Escapism, Eyeballs and Colourful Backsides

‘Life in Colour’ with David Attenborough shows the full spectrum of our natural world. But what allows us and other animals to actually see it?

A male Mandrill Baboon, showing his dominance through his natural facepaint

In a year of minimal travel, nothing transports you around the globe quite like David Attenborough’s TV series. Our senses are spoiled as we move through vast, magnificent landscapes, and then focus down onto the tiniest of details and organisms within them. Of course it is all wrapped together by that famous, calming voice; temporarily whisking you away from the problems of our COVID-19 life.

In this latest offering from the BBC, we are invited into the rich and vibrant world of colour. As the colour in the natural world has developed, so have the abilities of organisms to perceive it (and vice versa). This relationship is explored in Life in Colour, from sea life to jungle life. We meet poisonous frogs which have seemingly colour-coded themselves on a scale of toxicity, butterflies and Mantis shrimp carrying secret messages, and birds of paradise who come dressed to impress.

We are also introduced to the striking male Mandrill baboons, who develop more intense blue and red pigmentation in their faces with increasing social rank. Although humans — only a couple of branches away on the family tree — don’t develop multi-coloured behinds (if you do, you might want to get it checked), previous research has found that men are more attracted to women wearing red than other colours. In fact, wearing red in your dating profile picture can increase the number of messages you recieve.

How we see

Human eyes come in many shapes and colours, determined by our hereditary genes. We’re still trying to find out exactly why we might have different coloured eyes.

As someone who studies Ocular Drug Delivery, I am fascinated by the different types of eyes that we find in the natural world and how they work. Human eyes are similar to most mammals’. The anterior (front) tissues are designed to let light in and to direct it to the photo-receptor, ‘seeing’ cells at the back of the eye (in the retina). I work with the clear front of the eye — the cornea. I’m still in awe at how cells can form a clear tissue if they’re organised correctly.

At the back of the eye, humans have two types of photo-receptor cells — Rods for low light, and Cones for colour. These cells hold ‘opsin’ pigments which change shape when they are hit by a photon (the quantum particle of light). This change in shape creates an electrical current which is transmitted by a complex circuit of cells to the brain*.

The image that our brain recieves from the eye is actually upside down, as the light that travels through the lens is flipped, so our brains flip the image back so we ‘see’ the world as it is. You can actually train your brain to re-flip the image for a while by wearing mirror glasses!

*Researchers are actually trying to mimic this process to try and make bio-inspired, photosensitive materials.

Specialist eyes in the Natural World

Chameleons can move their eyes in different directions simultaneously.

Life in Colour touched on a couple of the ways that eyes in the animal world can differ from our own. Humans have what are called ‘camera’ type eyes (or ‘simple’ eyes), allowing us to focus in detail in one direction at a time. Some other animals make up for the restricted view from their ‘simple’ eyes by being able to move them effectively, such as chameleons sending their cone-shaped eyes in different directions, or owls rotating their heads through 360°.

Insects have compound eyes, which are like thousands of tiny simple eyes bunched together, and can therefore see in multiple directions at the same time — just not very well. There are also hybrid approaches — spiders and crickets have both complex and simple eyes, allowing them to look out for both predators and prey.

At the extreme end of the spectrum, Cubozoans — poisonous, jellyfish type creatures — are known for having a mix of 24 (!) complex, simple and so called ‘pit’ eyes (no lens, just photosensitive cells).

The development of vision has been a pivotal stage in the evolution of the vast majority of life forms on Earth. This is unsurprising, due to the fact the light from the sun powers our ecological systems and creates the opportunity for life. However, sunlight doesn’t reach everywhere equally. It was recently found that fish in pitch-black caves have actually lost their eyes through evolution, and not all animals rely on sight as their primary sense, relying on touch or smell to navigate instead.

There is also a huge variation in eye structure and neurology — the way the eyes connect to the brain — in environments where there are multiple forms of light around. In the middle ‘Mesopalagic’ zone of the ocean, there’s both the sunlight filtering down from the surface, alongside creatures creating their own glow (called bioluminescence). Here some eyes rely on being able to take in as much sunlight as possible, while others trade sensitivity for precision, to be able to pinpoint sparks of bioluminescence in the distance.

It’s great to have another Attenborough series so soon into 2021 — a light in the dark, if you will. I’m looking forward to next week’s episode and being able to nerd out about the beauty and diversity of life on our planet, a welcome escape from sepia tones of lockdown life!