How Do Animals See Colour? Dogs, Bees, Birds and the Mantis Shrimp
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Colour is not a property of light. It is something an eye and a brain make out of light, by comparing the responses of a few kinds of sensor tuned to different wavelengths, and the number and tuning of those sensors differ from one animal to the next. Humans have three. Dogs have two. Most birds have four and can see ultraviolet. The mantis shrimp has twelve, and the surprising thing about it is how little that helps.
Cones and comparison
The retina at the back of an eye contains two kinds of light-sensing cell. Rods respond to dim light and see in shades of grey; cones work in bright light and come in types, each containing a pigment most sensitive to a particular band of wavelengths. A single cone type cannot tell colour from brightness, because a dim green light and a bright red one can excite it equally. Colour appears when the brain compares the output of two or more cone types: if the long-wavelength cones fire more than the medium ones, the light is reddish, and so on.
The number of cone types therefore sets how many dimensions of colour an animal can see. Two types give a colour space like a line, three give a plane, four give a volume. Each extra type lets the animal tell apart colours that looked identical before.
Dogs, cats and most mammals
The ancestors of mammals spent the age of dinosaurs as small nocturnal animals, and somewhere in those hundred million years of night they lost two of the four cone types that other vertebrates keep. Most mammals still have only two, one sensitive to blue and one to yellow-green, which makes their colour vision like that of a red-green colour-blind person. A dog sees a red ball on green grass as two similar shades of yellow-brown, which is why so many dogs are slow to find red toys and quick to find blue ones.
Cats, horses, cattle and deer see the same way; the red cape at a bullfight is for the crowd, and the bull charges at the movement. Humans and the other Old World primates regained a third cone by duplicating a gene some thirty million years ago, most likely because telling ripe red fruit from green leaves was worth the cost, and the fact that the new pigment is tuned to exactly that distinction supports the idea.
Birds, bees and ultraviolet
Birds never lost their fourth cone, and it is tuned to ultraviolet. A bird's colour space has a whole dimension that ours lacks, and it uses it. Many species that look identical to us differ in ultraviolet plumage, so a male and female blue tit, which look alike to a birdwatcher, look plainly different to each other. Kestrels can see the ultraviolet reflected by the urine trails of voles in a field. Fruit and flowers advertise in ultraviolet as well as in visible colours.
Bees see a similar range shifted the other way: they have three cone types tuned to ultraviolet, blue and green, and cannot see red at all. Flowers have evolved to match. Many that look plain yellow to us carry ultraviolet patterns, bullseyes and landing stripes pointing to the nectar, that are invisible to us and vivid to a bee. A photograph of a dandelion under ultraviolet light shows a dark centre we never suspected.
The mantis shrimp and the limits of counting
The mantis shrimp, a small, spectacularly coloured predator of tropical reefs, has twelve types of colour receptor, plus others for polarised light, and for years it was assumed to have the richest colour vision on Earth. When researchers finally tested it in 2014, they found it could not distinguish colours that a human separates easily. The shrimp does not compare its receptors the way we do. Each one seems to act as a detector for its own narrow band, and the animal reads colour by which detector fires, like a bar code, rather than by mixing.
The reason may be speed. Comparing receptors takes brain processing, and the mantis shrimp, which strikes prey with the fastest punch in the animal kingdom, may have traded fine discrimination for a colour system that needs almost no computation. It is a reminder that more sensors do not mean better vision, only different vision. Colour vision across the animals:
- •Most mammals, including dogs and cats: two cone types, no red-green distinction
- •Humans and Old World monkeys: three, regained for finding fruit
- •Birds, many reptiles and fish: four, including ultraviolet
- •Honeybees: three, ultraviolet to green, blind to red
- •Mantis shrimp: twelve, read individually rather than compared
The takeaway
Animals see colour by comparing the outputs of different cone types, and the number and tuning of those types decide what they can see. Dogs and most mammals have two and miss red, humans regained a third for fruit, birds and bees have ultraviolet, and the mantis shrimp shows that a dozen receptors read separately give less discrimination than three read together.