Ecological Constraints for Sound Communication: From Grasshoppers to Elephants 61
amount to 10 dBm· 1 (Griffin 1971). Such high frequencies are used by bats for
echolocation, and high absorption coefficients have been shown to influence the
evolution of echolocation signals of insectivorous bats which hunt in open space.
Here bats detect their prey over much greater distances than those searching for
prey close to, or within, vegetation, but in order to avoid strong absorption over
these distances, their echolocation signals are much lower in carrier frequency
(Neuweiler 1989). This can be interpreted as an adaptation to the greatly reduced
range of high-frequency signals caused by absorption in open space.
Whereas the advantage of sound signals over visual signals is their usefulness at
night and in habitats with complex vegetation, the latter nevertheless has
attenuating effects on acoustic signals due to scattering. For insects or animals
using high frequencies which call within, or close to, meadows, bushland, or
forests, this can become an important source of excess attenuation, because such
vegetation includes surfaces the size of which is in the same order of magnitude as
the wavelength of sound. There is a strong correlation between sound frequency
and the degree of excess attenuation due to scattering. Leaves in the order of 5 em
scatter sound with a wavelength of 33 em (1 kHz) much less than sound of a
wavelength of3.3 em (10kHz).
The habitat therefore acts as a low-pass filter (Keuper and KUhne 1983 ). The
technique with a "biological microphone" has been used to study such frequencydependent sound propagation in insect habitats. It enables long-term recordings of
afferent nervous activity from identified auditory intemeurons of insects, and the
portable device allows the study of the perception and coding of sound signals in
any terrestrial habitat (Romer and Lewald 1992). Within a bushy thicket, pure tone
pulses with a carrier frequency of 5 kHz were attenuated by 22 dB over a distance
of 10 m, in contrast to 52 dB at ultrasound frequencies of 40 kHz. Similarly, as a
result of frequency-dependent scattering, coniferous forests exhibit less excess
attenuation than deciduous forests, these effects are more pronounced at higher
frequencies (Morton 1975; Marten and Marler 1977).
Obvious solutions to counteract excess attenuation due to vegetation are (I) a
reduction of the carrier frequency of the signal (lower frequencies and thus larger
wavelengths are scattered less) or (2) by signaling from a position above the
vegetation in open space, thus avoiding scattering of the signaL Both solutions are
by no means easy options for most animals and are constrained by other factors.
The carrier frequency of acoustic signals is in most cases negatively correlated
with body size, which renders it difficult for small animals, in particular insects, to
call at frequencies best adapted for transmission through vegetation. Calling on top
of the vegetation would significantly reduce excess attenuation (Rheinlaender und
Romer 1986; Romer und Lewald 1992), but such preferred broadcasting positions
make a diurnal signaler in tum more conspicuous for visually hunting predators.
This tradeoff between conspicuousness of the acoustic signal and visual
conspicuousness of the signaler is probably reflected in the calling behavior of the
bog katydid Metrioptera sphagnorum. This small insect produces a continuous call
at high sonic and ultrasonic frequencies of 17 and 34 kHz (Morris 1970, Fig. 1 A),
amount to 10 dBm· 1 (Griffin 1971). Such high frequencies are used by bats for
echolocation, and high absorption coefficients have been shown to influence the
evolution of echolocation signals of insectivorous bats which hunt in open space.
Here bats detect their prey over much greater distances than those searching for
prey close to, or within, vegetation, but in order to avoid strong absorption over
these distances, their echolocation signals are much lower in carrier frequency
(Neuweiler 1989). This can be interpreted as an adaptation to the greatly reduced
range of high-frequency signals caused by absorption in open space.
Whereas the advantage of sound signals over visual signals is their usefulness at
night and in habitats with complex vegetation, the latter nevertheless has
attenuating effects on acoustic signals due to scattering. For insects or animals
using high frequencies which call within, or close to, meadows, bushland, or
forests, this can become an important source of excess attenuation, because such
vegetation includes surfaces the size of which is in the same order of magnitude as
the wavelength of sound. There is a strong correlation between sound frequency
and the degree of excess attenuation due to scattering. Leaves in the order of 5 em
scatter sound with a wavelength of 33 em (1 kHz) much less than sound of a
wavelength of3.3 em (10kHz).
The habitat therefore acts as a low-pass filter (Keuper and KUhne 1983 ). The
technique with a "biological microphone" has been used to study such frequencydependent sound propagation in insect habitats. It enables long-term recordings of
afferent nervous activity from identified auditory intemeurons of insects, and the
portable device allows the study of the perception and coding of sound signals in
any terrestrial habitat (Romer and Lewald 1992). Within a bushy thicket, pure tone
pulses with a carrier frequency of 5 kHz were attenuated by 22 dB over a distance
of 10 m, in contrast to 52 dB at ultrasound frequencies of 40 kHz. Similarly, as a
result of frequency-dependent scattering, coniferous forests exhibit less excess
attenuation than deciduous forests, these effects are more pronounced at higher
frequencies (Morton 1975; Marten and Marler 1977).
Obvious solutions to counteract excess attenuation due to vegetation are (I) a
reduction of the carrier frequency of the signal (lower frequencies and thus larger
wavelengths are scattered less) or (2) by signaling from a position above the
vegetation in open space, thus avoiding scattering of the signaL Both solutions are
by no means easy options for most animals and are constrained by other factors.
The carrier frequency of acoustic signals is in most cases negatively correlated
with body size, which renders it difficult for small animals, in particular insects, to
call at frequencies best adapted for transmission through vegetation. Calling on top
of the vegetation would significantly reduce excess attenuation (Rheinlaender und
Romer 1986; Romer und Lewald 1992), but such preferred broadcasting positions
make a diurnal signaler in tum more conspicuous for visually hunting predators.
This tradeoff between conspicuousness of the acoustic signal and visual
conspicuousness of the signaler is probably reflected in the calling behavior of the
bog katydid Metrioptera sphagnorum. This small insect produces a continuous call
at high sonic and ultrasonic frequencies of 17 and 34 kHz (Morris 1970, Fig. 1 A),