To understand how animals see, we first need to understand what colour vision actually is. Scientifically speaking, the term 'colour vision' refers to an animal's capacity to distinguish between lights of varying wavelengths. This ability comes down to tiny light-detecting cells in the retina called cone pigments, which act as photoreceptors and are principally responsible for our colour vision. Most humans are "trichromats", meaning we have three types of these cones. But the animal kingdom is full of creatures that process light in completely different ways!
Why Humans and Primates See in Colour
Why do humans and other primates see in colour? Our evolutionary history gives us a clue. Having a strong ability to see red and green hues helps primates spot camouflaged food items, like ripe fruit, hidden among green foliage. Furthermore, being able to see a wide spectrum of colours helps us notice important changes in skin colour, such as blushing, sunburn, or bruising, which are crucial for social communication and health.
Dichromacy: Seeing the World Through Two Colour Channels
Imagine being able to match any colour you see with a mixture of just two primary colours. This condition is called "dichromacy". While this is a form of colour blindness in humans, it is completely normal for many animals. Dogs and cats, for example, are natural dichromats. They do not see in black and white as people used to think; instead, they experience a world painted mostly in shades of blue and yellow.
Tetrachromacy: The Power of Four Cones
If having three cones is good, is having four even better? Absolutely! This is known as "tetrachromacy", a state where an eye has four types of colour-sensitive cones. With an extra cone type to process different light frequencies, the brain receives a massive boost in colour information. While extremely rare in humans, tetrachromacy is the standard for many birds, bees, and butterflies.
Because of their fourth cone, these insects and birds can see ultraviolet (UV) light, which is completely invisible to the human eye! This helps them find nectar in flowers that have hidden "runway lights" visible only in UV.
Monochromacy: A World Without Colour
On the other end of the scale, we have "monochromacy" or total colour blindness, where an eye has only one channel for conveying colour, meaning the animal perceives only variations in brightness. Many deep-sea fish and nocturnal animals live in this high-contrast, black-and-white world.
The Mantis Shrimp: A Colour Vision Standout
The mantis shrimp stands out from the rest of the animal kingdom when it comes to colour vision. While a human relies on three types of cones, the mantis shrimp boasts up to 16 different types of photoreceptors!
