TAXES

What does an animal do when it has a specific need, such as food, a higher humidity environment, or shelter from the sun, and has information about the location of the needed resource? It may engage in an directed search, or taxis. There's a small family of terms surrounding the basic word "taxis". For example, an animal that moves towards light is "positively phototactic", while an animal that flies away from the wind would be "negatively anemotactic".

The key in tactic orientation is the localization of the source of stimulus. From a human perspective, its seems like a simple thing to visually identify the location of an object or to audtiorially find the source of a sound. We do this by triangulation--our two sensory organs (two eyes, two ears) simultaneously perceive the stimulus and we use the difference in perception between between the two organs to calculate distance/direction. In a simple sense visual triangulation gives us our ability to tell how far away an object is, although in reality we use addtional strategies to refine this information. Key to visual or auditory localization of a stimulus is simultaneous perception of the stimulus by the paired sense organs.

In this fanciful drawing, something vaguely resembling a frog determines the distance of its prey, a fly, by triangulating on it. The angles of the triangle depend on the distance of the prey from the frog. Triangulation allows the "frog" to accurately strike with its tongue.

Simultaneous perception isn't always possible. Again, putting this in a human frame of reference, we (along with most vertebrates) have only one nose, which corresponds to a single sense organ. This would seem to make triangulation impossible. What is the solution? With a single sense organ, sequential perception allows triangulation. By moving the sense organ from side to side, a triangle can be created in time sequence. Watch a dog with its nose to the ground, following a scent. It stays on course to find the scent be using small side-to-side movements of its nose, triangulating on the scent source.

Many animals (grasshoppers and lizards are great examples) combine simultaneous with sequential sensory strategies to maximize their accuracy in triangulation. By moving their heads back and forth while triangulating on a prey item or a spot to jump towards, they increase the angle, making their measurement more accurate. This makes it as if their eyes are further apart.