The incredible – and bizarre – spectrum of animal colour vision
A common butterfly has five times as many types of colour-sensing cells in their eyes as us. Why do they – and other animals – need so many? Belinda Smith looks into six extreme cases.
The natural world is awash with colour – the dazzling plummage of parrots, vast fields of wildflowers. But we humans only getting a small slice of the palette.
We perceive colour via special cells on our retinas called "photoreceptors" split into rods and cones, of which the latter are responsible for colour vision.
When light of a certain wavelength, or colour, hits them, cones transmit electrical signals to the visual cortex of the brain. There, different combinations of firing cells are cobbled together to be seen as colour.
Most humans have three types of cone – short-wavelength, medium-wavelength and long-wavelength; or red, green and blue – from which we can see millions of colours. For us to see orange, for instance, red cones fire the most, along with a little of the green, and pretty much no blue.
I say most humans because a small proportion of the population is colour-blind. That's not to say they can't see colour – but they lack one type of cone (usually red).
And some have a fourth type of cone. These "tetrachromats" seem to have an extra cone that peaks somewhere between red and green, allowing them to see different colours where others would see identical hues.
The animal world takes colour vision a massive step further.
This week, a butterfly was discovered to have a staggering 15 types of photoreceptor. Indeed, most animals have more or fewer types of colour photoreceptors than humans. Here are just a few – and why their colour vision is so different to ours.
The bluebottle butterfly has 15 types of photoreceptor
With one type of cone stimulated by ultraviolet, another by violet, three by varying shades of blue, one blue-green, four by green and five by red light, the common bluebottle butterfly has five times as many colour photoreceptors as we do.
Why so many? After all, many other insects make do with three.
Researchers believe the butterflies only use four of their photoreceptors for day-to-day colour vision, while the others 11 are used in specific environments, such as picking out objects hidden in vegetation. A similar system is found in a six-photoreceptor butterfly, the Asian swallowtail.
Mantis shrimp have 12 colour photoreceptors
These crustaceans are striking in more ways than one. Not only are they brightly patterned, they're formidable hunters with a punch so fast it produces an underwater shockwave, and is capable of shattering aquarium glass.
So why do they need 12 types of cone? And does this mean they have super colour vision?
A 2014 study showed that they don't, in fact, have particularly good colour vision. Each cone's firing range is narrow, meaning each picks up a specific colour.
This means the brain doesn't have to weigh up millions of inputs to determine colour, making it less fuel-intensive and faster for the mantis shrimp to recognise different-coloured prey.
Some birds see colour and ultraviolet
Having the full colour spectrum plus ultraviolet light brings a new dimension to bird plumage.
Where we see a brilliant palette of feather colours, birds see more. Not only do many species have better colour vision than us, but an extra ultraviolet cone picks up highlights invisible to us.
Bumblebees are trichromats
Just like humans, the good old bumblebee has three types of colour photoreceptor. But unlike us, their cones are shifted towards the ultraviolet end of the spectrum.
This lets them see markings on flowering plants directing them to nectar stores, like lights along airport runways.
Boas and pythons can 'see' in the infrared
While they don't have much in the way of detailed colour vision, some snakes can sense the long wavelengths of the infrared spectrum with a set of organs that act as infrared goggles.
These "pit organs" are usually situated on the snake's face between their eyes and nostrils, and contain thousands of infrared-sensitive receptors that respond to wavelengths longer than the visible spectrum. This allows snakes to see infrared wavelengths that are invisible to human eyes, although we do feel infrared as heat.
Starfish have 'eye spots' to detect light, but not colour
And all the way at the other end of the colour vision spectrum, starfish and some worms make do with rudimentary "eye spots", light-sensitive patches on their skin.
While not strictly colour vision, and pretty much useless to spot anything swimming by, starfish use them to look around for coral reefs – big, stationery blobs.