Birds, reptiles, amphibians and fish may see kinds of colours we cannot even imagine, say researchers whose experiments with wild hummingbirds show they perceive five so-called non-spectral colours.
Almost all of the colours we perceive correspond to a single wavelength. Such colours are called spectral colours as they are part of the visible spectrum, ranging from red and yellow to blue and violet.
The exception is purple, which can be evoked only by a combination of red and blue light, not by any single wavelength. For this reason, it is known as a non-spectral colour. “For us, purple is kind of special,” says Mary Caswell Stoddard of Princeton University.
Our eyes have three kinds of cells, called cone cells, that detect red, green and blue light. We see purple when only the red and blue cones, but not the green cones, in our eyes are stimulated.
Birds, reptiles, amphibians and fish also have an ultraviolet cone in their eyes. This means they might see five non-spectral colours, because there are five ways that two or three cones can be stimulated without any others. They might see ‘ultraviolet plus yellow’, for instance, when the ultraviolet, green and red cones, but not the blue cone, are stimulated.
To test this idea, Stoddard’s team recorded how often wild hummingbirds visited feeders containing either plain or sugar water. The researchers assumed that the birds always prefer sugar water, but they moved the feeders around so that the only way the birds could distinguish between them was by looking at the colours emitted by a special light next to each feeder.
The results show that hummingbirds can perceive five non-spectral colours: purple, ultraviolet plus green, ultraviolet plus red, ultraviolet plus yellow and ultraviolet plus purple. For instance, the birds had no trouble distinguishing ultraviolet plus green from either pure ultraviolet or pure green.
Most vertebrates can probably perceive these non-spectral colours, because most have four cones, but not mammals. The nocturnal ancestors of mammals lost two cones. Our primate ancestors re-evolved a third cone, but our colour vision remains poorer than many animals.
“The more philosophical question of what these colours look like to birds is impossible for us to answer,” says Stoddard. “We have no idea what these colours really look like to birds.”