Scientists studying the neurophysiology underlying animal behavior (neuroetholo-gists) have been amazed for many years at the ability of migrating animals unerringly to find their destinations in the apparent absence of obvious terrestrial or celestial orienting features. Factors such as the positions of stars, sun, and moon (when visible) and water chemical content (where relevant) have been implicated as navigational guides, and in some cases may be guides. However, behavioral evidence has accumulated showing that certain diverse animals such as yellowfin tuna, trout, sea turtles, pigeons, spiny lobsters, honeybees, and certain mollusks and bacteria use the Earth's magnetic field to guide their migratory and systematic movements, often in conjunction with certain other geophysical factors mentioned above.

The Earth's permanent magnetic field vector, Be, varies over the surface of the Earth. It is, of course, the geophysical phenomenon that in past centuries, has permitted planetary exploration. The Earth's magnetic field has two orthogonal vector components, one tangential to the Earth's surface (the horizontal component) and a second, vertical component directed radially downward (or upward). For example, in Cambridge, MA, the horizontal component is about 1.7 x 10-5 Tesla directed about 15° west of true (polar) north; the departure of the horizontal component of Be from true north is called its variation or declination. The vertical component of Be is about 5.5 x 10-5 Tesla. The dip or inclination angle of Be is about 73° (from horizontal) at Cambridge. (Note that 1 Tesla = 1 w/m2 = 104 Gauss.) By definition, the dip angle and vertical component of Be are zero on the Earth's magnetic equator. The declination, dip angle, and strength of the Earth's Be vary from place to place on the Earth's surface, and change slowly with time.

To orient itself in the Earth's weak dc magnetic field, an animal obviously must have neural sensors that can sense the Earth's magnetic field vector relative to its body direction. A migrating animal must also possess the cognitive apparatus that allows it to relate its present orientation determined magnetically with its desired travel direction, i.e., it may have a stored, magnetic "map sense." In the case of north/south migration, requirements on the magnetic sensor may be simple; the animal must follow a constant course until other physical cues tell it that it is near its destination, where a "fine approach" mode of navigation can be employed. There is also behavioral evidence that some animals use the inclination angle of Be in their magnetic orientation/navigation behavior. That is, they effectively sense Be and break it down into its horizontal and vertical components.

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