The Ecology of Humans (15-22) Evolution of Sight continued 3


An obvious significant variance in vision systems is eye placement, which depends upon whether the species is predator or prey. Predators have eyes front, to optimize focus and distance estimation. The trade-off is limited peripheral vision.

The strategic placement of the eyes of crocodiles and alligators allows them to stalk prey while keeping their bodies submerged and hidden. Eye placement is to the side, more optimized for field of vision. Crocodiles have keen hearing, albeit at a limited range of 50–4,000 hertz.

By contrast to predators, the survival of prey animals is greatly aided by field of vision. So, prey animals have eyes on the side of their heads – their minds create a panoramic mental image, optimized for motion detection but with suboptimal vision directly in front and poor depth perception. Prey animals often have a sensitive sense of smell and acute hearing to compensate for poor visual focal acuity.

The distribution of photoreceptors in an eye matches the area where visual acuity is needed. Animals that live on the African plains need an excellent line of sight, and so have a horizontal line of high-density photoreceptors. Tree-dwellers need good vision all around, and so sight tends to a symmetrical distribution, such that acuity decreases from the center.

Flight is especially taxing on the vision system. Males that mate mid-air have tremendous visual acuity, as they need to spot and assess potential mates against a flowing panorama.

At the other extreme, animals active at night tend to have larger eyes to increase light capture. Nocturnal animals tend to methodical movements as low light limits sight.

Many animals see far more of the world than humans can. Some moths can see color at night when we can hardly see at all. The color sense of moths is limited, as they are dichromats: able to see 2 color pigments and thus able to distinguish yellow, green, and blue, but unable to perceive red, which is a longer wavelength. Dichromats can see ~10,000 colors.

Most land mammals are thought to be dichromats. Marine mammals are cone monchromats: unable to perceive different color hues as they have only one type of cone cell. More particularly, both orders of sea mammals – pinnipeds (seals, sea lions, and walruses) and cetaceans (dolphins and whales), have cones that perceive mid-wavelength light, which corresponds to green, which travels best underwater.

Humans and closely related primates are trichromats: color conveyance from 3 different cone types, thus able to see red as well as green and blue. Each of the 3 different cone types in a human eye can perceive about 100 distinct color gradations. The combinatorial effect allows trichromats to see millions of colors. The human eye can distinguish over 7 million colors. Red cones predominate in the human eye as an evolutionary adaptation to see subtle changes in skin tone and facial expression.

There are exceptions to human trichromacy. Females have 2 X chromosomes, as contrasted to male XY chromosomal configuration. Via an anomaly in X inactivation (normally only 1 X chromosome is active), a tiny percentage of females carry cone cell pigment variants which provide for 4 kinds of cone cells. This aberration affords tetrachromacy, and thereby discrimination of 100 million shades of color.