foraging outside of the coils was unaffected. That pigeons and bees possessed a magnetic sense was
evident. However, how this was done was completely unknown.
In 1975 Blakemore reported an astonishing observation (23). Electron microscopy of certain
bacteria, known to have the ability to orient in the earth's magnetic field, disclosed the presence within
them of microcrystals of magnetite that appeared to be single domains (the smallest unit magnet). The
possibility that similar units existed in both bees and pigeons occurred to Gould et al. However, since
electron microscopy of even the bee's brain would be a lifetime task, they adopted an alternate
stratagem (24). Whole bees were examined by SQUID magnetometers and found to be magnetic; the
simplest explanation then being that somewhere in the bee was a similar collection of magnetite
crystals. Subsequently, the bees were dissected into various anatomical parts and each part examined.
The magnetic signal was found to be coming from the abdominal region, although as yet no
visualization of the presumed magnetite crystals has been reported. Using the same technique, Walcott
et al. began a study of the heads of homing pigeons (25). By a similar process of dissection and
subdivision using nonmagnetic tools, they finally located a deposit of magnetite between the brain and
the inner table of the skull, unilaterally ! This material was visualized microscopically and found to
consist of electron-dense structures of a size compatible with single domain crystals of magnetite,
imbedded in a connective tissue that was richly supplied with nerve fibers. While these observations do
not firmly establish that this structure is actually that used by the pigeon for navigation, it seems likely
that it is.
However, in common with all new scientific observations, this one raises more questions than it
answers. The presence of such a mechanism in such divergent animal types as bees and homing
pigeons would seem to indicate that the mechanism is a generalized one present in all species, although
perhaps more highly developed in those animals possessed of outstanding navigational ability. Is such a
structure, or its analog, present in mammals, including man, and if so, what functions does it serve?
More fundamental and perhaps more important is the question of how the information is "read out"
from this structure. It is apparent from Keeton's experiments that the magnetic compass of the pigeon
far surpasses any manufactured by man in accuracy. While it is known that the magnetic field varies
geographically in its characteristics and can be influenced by such local tactors as deposits of iron ore,
our instruments have never revealed anything resembling a "grid-like" formation in it. Yet it is this sort
of magnetic "map" that seems to be what the pigeon is sensing! It is possible that the earth's field has an
informational content that we are unaware of? Finally, while a number of mechanisms can be proposed
for the generation of signals by the magnetite, we have no idea how these signals are transposed into
appropriate navigational directions in the animal's nervous system.
Most recently, Baker found an "unexpected" sense of direction in humans which seemed to be
associated with a similar magnetic sense (26). In his experiment, blindfolded human volunteers were
taken on a complex journey over considerable distances and upon completion were asked to point out
the direction of the origin of the trip. Results were similar to those observed in the initial vector
directions of pigeons and salamanders after spatial displacement, indicating a similar directional ability
in the human, even when devoid of visual or auditory cues. Subjects wearing bar magnets ranging from
140 to 300 gauss in strength on their heads demonstrated essentially random vectors in the same type of
experiment. In a recent series of experiments, Gould and Able were unable to confirm this observation
(27). However, their experiment was conducted in Princeton, New Jersey, an area much more
electromagnetically "contaminated" with man-made signals than the rural experimental area of Baker.
While the use of the earth's magnetic field as a navigational aid for many living things seems
fairly well established, it is by no means the only component of the earth's electromagnetic field that
ELECTROMAGNETISM & LIFE - 55