Color vision is crucial for many invertebrate and vertebrate animals. It is achieved by photoreceptor cells called cone cells that contain opsins sensitive to different wavelengths of light (Fig. 36.17). The more numerous rod cells are also sensitive to light, and most sensitive to blue-green light. Because all rod cells have the same opsin (called rhodopsin), the brain interprets the light detected by rod cells as shades of gray, not as blue-green. The human retina contains about 6 million cone cells and about 125 million rod cells. Because of their greater number and sensitivity to light, rod cells enable animals to see in low light. Together, rod and cone cells constitute approximately 70% of all sensory receptor cells in the human body, highlighting the importance of vision in perceiving our environment.

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Vertebrate cone cells likely evolved from a rod cell precursor. Most non-mammalian vertebrates have four types of cone cell, each with a different opsin, whereas most mammals have only two types of cone cell, and two opsins. It is likely that two opsins were lost early in mammal evolution, when mammals were small, nocturnal, and burrowing. Old World primates, apes, and humans, as well as some New World primates, regained trichromatic (three-color) vision by means of gene duplication, likely in response to selection for better ability to locate fruit, a key part of their diet. Each human cone cell therefore has one of three opsins, which absorb light at blue, green, or red wavelengths (Fig. 36.18). Stimulation of cone cells in varying combinations of these three opsins allows humans and other primates to see a full range of color (violet to red). Fish, amphibians, reptiles, and birds also have good color vision, including for many the ability to see ultraviolet.

Cone cells require higher levels of light than rod cells to become stimulated. The low sensitivity of cone cells to light makes it hard to detect color at night. Cone cells are most concentrated within the fovea of the retina, the center of the visual field of most vertebrates (see Fig. 36.15). Cone cells provide the sharpest vision. Animals with particularly sharp vision, such as hawks and other birds of prey, have an extremely concentrated number of cone cells in the fovea, approaching 1 million/mm² (compared with about 150,000/mm² in the human fovea). Birds of prey can see small prey on the ground from high in the air. Many birds also have eyes with two foveae, one projecting forward for binocular vision and one projecting to the side to enhance the sharpness of their side vision.

In contrast, rod cells are absent in the fovea but predominate in the periphery, making it easier to detect motion in the periphery, particularly at night. The eyes of many nocturnal predatory mammals, such as foxes and cats, contain mainly rod cells, enhancing nighttime vision. These animals also have a pigment located behind the retina that reflects light past the photoreceptors, enhancing the ability to sense light under low-light conditions. The reflection of light by this pigment creates the “eyeshine” that you see when a flashlight or a car’s headlights shines on a nocturnal predator’s eyes at night. Great white sharks are able to hunt at night because they also have a reflective pigment within the retina.

Quick Check 4 What is the primary role of rod cells, and what are the two roles of cone cells in the retina?