Trichromacy
Three cone types enable color vision in humans and some animals.
Trichromacy means having three separate pathways in the eye to process color, each linked to a distinct type of cone cell. Creatures with this ability are known as trichromats. In the usual account, the retina holds three kinds of color receptors—cone cells in vertebrates—each tuned to a different range of light wavelengths. In reality, there may be more than three receptor types, since different ones can become active depending on light intensity. For vertebrates with three cone types, rod cells can also contribute to color vision when light is dim.
Humans and some other mammals are trichromats, a trait that partly traces back to pigments inherited from early vertebrates. Fish and birds, by contrast, use four visual pigments, some of which detect ultraviolet light. Over time, placental mammals lost two of these pigments and gained another, leading to trichromacy in certain primates. Humans and their close primate relatives are typically trichromats, as are some female New World monkeys and both male and female howler monkeys. Evidence now suggests that trichromacy may be widespread among marsupials as well. Studies on Australian marsupials—such as the honey possum and the fat-tailed dunnart—indicate that their medium-wavelength-sensitive cones come from an inherited reptilian retinal setup. Behavioral tests, genetic analysis, and immunohistochemistry have also revealed trichromacy and ultraviolet vision in nocturnal sugar gliders. This marsupial trichromacy may have a different evolutionary origin than that of primates, and further research could confirm whether it is a common marsupial feature.
Most other mammals are thought to be dichromats, with only two cone types, though limited trichromacy can occur at low light when both rods and cones are active. Studies of carnivores—including domestic dogs, ferrets, and spotted hyenas—generally show dichromacy. Some insects, like honeybees, are also trichromats, but they sense ultraviolet, blue, and green rather than blue, green, and red. Research suggests that trichromacy helps animals distinguish brightly colored fruit and young leaves from less useful vegetation. Another theory holds that detecting skin flushing—and thus mood—may have driven the evolution of primate trichromatic vision. The color red also influences primate and human behavior, as discussed in the article on color psychology.
Among placental
- definition
- Possession of three independent color channels from three cone cell types
- key organisms
- Humans, closely related primates, some New World monkeys, howler monkeys, honey possum, fat-tailed dunnart, sugar glider, honeybees
- cone types
- L (long-wavelength, ~560 nm), M (medium-wavelength, ~530 nm), S (short-wavelength, ~420 nm)
- evolutionary origin
- Partly inherited from early vertebrates; two pigments lost in placental mammals, one gained in some primates
- distinct from
- Dichromacy (most other mammals) and tetrachromacy (fish, birds)
Lore & Background
Trichromatic color vision is the ability to see different colors mediated by interactions among three types of color-sensing cone cells. The trichromatic color theory began in the 18th century when Thomas Young proposed that color vision resulted from three different photoreceptor cells. Hermann von Helmholtz later expanded on Young's ideas using color-matching experiments, and physiological evidence was given by Gunnar Svaetichin in 1956. Each of the three cone types contains a different photosensitive pigment composed of opsin and 11-cis retinal, with peak sensitivities in the blue (S cones), green (M cones), and yellow-green (L cones) regions.
Reader's Guide
Trichromacy is significant because it allows animals to distinguish brightly colored fruit and young leaves from other vegetation, and may have influenced the development of primate social behavior through detecting skin flushing and mood. In primates, S cones make up 5–10% of cones and form a regular mosaic with a separate signal pathway through the thalamus to the visual cortex, while L and M cones are randomly distributed in equal numbers. The mechanism relies on comparing signals from at least two cone types to determine both intensity and color; the average human can distinguish up to ten million different colors. Research indicates that trichromacy in marsupials may have a different evolutionary basis than in primates, and further tests may verify if it is common among marsupials.
Did You Know?
- Humans and closely related primates are usually trichromats, as are some female New World monkeys and both sexes of howler monkeys.
- Recent research suggests trichromacy may be common among marsupials, including the honey possum, fat-tailed dunnart, and nocturnal sugar glider.
- The three types of cones in primates have peak sensitivities at approximately 560 nm (L), 530 nm (M), and 420 nm (S).
- Trichromacy allows animals to distinguish brightly colored fruit and young leaves from other vegetation.
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