Ecological Constraints for Sound Communication: From Grasshoppers to Elephants 65
Measurements of temperature at various height above the ground in a typical open
African savanna show that strong temperature inversions form at the surface before
sunset, and decay with sunrise (Fig. 2A). After sunset, the air is stratified near the
ground and should lead to an enhancement of sound propagation for the infrasound
signals between 15 and 50 Hz produced by elephants. Additional wind-speed
measure-ments indicate that calm conditions do exist especially after sunset, with
the consequence that such layers of air differing in propagation speed are not
disturbed by topographic winds, because of turbulent mixing.
Such meteorological data has been used to calculate the effect of temperature
gradients on the range of elephant vocalizations. The authors used idealized
temperature profiles typical for midday superadiabatic conditions, late afternoon
adiabatic lapse and evening inversions. They defined the elephants' calling range
as the distance over which the call (117 dB SPL at a distance of 1 m ) decreased
by 67 dB to reach the hearing threshold (about 50 dB SPL) of receivers. This
range changes from about 2 to 9 km between late afternoon and early evening, as a
result of the sound channel created by a temperature inversion. Based on these
calculations and temperature measurements over the African savanna for a period
of2 months, the same authors determined the calling area (i.e., the area over which
the infrasound signal could possibly be detected by another elephant), which varies
from less than 50 km 2 to values of 300 km 2 • On any given day, the calling area
undergoes an expansion and contraction, largest areas appear after sunset, or
prevail at times of longer undisturbed weather conditions for most of the night
(Fig. 2C).
The acoustic communication distances of up to 1 0 km estimated by Garstang et
a!. ( 1995) are larger than those of 2-4 km reported by Lang bauer et al. (1991)
using a playback technique, but the latter study was performed with elephants
when they came to drink, without a control of the actual atmospheric conditions.
Optimum conditions for maximizing calling area should also occur in other
Savanna areas in Africa at dusk and at dawn, and if so, then these predictable
conditions may have exerted a selective pressure on animals other than elephants
to call at times when transmission is best. This is the case for lions (Schaller 1972)
and coyotes (Laundre 1981, Larom et al. 1997).
3.2 Bladder Grasshoppers in Trees
Although the size of the animal in the following section contrasts greatly with that
of an elephant, it makes use of environmental gradients for long-distance
communication as well. Bladder grasshoppers (Bullacris membracioides) are
endemic to southern Africa and the conspicuous nocturnal calls of males are
audible to humans over several kilometers. The abdomen of the male is an airfilled space and it acts as a resonator set into vibration by rubbing a file on the
hindleg against another row of teeth on the abdomen itself. This creates a very
Measurements of temperature at various height above the ground in a typical open
African savanna show that strong temperature inversions form at the surface before
sunset, and decay with sunrise (Fig. 2A). After sunset, the air is stratified near the
ground and should lead to an enhancement of sound propagation for the infrasound
signals between 15 and 50 Hz produced by elephants. Additional wind-speed
measure-ments indicate that calm conditions do exist especially after sunset, with
the consequence that such layers of air differing in propagation speed are not
disturbed by topographic winds, because of turbulent mixing.
Such meteorological data has been used to calculate the effect of temperature
gradients on the range of elephant vocalizations. The authors used idealized
temperature profiles typical for midday superadiabatic conditions, late afternoon
adiabatic lapse and evening inversions. They defined the elephants' calling range
as the distance over which the call (117 dB SPL at a distance of 1 m ) decreased
by 67 dB to reach the hearing threshold (about 50 dB SPL) of receivers. This
range changes from about 2 to 9 km between late afternoon and early evening, as a
result of the sound channel created by a temperature inversion. Based on these
calculations and temperature measurements over the African savanna for a period
of2 months, the same authors determined the calling area (i.e., the area over which
the infrasound signal could possibly be detected by another elephant), which varies
from less than 50 km 2 to values of 300 km 2 • On any given day, the calling area
undergoes an expansion and contraction, largest areas appear after sunset, or
prevail at times of longer undisturbed weather conditions for most of the night
(Fig. 2C).
The acoustic communication distances of up to 1 0 km estimated by Garstang et
a!. ( 1995) are larger than those of 2-4 km reported by Lang bauer et al. (1991)
using a playback technique, but the latter study was performed with elephants
when they came to drink, without a control of the actual atmospheric conditions.
Optimum conditions for maximizing calling area should also occur in other
Savanna areas in Africa at dusk and at dawn, and if so, then these predictable
conditions may have exerted a selective pressure on animals other than elephants
to call at times when transmission is best. This is the case for lions (Schaller 1972)
and coyotes (Laundre 1981, Larom et al. 1997).
3.2 Bladder Grasshoppers in Trees
Although the size of the animal in the following section contrasts greatly with that
of an elephant, it makes use of environmental gradients for long-distance
communication as well. Bladder grasshoppers (Bullacris membracioides) are
endemic to southern Africa and the conspicuous nocturnal calls of males are
audible to humans over several kilometers. The abdomen of the male is an airfilled space and it acts as a resonator set into vibration by rubbing a file on the
hindleg against another row of teeth on the abdomen itself. This creates a very