magnetic stimuli





The Role of Magnetic Stimuli in Animals
In as early in 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 than biological. There are suggestions that there may be at least a north/south klino- or tropptaxis to the magnetic field. It must be remembered that no-one has yet been able to give the slightest indication of what the magnetic-sensitive organs are, nor whether they have sufficient acuity for us to be able to speak of a menotaxis, let alone orientation. By contrast, the bird’s eye is a very highly developed sense organ. Recent work suggests that European robins do not even detect north from the polarity of the magnetic field but from its angle to the horizon (43).
Hypotheses that the earth magnetic field could provide a navigational grid date as far back as the work Viguier completed in 1882. The outcome of his work suggested that birds could detect and measure three components of the field, its intensity, inclination (the angle which a compass needle makes with the horizontal) and declination (the angle between magnetic and geographical north). These three components vary more or less with independence of one another so that their isolines would form a complex grid. Over the next few years, several different scientists restated this hypothesis, with minor variations. The complete lack of evidence for any direct reaction to a magnetic field in birds is a very questionable issue (Carthy 46). Can birds actually use magnetic stimuli as an internal compass? Well Casamajor (1927) and Wodzicki (1939) found that fixing magnets to the head of the Pigeon and the Stork, had no effect on their homing ability. There are many other theoretical difficulties that may provide an answer as to why the magnets did not affect the homing ability of the two animals in question (48). An important one is that measurement of declination requires an exact knowledge of geographical north. Elimination of the declination isolines from the magnetic grid reduces the plausibility of the whole scheme, since the inclination and intensity isolines generally cross one another at oblique angles making good ‘fixes’ impossible (Lincoln 79).
With these initial theoretical difficulties in mind the concept of direct sensitivity was therefore replaced by one of indirect sensitivity to the earth’s magnetic field, and the whole hypothesis was resurrected (Lincoln 89). In the year 1947, Yeagley suggested that the flying bird, which acted as a linear conductor moving through the lines of force field, could detect the earth’s field. Theoretically this would result in a small potential difference being set up between the two ends of the conductor, though at this time had not been demonstrated in practice (90).
While the theoretical case against