore today might just be my favourite.
This is a phenomenon called iridescence.
iridescent | ˌɪrɪˈdɛsnt |
adjective
showing luminous colours that seem to change when seen from different angles.
We find iridescence in loads of places:
here too…
…also there
Colour has many functions in nature. Sometimes it’s for getting attention, sometimes for staying concealed, and sometimes for reasons we don’t understand. But iridescence is uniquely mind-blowing because the colours that we see aren’t really there. They come from a weird trick of physics.
To figure out how it works, I’m exploring the physics of a bird, a beetle, and a rock to show us some of Londolozi’s most incredible examples of iridescence.
Out here starlings often get all the attention when it comes to iridescent plumage, but I think that the fanciest rainbow feathers belong to even smaller members of the class Aves, the sunbirds.
The fundamental definition of iridescence is that the colour changes depending on the direction that you are looking at it. The light hits this Marico Sunbird perfectly as it rules over a patch of aloe trees.
Sunbirds have these amazing colours largely because they’re using them to attract mates – as being pretty might indicate the best genes, the best potential to find food, and the best ability to care for offspring. What is happening inside of these feathers that helps create these colours? It’s just a little bit of a trick of physics. Three things make up the basics of this iridescent colour in these sunbirds:
- Melanin – this is the same pigment that colours our hair
- Keratin – this makes up the feather and is similar to our fingernails
- Air
Melanin (in the form of melanosomes; tiny pancake-like structures) acts as a kind of prism, scattering light into different wavelengths and producing an array of colours. The way light dances off of sunbirds and the colours we see depends on the angle of the light and the position we are observing the bird.
All of the light enters the feathers of the White-bellied Sunbird, but only some light is allowed to come out. So when you look at the feather from different angles, different waves of light line up as they’re bounced back to your eye. That is what creates the sensation of shimmering, changing colour.
While sunbirds are spectacular, I don’t think they hold the crown for the most incredibly iridescent animal. Arguably, I’d have to say that beetles are the true monarchs of iridescence in the animal kingdom.
“If there is a Creator, he must have an inordinate fondness for beetles”- JBS Haldane
Biologist Haldane was on to something in his above quote which is in reference to over 400,000 known species of beetles in the world, accounting for almost 40% of all known insect species and 25% of all known animal life forms. Not every beetle is iridescent, but the thousands that are, have some of the most unbelievable colours in nature.
The outer layer of a beetle’s exoskeleton is made of a super stiff polymer called chitin – when light hits this layer, or the layers just below it, the light bends through a process called refraction. If the beetle’s layers are spaced out just right, certain colours of reflected light waves will interfere and only certain colours of light escape at certain angles.
Dung Beetles are one of the most loved insects out here. Sometimes, the refraction reflectors are in the farthest outer layer of the beetle’s body, or they can be buried a little bit deeper inside. That’s what creates the huge range of iridescence that we see in different beetle species.
Being shiny and iridescent may look cool, but one of the most important questions we have to ask in biology is why something is the way it is – nature doesn’t bring about any kind of change that doesn’t have a purpose. So there must be some surprising functions that these flashy suits of armour have. For example, the Jewel Beetle, with its bright metallic greens, blends great into its habitat consisting of lush green vegetation, often with a lot of residual water. So being shiny and reflective in just the right habitat can be greatly beneficial.
Lilac-breasted rollers have exceptional eyesight. Another benefit of iridescence in insects may be, depending on the colour patterns, an indication you don’t taste very good so a bird would leave you alone.
In some cases, iridescence might actually be about a non-visual thing altogether. Something like thermal regulation is super important in insects because they cannot control their temperature and are impacted by the temperature around them. So having the ability to reflect some of that UV back so they don’t overheat is important. There are lots of speculated reasons regarding iridescence, but we still don’t have definitive answers as to why one particular species of beetle is this way versus another species.
But it obviously serves them, or it would have dropped out of the population.
It’s not only members of the animal kingdom that have this biological beauty bestowed upon them. Geological features around the world are renowned for their iridescence, and people pay large sums of money to drape themselves in shimmering jewels and diamonds. Fortunately for us at Londolozi, we sit atop an underlying layer of granite rock, which is made up of quartz, feldspar and mica.
Quartz is well known for its stunning sheen and just like with the birds and beetles discussed above, all of its iridescent colours are the result of physical structures that bend light, not pigments or fluorescence or any of the other ways that colour is created in nature. Just the strange and beautiful result of light waves interfering with each other.
Quartz is made up of silicon and oxygen atoms that form a repeating pattern. If light passes through a thin layer or small structure on the surface of the quartz, it can scatter and bend, creating iridescent colours. This effect can become more intricate and vibrant if the quartz has impurities or defects in its structure.
What I think is the coolest thing about iridescence is how completely distantly related things in nature can stumble on the same physics for making colour. Light is bending and reflecting and interfering in almost the same way with the bird’s feathers, the beetle’s shell, and the rock’s layers, using a totally different material. All these things are so different but are tied together by this colourful bit of physics.
Robyn Morrison
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