A Gallery of Animal Eyes

The most complex and efficient eyes belong to higher animals–especially vertebrates such as fish, birds, and mammals. Like humans, most animals have adjustable lenses and myriads of light-sensitive cells for recording sharp images.

But while human eyes are basically the same except for the color of their irises, the eyes of animals take on an almost infinite range of colors, shapes and sizes. The variations reflect evolutionary adaptations to the animal’s habits and environment.

Night-prowling animals most often have large, circular pupils because their eyes must be able to catch every stray glimmer of light. But animals like the cat, which hunts both night and day, have made special adaptations to allow them to see clearly in both dim and intense light. The cat’s pupil is not around but oval-shaped. At night the oval opens wide, but in strong light it closes to a tight slit. Similarly, the gecko–a lizard which feeds at night–shuts its pupils in daylight but leaves four tiny diamond-shaped holes to admit light.

The green whip snake has an odd pupil shaped like a keyhole, allowing it a sweeping view forward through a horizontal slit and a more direct view to the side through the round portion. Among sea dwellers, the skate has a fringed awning to protect its eyes from strong sunlight. But the oddest adaptation of all belongs to a tropical fish, the Anableps, which swims along the surface with its eyes half in and half out of the water. Each eye has two pupils, one for looking upward into the air and the other for gazing down into the water.

Life Science Library - Light and Vision