Unconventional colour vision

Justin Marshall, Kentaro Arikawa

Colour vision in humans is 'middling' at best, both figuratively and literally in the animal visible spectrum of 300-750 nm. This comes as a surprise to many of us as we cannot imagine the need to see more colours than the millions we can manage. The fact is that many animals have colour vision that exceeds our red-green-blue (RGB)-based trichromacy. Birds and reptiles, along with several freshwater fish, have four colour receptors, for example, extending both ends of the human visible spectrum (400-700 nm), and may be termed tetrachromats. Horses, dogs, some primates and barracuda, on the other hand, have only two spectral classes of photoreceptors, and may be likened to red-green colour blind humans in performance; they are dichromats. Some animal groups, including insects, smaller fish, most birds and even mice make use of the ultraviolet (UV), a part of the spectrum we avoid, while others may see the spectrum that we have available to us but in more detail (Figure 1). The past two decades have also revealed animals with the potential for 'penta'-chromacy and beyond. Stomatopods (mantis shrimp) and butterflies possess up to twelve spectral sensitivities in their eyes and our mind boggles at the potential for 'dodeca'-chromatic colour space. How does a shrimp's brain decode a twelve-dimensional colour space, if indeed it does? Do butterflies require a higher level of colour vision to interpret the information of colours their wings seem to contain? Are we missing something?