FREE ESSAY ON MAGNETIC STIMULI

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 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 therefore essential
that the magnets should move relative to the bird's body. It was imperative to attach
small, powerful magnets to the wins of Pigeons, sewing them on through the metacarpal
joints (70). The fluctuating e.m.f. induced in the bird's body when the wings were
beating would swamp any measurement of that induced by the movement of the body through
the earth's field (71). By using only ten Pigeons treated in this way and ten control
birds with copper bars, Yeagley claimed to have established that the magnets had a
strongly deleterious effect on homing (73). 
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).