62
Heiner Romer
which are strongly attenuated when males call close to the ground within dense
vegetation. Males prefer to sing during late afternoon on small spruce trees
scattered around in the habitat, but their preferred height is not the top of these
trees, but a site on average about 30-50 em below (Fig. IB). This reduces their
visual conspicuousness (at least to the searching entomologist) dramatically, but
has the effect that there is only little excess attenuation of the amplitude of their
call.
Male bushcricket T. viridissima, however, establish positions which are less than
ideal for maximal sound transmission. Males could have increased the detection
range of their signal almost threefold by singing from even higher positions than
those observed in the field (Arak and Eiriksson 1992). This probably reflects the
common tradeoff between the costs of singing at higher, exposed elevations and
the increased detection range of the signal.
2.2 Frequency Filtering
Metrioptera sphagnorum also provides an example for the frequency filtering
effect of the transmission channel. Since higher frequencies with smaller
wavelengths suffer from multiple scattering more strongly than lower frequencies,
the transmission channel acts essentially as a low-pass filter. In the katydid M
sphagnorum, the amplitude of the ultrasonic component close to the source is 5 dB
higher compared to the "audio" -component at 17 kHz, but after transmission over
I 0 meters in the bog habitat (source and receiver 50 em above ground) its
amplitude is below the detection threshold of a receiver, as shown by the activity
of the omega-neuron at this distance (Fig. 1 C). By contrast, the 17 kHz component
is still perceived, resulting in a patterned response of the neuron at large distances.
Since the neuron is equally sensitive to both components, it does respond even
more strongly to the ultrasonic component close to the source, because of its
higher amplitude (Romer and Morris, unpublished). Since the 17 kHz component
is the only one available at larger distances, but has been shown not to be sufficient
for female phonotaxis, the lower frequency has been suggested to function in male
spacing (Morris eta!. 1975).
2.3 Directionality
Apart from frequency-dependent attenuation, the other effect of dense vegetation
on the transmission channel is on the directionality of the signal. In the most
extreme situation, multiple reflections and scattering will tum the sound field
around a listening receiver into a diffuse sound field where sound waves arrive
with equal amplitude from all directions. Directional hearing under these
conditions would be strongly impaired.
Heiner Romer
which are strongly attenuated when males call close to the ground within dense
vegetation. Males prefer to sing during late afternoon on small spruce trees
scattered around in the habitat, but their preferred height is not the top of these
trees, but a site on average about 30-50 em below (Fig. IB). This reduces their
visual conspicuousness (at least to the searching entomologist) dramatically, but
has the effect that there is only little excess attenuation of the amplitude of their
call.
Male bushcricket T. viridissima, however, establish positions which are less than
ideal for maximal sound transmission. Males could have increased the detection
range of their signal almost threefold by singing from even higher positions than
those observed in the field (Arak and Eiriksson 1992). This probably reflects the
common tradeoff between the costs of singing at higher, exposed elevations and
the increased detection range of the signal.
2.2 Frequency Filtering
Metrioptera sphagnorum also provides an example for the frequency filtering
effect of the transmission channel. Since higher frequencies with smaller
wavelengths suffer from multiple scattering more strongly than lower frequencies,
the transmission channel acts essentially as a low-pass filter. In the katydid M
sphagnorum, the amplitude of the ultrasonic component close to the source is 5 dB
higher compared to the "audio" -component at 17 kHz, but after transmission over
I 0 meters in the bog habitat (source and receiver 50 em above ground) its
amplitude is below the detection threshold of a receiver, as shown by the activity
of the omega-neuron at this distance (Fig. 1 C). By contrast, the 17 kHz component
is still perceived, resulting in a patterned response of the neuron at large distances.
Since the neuron is equally sensitive to both components, it does respond even
more strongly to the ultrasonic component close to the source, because of its
higher amplitude (Romer and Morris, unpublished). Since the 17 kHz component
is the only one available at larger distances, but has been shown not to be sufficient
for female phonotaxis, the lower frequency has been suggested to function in male
spacing (Morris eta!. 1975).
2.3 Directionality
Apart from frequency-dependent attenuation, the other effect of dense vegetation
on the transmission channel is on the directionality of the signal. In the most
extreme situation, multiple reflections and scattering will tum the sound field
around a listening receiver into a diffuse sound field where sound waves arrive
with equal amplitude from all directions. Directional hearing under these
conditions would be strongly impaired.