Art: Anaelle Stenman
Colour design: Dion Susanto
The human eye is a magnificently complex feat of evolution, and for the most part, it provides the brain with an accurate picture of the world around it. Your retina is inlaid with a mosaic of photoreceptors – rods to detect brightness and movement, dotted with the less numerous cones, which come in red, green and blue varieties to perceive colour. The fields of view that each of our eyes have overlap, giving us binocular vision, endowing us with tremendous depth perception and keen long-range sight.
But our eyes are far from perfect. Light must pass through a network of nerves to reach the receptors, there’s a blind spot where the optic nerve connects to the retina, and on top of these “defects,” we are limited to observing the narrow slit in the electromagnetic spectrum that we call visible light. Whilst our eyes have evolved to suit our lifestyles, there are other creatures whose evolutionary histories have given them a very different perspective on the world.
To you and me, a garden in spring is jostling with every colour of the rainbow, flowers bursting and blooming, each one more eye-catching than the last. But it is not our eyes they need to please; we are just lucky observers. Instead, the show is intended for those perfect little pollinators – insects. Through a compound eye consisting of 5,000 individual “pixels,” a bee sees hidden patterns in the petals of flowers, which glow sharply against darker, leafy backgrounds. Ultraviolet-reflecting landing strips guide the bee towards the nectar, and some spiders have even evolved to weave their own UV designs into their webs to deceive bees in search of food. If the bee manages to avoid the trap, she can track the shapes formed by polarised light in the sky to navigate back home. However, the complexity of colour they experience is similar to our own, as they too possess three colour receptors, although tailored towards shorter wavelengths: green, blue and ultraviolet.
On the other end of visible light lies the infrared spectrum. Certain snakes such as pythons, boa and pit vipers see in pretty much the same way as we do, albeit less vividly, and with the added ability to form an infrared map of space that they can superimpose onto their visual map. They can boast this ability thanks to ‘pits’ in the skin near their nostrils, which contain heat-sensitive nerve endings. This extra sense gives these snakes a tremendous predatory advantage by allowing them to track their prey’s body heat from up to a metre away, facilitated by an impressive night vision, courtesy of their highly developed rods.
Returning to the realms of the humanly visible, it is clear that our eyes are far from the best that the animal kingdom has to offer. Kingfishers hunt in two worlds; in the air, they spot and track their prey, and as they plunge like a stone headfirst into the water, they must continue tracking and close their beaks, all in the time it takes for the fish to realise it has become a meal. Glare can be a nuisance for any aerial hunter looking for fish, but evolution has gifted kingfishers with many more red droplets in the cones of their eyes, which act as chromatic filters. Light travels at different speeds in air and water, and in order to make the transition without losing focus the kingfisher has two foveae in each eye, whereas we possess only one per eye. The fovea is the area of the eye that is most dense in photoreceptors. We use this region to optimise our vision. Right now, you are reading this sentence by subconsciously using your foveae. By switching from one fovea to the other as they break the water’s surface, kingfishers can compensate for the change in refractive index, keep the second half of their eye on the prize, and evolution rewards them with lunch.
At night, our eyesight is hopeless. But there is a whole host of nocturnal creatures whose eyes are dedicated to enjoying the view when we are blind. If you’ve ever taken a photo of a cat with flash you will have noticed its eyes glow. This comes from the reflective tapetum lucidum, a layer at the back of their eyes that mirrors light into their retina, making their night vision six times better than our own. In the same picture, our pupils appear red as the flash illuminates our blood-rich retina. Impressive, but not if you’re a leaf tailed gecko which have night vision 350 times sharper than humans. Their eyes have far more colour-sensitive cones than ours do, allowing them to perceive colour in almost complete darkness.
Art: Anaelle Stenman
Colour design: Dion Susanto
However, out of all the critters that run, fly, swim, burrow and, crucially, see, without a doubt the most fantastic eyes in the animal kingdom belong to the humble mantis shrimp. Thanks to having three types of colour receptors detecting red, green and blue light, our human eyes can distinguish up to 10 million different colours. Now try to imagine if, like the mantis shrimp, you had compound eyes containing 12 types of colour receptors. Beyond the wavelengths of visible light, the mantis shrimp sees ultraviolet, infrared, polarised light – colours and shades we cannot even conceptualise. Perhaps most incredibly, they can recognise circularly polarised light and are the only organisms known to do so. Outside the natural world, there are other things capable of detecting circularly polarised light, such as DVD players, yet not as well as the mantis shrimp. As if this wasn’t enough, the many various receptors they possess are split into three independent regions on each of their eyes which themselves sit on independently moving stalks, equipping the mantis shrimp with trinocular vision. Even if one eye is damaged beyond use, the remaining eye is still able to scan the environment with full depth perception.
But what does a solitary shrimp, which spends most of its life in a burrow, want with an arsenal of sensors to rival a military drone? As often is the case in biology, it comes down to sex. These colourful crustaceans are thought to flash an array of mating patterns on their bodies that are invisible to predators, creating a private show only its mate can enjoy – the romantic little shrimp!
