signal can reduce the chances that it will be detected by a receiver either
because the amplitude of the received signal falls below the auditory threshold of the receiver or because the signal-to-noise ratio has decreased to a
level at which the receiver no longer recognizes the signal.
The temporal and spectral structures of a signal will also be altered as it
travels from sender to receiver (Wiley and Richards 1978; Richards and
Wiley 1980; Michelsen and Larsen 1983). For example, reverberations from
objects in the environment or irregular amplitude fluctuations caused by
atmospheric turbulence can contribute to the degradation of a signal’s temporal structure. Some frequencies attenuate more rapidly than others do;
this “frequency-dependent attenuation” can alter the spectral form and thus
the perception of a signal. Over long distances or in harsh environments,
such loss of fidelity can render a signal unrecognizable to the receiver.
All sounds experience these changes during transmission, but the amount
of change is partially determined by the spectral and temporal structures
of the signal itself. Selection can therefore act on the form of a signal to
increase the distance over which it can be heard and recognized (Morton
1975; Sorjonen 1986; Wiley 1991; Endler 1992). Endler (1992) described 12
“rules” for the design of communication signals, most of which were considerations of the effects of the environment on sound transmission. We
present a modified list of design rules in Table 5.2. Although these guidelines are applicable to many communication signals, they should be especially important for calls that must travel over long distances.
The frequency of a signal has a large effect on its propagation distance.
Sound-transmission experiments in various habitats have repeatedly
demonstrated that lower-frequency sounds generally experience less attenuation than higher-frequency sounds (e.g., Morton 1975; Marten and
Marler 1977; Marten et al. 1977; Waser and Brown 1986). This is because
higher-frequency sounds are more susceptible to absorption by objects in
5. Selection on Signals
233
Table 5.2. Guidelines for acoustic signal production. (Modified from Endler 1992.)
1. Use frequencies lower than 2 kHz in order to minimize reverberation, attenuation, and
scattering during transmission.
2. For animals that must call near the ground, use frequencies above 0.5–1 kHz to avoid
ground attenuation.
3. Use species-specific frequency bands and tuned receptors to minimize interference from
other species and abiotic sounds.
4. Use greater-amplitude signals to maximize the signal-to-noise ratio, increase transmission
distance, and increase the probability of detection.
5. Use frequency modulation rather than amplitude modulation to encode information.
Reverberations and air turbulence alter amplitude more than frequency.
6. Send signals from above the ground to minimize ground attenuation and the effects of
vegetation and wind and temperature gradients.
7. Use redundant signals to offset the effects of discontinuous background noise and the
effects of reverberations and amplitude fluctuations.
8. Call in locations or during times that minimize turbulence and/or background noise.
9. Use alerting signals to attract the receiver’s attention before sending the main signal.
because the amplitude of the received signal falls below the auditory threshold of the receiver or because the signal-to-noise ratio has decreased to a
level at which the receiver no longer recognizes the signal.
The temporal and spectral structures of a signal will also be altered as it
travels from sender to receiver (Wiley and Richards 1978; Richards and
Wiley 1980; Michelsen and Larsen 1983). For example, reverberations from
objects in the environment or irregular amplitude fluctuations caused by
atmospheric turbulence can contribute to the degradation of a signal’s temporal structure. Some frequencies attenuate more rapidly than others do;
this “frequency-dependent attenuation” can alter the spectral form and thus
the perception of a signal. Over long distances or in harsh environments,
such loss of fidelity can render a signal unrecognizable to the receiver.
All sounds experience these changes during transmission, but the amount
of change is partially determined by the spectral and temporal structures
of the signal itself. Selection can therefore act on the form of a signal to
increase the distance over which it can be heard and recognized (Morton
1975; Sorjonen 1986; Wiley 1991; Endler 1992). Endler (1992) described 12
“rules” for the design of communication signals, most of which were considerations of the effects of the environment on sound transmission. We
present a modified list of design rules in Table 5.2. Although these guidelines are applicable to many communication signals, they should be especially important for calls that must travel over long distances.
The frequency of a signal has a large effect on its propagation distance.
Sound-transmission experiments in various habitats have repeatedly
demonstrated that lower-frequency sounds generally experience less attenuation than higher-frequency sounds (e.g., Morton 1975; Marten and
Marler 1977; Marten et al. 1977; Waser and Brown 1986). This is because
higher-frequency sounds are more susceptible to absorption by objects in
5. Selection on Signals
233
Table 5.2. Guidelines for acoustic signal production. (Modified from Endler 1992.)
1. Use frequencies lower than 2 kHz in order to minimize reverberation, attenuation, and
scattering during transmission.
2. For animals that must call near the ground, use frequencies above 0.5–1 kHz to avoid
ground attenuation.
3. Use species-specific frequency bands and tuned receptors to minimize interference from
other species and abiotic sounds.
4. Use greater-amplitude signals to maximize the signal-to-noise ratio, increase transmission
distance, and increase the probability of detection.
5. Use frequency modulation rather than amplitude modulation to encode information.
Reverberations and air turbulence alter amplitude more than frequency.
6. Send signals from above the ground to minimize ground attenuation and the effects of
vegetation and wind and temperature gradients.
7. Use redundant signals to offset the effects of discontinuous background noise and the
effects of reverberations and amplitude fluctuations.
8. Call in locations or during times that minimize turbulence and/or background noise.
9. Use alerting signals to attract the receiver’s attention before sending the main signal.