Why most mammals see the world in only two primary colors

Health · Published 3 October 2026 Swipe more like this →

In short: While humans generally rely on three types of cone cells to perceive a full spectrum of color, most placental mammals are naturally dichromatic. This limitation, which reduces color vision to two dimensions, was likely an evolutionary adaptation to the dim light conditions faced by our distant, nocturnal ancestors.

Dichromacy is defined by the presence of only two functional cone cells in the retina. Unlike trichromats, who require three primary colors to represent their visible gamut, dichromats can evoke every color in their range using only two. This visual system is simpler than the trichromatic vision typical of humans, yet it remains the standard for most placental mammals. The common ancestor of all vertebrates was originally tetrachromatic, possessing four opsin classes. However, early mammalian evolution involved a nocturnal bottleneck that led to the loss of two of these opsins, as dichromacy may have provided an advantage for distinguishing colors in low-light environments.

In humans, dichromacy is a form of color vision deficiency resulting from the absence or malfunction of one of the three standard cone cells. Protanopia and deuteranopia, which involve the loss of L-cones and M-cones respectively, are sex-linked conditions affecting about 1% of males each. Tritanopia, the rarer blue-yellow deficiency, occurs in approximately 1 in 100,000 people and is not sex-linked. While researchers estimate that a dichromat might distinguish around 10,000 colors—a fraction of the millions perceived by trichromats—these individuals often possess superior abilities in detecting flickering stimuli at higher frequencies, such as 10 or 16 Hz.

The mammalian rule of dichromacy has notable exceptions. Old World monkeys and apes have re-evolved trichromacy, while marine mammals have shifted toward cone monochromacy. New World monkeys show mixed patterns: while most males are dichromats, roughly 60% of females are trichromats, with the exception of owl monkeys, which are monochromats, and howler monkeys, which are trichromatic in both sexes. Even among marsupials, where trichromacy is widespread, some species like the South American Didelphis albiventris remain dichromatic. This diversity highlights that while dichromacy is the ancestral mammalian baseline, evolutionary pressures have frequently driven shifts in visual capability across different lineages.

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