The Role of Magnetic Stimuli in Animals

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In as early as the year 1855 Minddendorf proposed the idea of broad front, one-direction migration also suggested a means of orientation, that birds were capable of detecting the magnetic poles and of maintaining their bearing therefrom. Since then many similar ideas have continued to pop up at random intervals (Carthy 56). An immediate difficulty is the lack of any structure or tissue that could possibly react to the magnetic field. In the year 1948, the discovery of certain forces were indeed produced by placing 'non-magnetic' material in a magnetic field, however they were far too minute to merit any serious consideration (Carthy 59). Some reports speak of heightened locomotor activity and heartbeat, when in close proximity to increased magnetic fields; a fact which might mean that a kinesis-based magnetism is a possibility. A study was done in which magnets were attached to birds and released in sunny (or starry) conditions have repeatedly been shown to have no effect on orientation (Dorst 24). However recently it has been shown that pigeons repeatedly released under conditions of heavy overcast (in areas where the recognition of landmarks could not rigorously excluded) have an orientation which is disturbed by magnets. Most workers with caged birds have failed to find any tracer of orientation in a planetarium with all the stars blocked off or in any closed room (32). This phenomenon definitely shows evidence that some if not all birds use celestial bodies. One group studying magnetic orientation in birds has consistently claimed to the contrary. Their accumulated data does seem to show some directional tendencies but the scatter distribution is so wide that their significance could be said to be more statistical... ... middle of paper ... ... navigational errors, but considering compass information alone the changes are relatively small (Lincoln 102). In the Northeastern United States, declination changes one degree in about 80 kilometers and in the regional anomalies mapped by variations of more than five degrees are infrequent. At geographically small, high-amplitude anomalies, the deviation of a magnetic compass can be larger (102). In conclusion, the interpretations of the observed magnetic effects on animal orientation seem paradoxical. Theories that might explain the animal's extreme sensitivity appear to be ruled out by the earth's field. Vector navigation, on the other hand, restricts the use of the earth's magnetic field to compass information only, but this theory does not readily explain either the animal's sensitivity to magnetic fields or sight-specific magnetic effects (Carthy 86).

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