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It used to be confidently claimed that most mammals other than humans and our fellow primates were monochromats. In the last half-century, however, evidence of at least dichromatic color vision in a number of mammalian orders has accumulated. Two of the orders of sea mammals, the pinnipeds (which includes the seal, sea lion, and walrus) and cetaceans (which includes dolphins and whales) clearly are cone monochromats, since the short-wavelength sensitive cone system is genetically disabled in these animals. The same is true of the owl monkeys, genus Aotus.
Other animals may have more complex color vision systems than humans (for review see Kelber et al. 2003). These are many tropical fish and birds. In the latter case tetrachromacy is achieved through up to four cone types, depending on species. Brightly colored oil droplets inside the cones shift or narrow the spectral sensitivity of the cell. Mammals other than primates generally have less effective two-receptor color perception systems, allowing only dichromatic color vision; marine mammals have only a single cone type and are thus monochromats. Many invertebrates see colors. Honey- and bumblebees have trichromatic color vision. Papilio butterflies apparently have tetrachromatic color vision despite of possessing six photoreceptor types (Arikawa 2003). The most complex color vision system in animal kingdom has been found in stomatopods with up to 12 different spectral receptor types (Cronin & Marshall 1989) which are thought to work as multiple dichromatic units.
The mantis shrimps are also world-renowned as having the world's most sophisticated vision. According to Dr Justin Marshall, the stomatopod eye "contains 16 different types of photoreceptors (12 for colour analysis, compared to our 3 cones), colour filters and many polarisation receptors, making it by far the world's most complex retina." Mantis shrimps can thus see polarized light and 4 colors of UV (ultraviolet) light, and they may also be able to distinguish up to 100,000 colors (compared to the 10,000 seen by human beings).