Positional and Navigational Aides
It is clear that the existence of the biological cycle phenomenon is dependent upon the living subject having precise knowledge of its position on the earth. Since it also appears that the earth's electromagnetic field is the most important single signal for this function, it seems likely that it is similarly involved in the migrational and direction-finding abilities of many animals. This possibility has been confirmed by recent studies. Within the past few years a marked increase in interest in this area has occurred following some truly remarkable findings.
That various species of animals migrate with precision along definite geographic routes or with extreme temporal precision has no doubt been known to mankind since the dawn of civilization. While the true extent of this phenomenon was not apparent until it began to be studied by biologists, the Egyptians made use of the navigational ability of the homing pigeon as early as 2000 B.C. We now have a much better idea of the ability of certain animals in this regard. The common monarch butterfly, for example, annually travels over 2000 miles from Hudson Bay to South America, crossing several hundred miles of the Caribbean Sea, devoid of landmarks, in the process. The Arctic tern breeds in the North American sub-arctic region and travels to the antarctic pack ice for the northern hemisphere winter season, a distance of 11,000 miles. Species of salamanders, after hatching out of eggs laid in the mountain streams of California, will move out as far as 30 miles away across rugged terrain to grow to maturity, only to return to precisely the same stream and the same spot that they hatched out in years before. The palolo worm of the South Pacific Ocean migrates only the short vertical distance from the coral reef to the surface to breed, but it does so only on the first day of the last quarter of the October-November moon.
The obvious question is how can so simple an animal as a butterfly, for example, whose brain can literally rest on the head of a pin, accomplish this navigational feat? Experimentally, the earliest interest was directed to the ability of the homing pigeon to navigate with precision over long distances and the foraging activities of the honeybee, which demonstrated a similar ability over distances of several hundred feet. Both of these phenomena were more amenable to experiment than the annual migrations of the other species.
In 1973 Karl von Frisch won the Nobel prize for a series of studies done in the 1940's on the navigational ability of the honeybee. He found that they utilized both a sun angle compass and a polarized light system for navigation. Perhaps more amazing was their ability to communicate the vector and distance of a food source to other workers in the hive by means of a "dance" that used both the sun angle and the gravitational vector. While the sun angle and polarized light were quite efficient they would be absent on cloudy days. However, the bees were still able to navigate with the same precision under those conditions. There obviously had to be a back-up system of some kind available to these animals that was totally independent of these two cues.
In the initial studies on the homing pigeon, Kramer in 1953 observed that shortly after release these animals adopted a vector direction of flight that was appropriately homeward (18). Therefore, these animals must possess not only a "map" but a "compass" as well. Shortly thereafter pigeons were shown to have a solar compass similar to von Frisch's bees; however they were also able to navigate unimpeded on cloudy days, indicating the presence of a similar back-up system. In 1947 Yeagley had proposed that the pigeon possessed a "magnetic sense" that enabled it to utilize the earth's magnetic field in the same fashion that man utilized his magnetic compasses (19). This was of course promptly challenged. In the following year, for example, Clark and Peck, in a totally inadequate experiment involving one pigeon exposed to a variety of electromagnetic fields, stated that the animal displayed no discomfort and therefore seemed not to possess a magnetic sense (20)! In other experiments, magnets were attached to the heads or wings of pigeons, but no effects were observed.
The question remained open until Keeton in 1971 reasoned that the magnetic sense, if it existed, had to be the back-up system to the sun angle and polarized light systems. In that case, any attempt to confuse the magnetic system with attached magnets would fail if the pigeons flew in the daylight on a clear day! He observed that this was indeed true when small magnetics were attached to the back of the pigeons head on a clear day. But if the same pigeons were released on a cloudy day, they failed to display their usual navigational ability and were lost (21). In order to study this phenomenon at any time, Keeton devised translucent contact lenses for the pigeons that blocked both the sun angle and polarized light. The same disorientation was observed when the birds were fitted with these and also with the small magnets. However, pigeons wearing translucent contact lenses without magnets attached to their heads navigated over distances of hundreds of miles with perfect precision. The only navigational system available to them under these circumstances was their magnetic sense. These animals experienced difficulties only after appearing over their home loft at Cornell University, since the lenses prevented them from seeing the ground. They would fly in tight circles over the loft, slowly decreasing their altitude until close to the surface, when they would flutter to a landing similar to a helicopter.
In further studies, Walcott and Green fitted homing pigeons with small pairs of Helmholtz coils that permitted them to vary both the field magnitude and vector (22). As expected these animals navigated well on sunny days but became disoriented on cloudy days, flying directly away from the home loft if the field vector of the coils had the north pole directed up. If however the coils were set with the south pole directed upwards the birds were still able to navigate properly even on a cloudy day. Walcott interpreted these results to mean that the birds were using magnetic north as a reference point. During the same period of time Helmholtz coils were also used to study the bee's magnetic sense. When hives were enclosed within such coils the communicating "dance" became disoriented, but 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 serves such a function. The presence of an electrosensing mechanism is common among oceanic fish and the suggestion has been made that this capability was related to the direction-sensing associated with their migratory behavior, either by directly sensing the earth's electric field or by sensing the currents and voltages generated by the movement of water currents (e.g., the Gulf Stream) through the earth's magnetic field.
The American eel is one such migratory species, hatching out of eggs laid in the fresh water streams of the eastern seaboard and migrating as small larval "elvers" about one inch long out to the open ocean. Ultimately they travel to the Sargasso Sea, navigating with precision over a thousand miles of open ocean. In the Sargasso Sea the elvers grow to adults and when sexually mature they reverse the pathway, traveling back to the same streams they were hatched in to mate. In 1972 Rommell and McCleave studied the sensitivity of these animals to electrostatic fields using a conditioned reflex experimental format (28). The animals displayed a sensitivity to DC fields of 0.67 µV/cm and 0.167 x 10-2 ,µamp/cm2, values well within those generated by water currents flowing through the earth's magnetic field. The eels were found to be sensitive to these electrical parameters only when the field was oriented parallel to the long axis of the animal; fields perpendicular to the long axis were not sensed. As Rommell and McCleave point out, if one assumes the ability to distinguish polarity, the animals had only to orient themselves to optimize the signal of the appropriate polarity and they could migrate in both directions (to and from the Sargasso Sea) with ease.
From all the foregoing reports it is obvious that the present normal earth magnetic field is an important parameter of the environment for living things. Changes in the fields in the past have been shown to exert evolutionary pressure and possibly even to have been associated with biogenesis. All living things are at present intimately tied to various aspects of the earth's field, and it seems quite possible that even more dramatic findings will be reported in the future. It must be kept in mind that the relationship is a subtle one, in contrast to the more obvious parameters of the environment. Since the present relationship between living things and the electromagnetic environment is the result of several billions of years of development, the question of the biological effects of abnormal electromagnetic parameters introduced into the environment by man's activities becomes of some importance.