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
ELECTROMAGNETISM & LIFE - 54