Electromagnetic Radiation Effect on Foraging Bats - Recent Advances and Issues in Environmental Science - page 73

each rotation). The rotation of the radar antenna would reduce the time that bats were
exposed to pulse-modulated microwave radiation and would therefore attenuate any
potential deterrent effect. When the radar antenna was fixed to emit a unidirectional
signal a fourfold increase in field strength was observed at all distances.
When foraging sites were exposed to a short pulse length signal from a fi xed an-
tenna there was a signifi cant reduction in bat activity during experimental trials (bat
counts and bat passes dropped by 15.5% and 13.3% respectively). Although, once the
Bonferroni correction had been applied, the difference in bat passes between control
and experimental trials was no longer signifi cant. An even greater level of signifi cance
was however observed when foraging sites were exposed to a medium pulse length
signal from a fi xed antenna (bat counts and bat passes dropped by 38.6% and 30.8%
respectively). Clearly, this represents a substantial reduction in bat activity. However,
bats continued to forage at each site during experimental trials, and on no occasion
were bats observed behaving abnormally or actively avoiding the beam of the radar.
However, temporal and spatial fl uctuations in bat foraging behavior are common [23,
24] and therefore results have to be treated with caution. Despite this caveat the sig-
nifi cant reduction in bat activity during all experimental trials with a fi xed antenna
supports our hypothesis that electromagnetic radiation exerts a deterrent effect on for-
aging bats. This raises questions regarding the mechanisms through which bats could
perceive electromagnetic fi elds and why they would seek to avoid them.
Nicholls and Racey [21] suggest that the aversive behavioral response of foraging
bats to electromagnetic radiation may be a result of thermal induction. Studies inves-
tigating the behavioral response of laboratory animals to the presence of electromag-
netic fi elds have shown that even short-term exposure can produce a thermal burden in
an organism that can result in signifi cant behavioral and physiological changes, some
of which may be harmful [36]. Behavioral effects of such exposure include perception,
aversion, work perturbation, work stoppage and convulsions [37]. The wing mem-
branes of bats present a large surface area over which radiation might be absorbed,
increasing heat load on the animal. This, combined with the heat energy produced
during fl ight, makes bats particularly susceptible to overheating [38, 39]. Furthermore,
observations of captive bats have noted their aversion to even a moderate infra-red
heat source [40].
However, the pulsed microwave radiation characteristic of radars is a rather inef-
fi cient source of energy. The energy produced by a radar signal can reach very high
values of peak power density, at relatively low levels of power density averaged over
time. This is because the pulse length of the radar signal is hundreds of times shorter
than the pulse repetition rate, therefore the average value of power density is hundreds
of times lower than the peak value of the radiation. Therefore, it would seem unlikely
that the energy in the radar signal would be suffi cient to induce a thermal burden in
bats foraging within the beam. However, several studies have reported signifi cant be-
havioral and physiological effects resulting from exposure to pulsed microwave radia-
tion even when the average power density of the signal was relatively low [41-43]. The
mechanism through which pulsed microwave radiation could affect behavior in this
manner is unclear although one possibility is an auditory response commonly referred
to as the auditory microwave hypothesis.