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Humans and other animals that are trichromats
Humans and other mammals have evolved trichromacy based partly on pigments inherited from early vertebrates. In fish and birds, for example, four pigments are used for vision. These extra cone receptor visual pigments detect energy of other wavelengths, including sometimes ultraviolet. Eventually two of these pigments were lost (in placental mammals) and another was gained, resulting in trichromacy among some primates. Humans and closely related primates are usually trichromats, as are some of the females of most species of New World monkeys, and both male and female howler monkeys. Recent research suggests that trichromacy may also be quite general among marsupials. A study conducted regarding trichromacy in Australian marsupials suggests the medium wavelength sensitivity, MWS, cones of the honey possum (Tarsipes rostratus) and the fat-tailed dunnart (Sminthopsis crassicaudata) are features coming from the inherited reptilian retinal arrangement. The possibility of trichromacy in marsupials potentially has another evolutionary basis than that of primates. Further biological and behavioural tests may verify if trichromacy is a common characteristic of marsupials. Most other mammals are currently thought to be dichromats, with only two types of cone (though limited trichromacy is possible at low light levels where the rods and cones are both active). Most studies of carnivores, as of other mammals, reveal dichromacy, examples including the domestic dog, the ferret, and the Spotted Hyena. Some species of insects (such as honeybees) are also trichromats, being sensitive to ultraviolet, blue and green instead of blue, green and red. Research indicates that trichromacy allows animals to distinguish red fruit and young leaves from other vegetation that is not beneficial to their survival. Another theory is that detecting skin flushing and thereby mood may have influenced the development of primate trichromate vision. The color red also have other effects on primate and human behavior as discussed in the color psychology article.

Types of cones specifically found in primates
Primates are the only known placental mammalian trichromats. Their eyes include three different kinds of cones, each containing a different photopigment (opsin). Their peak sensitivities lie in the blue (short-wavelength S cones), green (medium-wavelength M cones) and yellow-green (long-wavelength L cones) regions of the color spectrum. (Schnapf et al, 1987). S cones make up 5–10% of the cones and form a regular mosaic. Special bipolar and ganglion cells pass those signals from S cones and there is evidence that they have a separate signal pathway through the thalamus to the visual cortex as well. On the other hand, these L and M cones are hard to distinguish by their shapes or other anatomical means – their opsins differ in only 15 out of 363 amino acids, so nobody has yet succeeded in producing specific antibodies to them. But Mollon and Bowmaker did find that L cones and M cones are randomly distributed and are in equal numbers.



Each of the three types of cones in the retina of the eye contains a different type of photosensitive pigment, which is composed of a transmembrane protein called opsin and a light-sensitive molecule called 11-cis retinal. Each different pigment is especially sensitive to a certain wavelength of light (that is, the pigment is most likely to produce a cellular response when it is hit by a photon with the specific wavelength to which that pigment is most sensitive). The three types of cones are L, M, and S, which have pigments that respond best to light of long (especially 560 nm), medium (530 nm), and short (420 nm) wavelengths respectively.

Since the likelihood of response of a given cone varies not only with the wavelength of the light that hits it but also with its intensity, the brain would not be able to discriminate different colors if it had input from only one type of cone. Thus, interaction between at least two types of cone is necessary to produce the ability to perceive color. With at least two types of cones, the brain can compare the signals from each type and determine both the intensity and color of the light. For example, moderate stimulation of a medium-wavelength cone cell could mean that it is being stimulated by very bright red (long-wavelength) light, or by not very intense yellowish-green light. But very bright red light would produce a stronger response from L cones than from M cones, while not very intense yellowish light would produce a stronger response from M cones than from other cones. Thus trichromatic color vision is accomplished by using combinations of cell responses.

It is estimated that the average human can distinguish up to ten million different colors.