68
Heiner Romer
distances up to 450 m. Extrapolating the male call transmission trajectory until the
SPL of the transmitted call intersects the hearing threshold of a female gives a
potential transmission distance to conspecific females ranging between 1400 and
1900 m.
The data and calculations in the elephant study predicted an enhancement of 1215 dB for the infrasound frequencies, whereas the experimental data for night-time
transmission of the grasshopper call show little evidence for a direct
enhancement of the call, i.e., SPLs higher than predicted by geometrical spreading
alone. However, the measured SPL at each distance is the additive effect of all
enhancing and attenuating conditions affecting the sound wave between sender and
receiver, and excess attenuation due to high grass vegetation on the transect most
likely reduced the enhancing effect of temperature inversions. Moreover,
compared with the infrasound frequencies of African elephants, enhancement
effects should be reduced at the 1.7 kHz carrier frequency typical of the calls of
this grasshopper.
Despite this difference, the design features of this communication system agree
with all expectations of natural selection to maximize broadcast range while
avoiding predation and intraspecific competition. For the female receiver the
amplitude of the male signal is maximized relative to background noise by
repetition of a temporally stereotyped signal, by an increased broadcast amplitude
resulting from the abdominal resonator, by an increase of the transmitted signal
amplitude using optimal weather conditions, and by (for an insect) extremely
sensitive hearing.
3.3 Crickets on the Ground
If we go further down in the size of the animal, and close to the ground of
terrestrial habitats, we find grasshoppers and crickets communicating by sound
close to the ground, although physicists tell us that they should not do this. In fact,
these insects face a complicated situation, where the sound waves they produce
interact with the ground in an angle which is almost zero and sound waves travel
almost parallel to the ground. The wave reflected from the ground will be out of
phase with the direct wave at some distance, and when the two interact they cancel
each other. Therefore sound propagation parallel to the ground is often referred to
as the forbidden mode of propagation since it creates another type of sound
shadow zone (for review see Embleton 1996).
Gryllus campestris is such a species living in and calling on the ground, and the
broadcast range of a male signal would be limited to just 1 or 2 m, at least from
theoretical calculations for ground waves. A field measurement confirmed
attenuation in excess to geometric spreading (Popov et a!. 1972), but the distance
over which a female could probably hear the song was about I 0-15 m and thus
considerably larger than the forbidden mode of propagation would suggest. I
Heiner Romer
distances up to 450 m. Extrapolating the male call transmission trajectory until the
SPL of the transmitted call intersects the hearing threshold of a female gives a
potential transmission distance to conspecific females ranging between 1400 and
1900 m.
The data and calculations in the elephant study predicted an enhancement of 1215 dB for the infrasound frequencies, whereas the experimental data for night-time
transmission of the grasshopper call show little evidence for a direct
enhancement of the call, i.e., SPLs higher than predicted by geometrical spreading
alone. However, the measured SPL at each distance is the additive effect of all
enhancing and attenuating conditions affecting the sound wave between sender and
receiver, and excess attenuation due to high grass vegetation on the transect most
likely reduced the enhancing effect of temperature inversions. Moreover,
compared with the infrasound frequencies of African elephants, enhancement
effects should be reduced at the 1.7 kHz carrier frequency typical of the calls of
this grasshopper.
Despite this difference, the design features of this communication system agree
with all expectations of natural selection to maximize broadcast range while
avoiding predation and intraspecific competition. For the female receiver the
amplitude of the male signal is maximized relative to background noise by
repetition of a temporally stereotyped signal, by an increased broadcast amplitude
resulting from the abdominal resonator, by an increase of the transmitted signal
amplitude using optimal weather conditions, and by (for an insect) extremely
sensitive hearing.
3.3 Crickets on the Ground
If we go further down in the size of the animal, and close to the ground of
terrestrial habitats, we find grasshoppers and crickets communicating by sound
close to the ground, although physicists tell us that they should not do this. In fact,
these insects face a complicated situation, where the sound waves they produce
interact with the ground in an angle which is almost zero and sound waves travel
almost parallel to the ground. The wave reflected from the ground will be out of
phase with the direct wave at some distance, and when the two interact they cancel
each other. Therefore sound propagation parallel to the ground is often referred to
as the forbidden mode of propagation since it creates another type of sound
shadow zone (for review see Embleton 1996).
Gryllus campestris is such a species living in and calling on the ground, and the
broadcast range of a male signal would be limited to just 1 or 2 m, at least from
theoretical calculations for ground waves. A field measurement confirmed
attenuation in excess to geometric spreading (Popov et a!. 1972), but the distance
over which a female could probably hear the song was about I 0-15 m and thus
considerably larger than the forbidden mode of propagation would suggest. I