Magnetic Stimuli Essay, Research Paper
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 the detection and measurement of the earth’s magnetic field by indirect methods is overwhelming, a good deal has also been done to test the hypothesis from a practical point of view. When dealing with certain biological systems the results of such experiments are always more convincing than physical arguments that may be based on false premises (Carthy 112). Griffin reported two techniques aimed at disturbing an electro-magnetic apparatus in 1940. The first passed electric currents through the heads of Pigeons before the release and the other subjected Leach’s Petrels to an intense electro-magnetic field for a few seconds before the beginning of the outward journey from home (Griffin 61). In both cases no effects on homing were apparent but the techniques were not very critical as it is really required that the bird should be subjected to ‘interference’ during the actual flight (62). Fixing magnets rigidly to the head will not be a satisfactory test since the additional field would be constant which could be taken in to account by the analyzing mechanism. It is
To the contrary of Yeagley’s findings, many retests that involved the variables of his experiments proved countless times that his hypothesis was completely unacceptable. Unacceptable not only because of its theoretical impossibility, but also because the massive field experiments have produced entirely negative results (Yeagley 1036). At the same time it is well to be careful of dismissing possible extensions of known senses. In the year 1951, Lissmann demonstrated a remarkable form of proximate orientation in certain fish. These fish set up a weak electrical field around themselves and apparently were able to detect not only their surroundings, but also their prey by changes in impedance. It has been proven that fish of this nature will react to a moving magnet. In 1953, Griffin showed that a form of echo-sounding is used by a bird nesting in dark caves (Lissmann 201).
Inspection of maps of the United States shows frequent anomalies in the horizontal and vertical components, having strengths of several hundred gamma and extending over several hundreds of kilometers. There seems to be no reason for one to suppose that regional anomalies in other parts of the world would be of any different character. This evidence suggests that position-fixing by bicoordinate navigation using any of the magnetic elements may be possible (Griffin 73). If no regular gradients exist, over the distances which pigeons navigate, it is difficult for one to imagine how the strategy would be successful. No physical mechanism is able to separate the earth’s main field from the anomaly field, and it is not clear what signal-processing a pigeon could use to achieve this (74).
It is a simple and well-known fact that pigeons home; they do so almost regardless of what we may do to them. Therefore, natural and manipulated changes of the magnetic field have been shown to affect their homing behavior to a varying degree (Dorst 66). In the year 1974, Walcott and Green concluded that artificial fields applied to the pigeon’s head on the release sight, under overcast conditions, disrupt the bird’s ability to maintain a constant compass course. The artificial field in the order of strength of the earth’s magnetic field obviously upsets the bird’s magnetic compass, either by changing its objective north direction, or by field strengths above or below the appropriate level (71).
In 1978, Kiepeenheuer proposed the inversion of the vertical or the horizontal magnetic field component during transport, results in a diverted or random orientation on release (Kiepenheuer). Aside from such effects, after severe manipulations of the magnetic field, much more subtle changes in magnetic field strengths in the order of one percent or even far less of the normal field have been demonstrated to affect the orientation of homing pigeons. Temporal fluctuations, as well as slight topographical changes, may result in a shift of the mean of vanishing bearings or even in random orientation of the pigeons on release. It appears improbable that such small variations in magnetic field strength have any influence on the magnetic compass of the bird, since, according to the results of Wiltschko dealing with robins and other small birds, the magnetic compass seems to be somewhat resistant to deficiencies of the field which is much larger than some of the ones in question. We therefore may have to conclude that the pigeon does not rely on some type of magnetic compass, but that, at least to some extent, its navigational abilities are influenced by very slight changes in the earth’s magnetic field. We might even speculate that systemic variations, or topographical peculiarities of the magnetic field might serve the pigeons as a grid or even as landmarks by which they are able to navigate (Kiepenheuer).
In this context, vector navigation faces fewer difficulties. In order to gain magnetic compass information, a pigeon would have to measure the direction of the earth’s field with far less precision than if it were using this directly as a position-fixing cue. Changes in declination would result in 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).