Electromagnetic Radiation Effect on Foraging Bats - Recent Advances and Issues in Environmental Science - page 74

The auditory perception of pulsed microwaves is now widely accepted. The effect
is generally attributed to the thermoelastic expansion of brain tissue following the
small but rapid increase in temperature due to the absorption of the incident energy.
This generates a sound wave in the head that subsequently stimulates the cochlea.
Repeated or prolonged exposure to these auditory effects is considered stressful [44].
Laboratory experiments have shown that the frequency of the induced sound is a
function of head size and of the acoustic properties of the brain tissue. The estimated
fundamental frequency of vibration in guinea pigs, cats, and adult humans are 45, 38,
and 13 kHz respectively [45, 46]. It is therefore not only plausible but probable that
bats exposed to an RF pulse of suffi cient power would effectively hear this pulse and
the frequency detected would lie within the range of frequencies used for orientation,
prey detection, and capture for the majority of bat species. It is possible that, as re-
ported in other studies, exposure to these auditory effects may be stressful for bats or
indeed it may interfere with their echolocation, inhibiting prey detection, or capture.
During the present study, foraging rate per unit time was signifi cantly reduced during
experimental trials indicating that bats foraging within the exposed area were feeding
at a reduced rate in comparison to those foraging during the control trials. This is par-
ticularly surprising given that exposure to the radar had no signifi cant impact on the
abundance of aerial insects, and the observed reduction in foraging rate is therefore
unlikely to be linked to a decline in insect abundance. It is therefore possible that the
auditory perception of the radar signal during experimental trials could have interfered
with the bats ability to detect or capture prey. However, further experimentation would
be required to accurately identify the causal relationship between exposure to electro-
magnetic radiation and the observed reduction in both bat activity and foraging rate.
Although, we have demonstrated a clear biological effect, one of the limitations of
the present study was the use of a commercial marine radar that was not specifi cally
designed for the task. With only a limited control over the parameters of the radar sig-
nal, it is diffi cult to determine which parameters are most effective in deterring bats.
To better understand the response of bats to electromagnetic radiation, and to identify
an optimum signal capable of deterring bats, will require radar engineers to work
with bat biologists to develop a portable radar which can be manipulated to produce
a wider range of electromagnetic outputs. The parameters most likely to be important
are the frequency, pulse length/pulse repetition rate and power output of the signal.
Similarly, the radar used in the present study was only effective when the antenna
was fi xed to produce a unidirectional signal with a horizontal beamwidth of 1.9°. A
narrow unidirectional signal is clearly not appropriate to deter bats from approaching
wind turbines. In order to provide an effective deterrent it would be necessary to emit
a multidirectional electromagnetic signal capable of encapsulating the large volume of
the rotor-swept zone.
CONCLUSION
We have demonstrated that pulsed electromagnetic radiation from a small, affordable
and portable radar system can reduce bat activity within a given area. Results were
most effective when the radar antenna was fixed to produce a unidirectional signal