In order to understand how different species see the world, it is important to understand how the eye works. Images, in the form of light, are received through the pupil, using the dilation of the iris to regulate the amount of light that enters. At the back of the eye is the retina, which is made up of photosensitive cells or photoreceptors (rods and cones in the case of most mammals). These cells transform the light into information. The information passes as a signal through the optic nerve to the brain, where it is then interpreted.
In many ways, the position and shape of an animal’s eyes determine its survival. For example, predators that hunt by means of ambush usually have eyes on the front of their heads. Along with a vertical pupil, this frontal position provides them with better focus and a greater ability to calculate distances. In contrast, lateral eyes and horizontal pupils – typical of herbivores like deer and goats – give a broader field of vision and provide protection against the sun’s glare. A further feature, the rotation of their pupils, ensures that they never lose sight of a potential predator.
So, what is light? And what is colour?
In very simple terms, we can say that light is a form of energy with two elements: one related to electromagnetic waves and another related to photons. The length of these waves is what determines the colour of the light. The number of colours that each species can see is determined by how its eyes have adapted to capture light and the type of photoreceptor cells they contains. For example, humans can see (visible spectrum) wavelengths ranging from 390 to 750 nanometers (the colours of the rainbow). However, we cannot perceive infrared light like some snake species, or ultraviolet light like many insects.
The giant eyes of the Tarsiers
In relation to their body, these small primates boast the biggest eyes and corneas of all mammals. Each of the tarsier’s eyes is equal in size to its brain and weighs even more. They are fixed in cranial cavity and cannot rotate. To compensate for this, tarsiers have evolved to rotate their heads up to 180 degrees to locate the small animals they prey on. Also, their huge corneas and the tapetum lucidum (a reflective layer of tissue on the back of the eye that reflects light) allow them to take full advantage of the light. This gives them excellent night vision. It’s believed that they can perceive ultraviolet light, however, like other nocturnal animals, their perception of colour is quite poor.
Barreleye fish and their strange double eyes
These animals, which inhabit the deep seas, have perhaps the strangest eyes in the animal kingdom. The heads of these fish are completely transparent and filled with liquid. Inside, there are two ocular structures each consisting of two parts. The first is similar to the eye of any other living creature. The second is a diverticular eye, which is separated from the main eye and contains a curved mirror formed by layers of guanine. This mirror captures the light and reflects it back into the main eye, allowing them to see up and down at the same time! They’re the only animals in the world with mirrors in their eyes.
The incredible sight of the mantis shrimp
The eyes of this crustacean are among the most complex in the animal kingdom. Most animals have between two and four photoreceptors or colour channels. The mantis shrimp has 12 (second only to a species of butterfly – Graphium sarpedon -which has 15)! Nevertheless, this doesn’t mean that their colour vision is better than that of other animals. As a crustacean, their brains do not have the capacity to process so much information. Having all these receptors takes away from their ability to distinguish between different colours, but they are much faster at recognising basic colours. As a result, energy consumption is decreased, and they are much quicker at responding to important, such as those related to territorial fights or mating.
The multi-directional vision of chameleons
Chameleons’ sight is not particularly good. They distinguish colours and have pretty good vision during the day, but at night this decreases considerably. What makes them unique is the mechanics of their eyes. Their cone-shaped eyelids are fused to their pupils, leaving only a small part of the eye exposed. Their eyes can be moved independently, allowing them to monitor 360 degrees around them without moving to remain camouflaged. When they’ve lock on to their target, they project their long tongue to catch it.
Diopsidae. Stalk-eyed flies
These flies are characterised by their remarkable eyes. They stick out of their heads on the end of stalks. And diopsidae antennae are attached to these stalks, instead of their heads like other fly species. Interestingly, this adaptation doesn’t seem to bring any type of advantage in terms of vision. However, their protruding eyes do play a role when it comes to sexual selection. Females prefer to mate with males with bigger bulges and this has driven the evolution of this feature. As a final touch, they are able to draw air into their heads and inflate their stalks, as shown in this video.
The psychedelic eyes of the satanic leaf-tailed gecko
Gecko eyes are up to 350 times better than human eyes at seeing colour, even at night. Our retinas are made up of cones. (the cells that perceive colour) and rods (photoreceptors more sensitive to light). However, gecko eyes are formed exclusively of cones. Their pupils are vertical and surrounded by cavities that increase or decrease in size depending on the intensity of light, allowing them to focus on objects at different distances at the same time. They have no eyelids. Their eyes are protected by a clear membrane that they clean with their tongue. The psychedelic pattern of their retinas keeps them camouflaged, along with their bizarre leaf-shaped tail.
Dragonflies and their 360 degree vision
Dragonflies have two large eyes, consisting of about 30,000 cells called ommatidia. They contain a lens and photosensitive cells. Each of these cells are responsible for processing part of the image, so as to create a 360 degree view. Dragonfly eyes are very sensitive to movement. Thanks to specially adapted neurons, they can focus all their visual attention on a single object, making them the deadliest of all insect-hunters. They also have three smaller, more simple eyes called ocelli. These detect movement even faster and send visual information to the motor system, giving the dragonfly lightning reactions. Also, they can see a wide spectrum of colours, including polarised or ultraviolet light.
Cuttlefish and polarised light
The pupils of the cuttlefish are W-shaped. To focus on an object, they change the actual shape of their eye, not just the shape of the lens as we do. As is the case with octopus or squid, cuttlefish are colour blind. However, they better than any other animal at capturing polarised light. The polarisation of the light occurs when it passes through a filter. In the case of a camera, the polarising filter helps reduce the unwanted glare from sources of light. In the ocean, with water as a filter, capturing polarised light helps cuttlefish to see their environment with better contrast.