The ability to see in color is not unique to humans, but there are many animals that can only see in black and white. Color vision is possible because of the presence of cone photoreceptors in the eye; the different types of cone cells respond to different wavelengths of light, resulting in the perception of different colors. Cone cells are not active in low-light conditions, unlike the more sensitive rod photoreceptors.
Monochromats, Dichromats and Trichromats
Humans, along with several other primates, are trichromats when it comes to cone receptors – we have three different types. It was once thought that most mammals only saw in black and white, but we now know this is not the case – dogs and cats, for example, are dichromatic with limited color vision. Animals that are monochromatic, with only one type of cone, can typically only see in shades of black, white and grey.
Diurnal and Nocturnal Animals
The amount and ratio of rod to cone cells varies among animal species. In terrestrial animals, these factors are largely affected by whether the animal is diurnal or nocturnal. Diurnal species, such as humans, usually have a higher density of cone cells than nocturnal species, which have a greater number of rod cells to help them distinguish shapes and movement in low light. Monochromatic nocturnal mammals include various bats, rodents such as the golden hamster and flat-haired mouse, and the common raccoon.
Old World primate species, such as chimpanzees, gorillas and orangutans, have trichromatic vision as do humans, but New World monkeys exhibit various ranges. Howler monkeys have three cones, but male tamarins and spider monkeys only have two, with females split between trichromancy and dichromancy. Night monkeys, or owl monkeys, are monochromatic. As their name suggests, they are nocturnal, with better vision in dim light than other primates have.
Fish and Marine Mammals
Most marine mammals are monochromatic; this includes pinnipeds, such as seals, sea lions and walruses, and cetaceans, such as dolphins and whales. Most fish are trichromatic, with good color vision, but there are some exceptions. The only animals known to have no cones at all, and therefore that are incapable of color vision, are skates, cartilaginous fishes related to rays and, more distantly, to sharks. Sharks are also monochromatic, but rays are thought to have relatively good color vision. Marine mammals and fish may have lost their color vision over time as it was not advantageous in the water.