Sunday, January 14, 2007

Keeping Watch In Two Directions At Once

Chameleons are among the few creatures that can move each eye independently, enabling them to look in two directions at once. These small lizards, commonest in Africa and Madagascar, have large, distinctive eyes that are covered in skin except for a small central area. The eyes are positioned one on each side of the head, and a chameleon can use one to watch for insects to feed on while the other keeps watch for approaching enemies, such as birds of prey.

A chameleons's ability to swivel its eyes instead of swinging its whole head round is a great advantage, because it spends most of its life balancing on tree branches high above the ground. Once a chameleon spots a meal, both eyes swivel round and lock on the target. In this position the fields of vision from both eyes overlap to give an accurate three-dimensional image. When the prey is in range, out shoots the chameleon's long, sticky tongue to capture it. A chameleon can extend its tongue for a distance nearly as long as its 7 - 10 in (17 -25 cm) long body. Dwarf chameleons of southern Africa direct their tongues just ahead of the target, anticipating the victim's flight path.

An insect resting on a branch out of the chameleon's range is not necessarily safe, because the lizard remembers its position. Even if it has to make a detour and the target is out of sight temporarily, it still approaches from the right direction and catches they prey the moment it is within tongue range.

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