Eyes to See With
The lens of the vertebrate eye focuses incoming light onto the retina lining the eye, where two different cells — called rods and cones because of their shapes — distinguish light levels and identify color, respectively. While a cone-poor retina is probably the ancestral mammalian condition, all mammals have some cones in their retinas and thus some capacity for color vision. But only primates, including some New World monkeys, Old World monkeys and humans, have trichromatic vision, allowing them to detect the red, green and blue portions of the spectrum.
The retinas of some diurnal mammals, such as squirrels, along with those of reptiles and birds, are almost completely packed with cones. Resulting acuity and color vision may be strong, but night vision suffers from lack of rods. Nocturnal animals with retinas that rely mostly on rods see principally in monochrome.
In rats and mice, rods and cones are distributed all over the retina, but in general these photosensitive cells tend to predominate in one area or another. In mammals, the cones are concentrated in the fovea, located in the center of the retina. Many of these cones have a one-to-one connection with nerve fibers transmitting impulses to the optic nerve and the brain; thus the fovea offers the greatest acuity. The fovea of mammals that live in forests and other habitats with contrasting elements is a circular structure, whereas that of mammals that live in grasslands and savannas, such as cheetahs and gazelles, tends to be an elongated strip, which allows for a wider focus area.
When light enters the eyes of nocturnal mammals, it stimulates the rods, then bounces off a reflecting membrane, the tapetum, behind the retina. Some light bounces back, giving the rods a second chance to absorb the rays. This membrane is responsible for the eye-shine of animals caught in headlights. Retinal pigments — and therefore the color they reflect — differ across species. For example, deer reflect white light, whereas lynx and other cats reflect gold, skunks amber, raccoons yellow, and opossums and bears various shades of orange or red.
Binocular Vision
Many mammals have front-facing eyes, a sign of a true predator. Binocular vision from the overlap of the fields of vision of each eye, along with the depth and distance perception this affords, help canids and felines to scope out and chase prey. Animals that are frequent prey, such as snowshoe hares, have eyes closer to the sides of the head and rely more on peripheral vision; many can scan the full 360-degree visual range for danger without moving their head or rolling their eyes.
