Animals that must live in darkness–in the gloom of caves, underground burrows and the night world–have special sight problems. Over millennia of evolution, they have been solved either by the development of oversized eyes or by the sharpening of other senses to supplement weak, ineffective eyes.
The night-hunting owl has eyes so large they cannot turn in their sockets. To follow the path of a scurrying field mouse, the owl swivels its head, and can twist it around to look directly backwards. Certain owls have eyes which are so sensitive that they can detect shapes in light many times dimmer than the minimum required by human eyes.
There are other adaptations that help nocturnal animals to see in the dark. A cat's eyes glow in the beam of approaching headlights because a mirrorlike lining at the rear of each eye reflect the light forward again, giving the receptors in the cat's eye a second chance to register each particle of light. The cat also has an especially large number of cells sensitive to dim light, although it cannot distinguish colors.
Moles, bats and shrews, which live in almost complete darkness, have eyes which have degenerated to such an extent that they are almost sightless. The mole rat is virtually blind; its two rudimentary eyes, no bigger than pinheads, detect only shades of light and dark, and are useful mainly as a danger signal when its burrow is broken into from above.
Life Science Library - Light and Vision
2/14/2010
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