toad vocal system, systematically altering them to study their effects on
signal production. He showed that larger toads generally have larger vocal
cords and that these larger vocal cords produce sounds with lower dominant frequencies. Of course, the relationship between body size and frequency is not perfect—Martin (1972) also showed that adding mass to vocal
cords independent of body size can lower the dominant frequency of a call.
In addition, call frequency can be changed by changing the tension on the
vocal cords—the frequency-modulated calls of many frogs are an example
of this process.
The interaction of calling efficiency and body size constrains the dominant frequency of acoustic signals. As the frequency of a signal decreases
relative to the body size of the sender, the efficiency of signal production
drops (e.g., Section 2.1 of this chapter; Bradbury and Vehrenkamp 1998).
Taken together, the constraints of body size and energetic efficiency indicate that small animals should use the more efficiently produced higher
frequencies for communication. However, as we will see in the following
sections, natural and sexual selection can act on signal structure in ways that
oppose this influence of sender morphology.
3. Environmental Constraints
During transmission, acoustic signals must travel through the external
environment. Over distance, these signals will be altered in ways that can
influence the response of receivers; all signals will eventually become so
degraded that potential receivers fail to recognize them altogether. The
temporal or spectral structure of signals, however, can influence the amount
of change they experience and thus the distance over which they can be
used. Selection can therefore act on the form of long-distance communication signals to decrease signal degradation and increase transmission distance. In this section, we will first review some of the basic properties of
signal transmission in the atmosphere (see Bass and Clark, Chapter 2,
for a discussion of underwater acoustics). We will then examine some of
the evidence that long-distance communication signals have evolved in
response to selection for increased propagation efficacy.
3.1. Signal Design for Maximum Range
The effects of transmission on acoustic signals have generally been partitioned into two main categories—loss of amplitude and loss of fidelity. Both
contribute to the degradation of a signal’s influence with distance. Because
of spherical spreading alone, signal amplitude will decrease, or attenuate,
by 6 dB for each doubling of distance, even in an ideal environment.
Absorption and scattering of sound waves by the air, ground, and vegetation cause additional, or “excess,” attenuation in most natural environments
(Wiley and Richards 1978). Attenuation of a long-distance communication
232
M.J. Ryan and N.M. Kime
signal production. He showed that larger toads generally have larger vocal
cords and that these larger vocal cords produce sounds with lower dominant frequencies. Of course, the relationship between body size and frequency is not perfect—Martin (1972) also showed that adding mass to vocal
cords independent of body size can lower the dominant frequency of a call.
In addition, call frequency can be changed by changing the tension on the
vocal cords—the frequency-modulated calls of many frogs are an example
of this process.
The interaction of calling efficiency and body size constrains the dominant frequency of acoustic signals. As the frequency of a signal decreases
relative to the body size of the sender, the efficiency of signal production
drops (e.g., Section 2.1 of this chapter; Bradbury and Vehrenkamp 1998).
Taken together, the constraints of body size and energetic efficiency indicate that small animals should use the more efficiently produced higher
frequencies for communication. However, as we will see in the following
sections, natural and sexual selection can act on signal structure in ways that
oppose this influence of sender morphology.
3. Environmental Constraints
During transmission, acoustic signals must travel through the external
environment. Over distance, these signals will be altered in ways that can
influence the response of receivers; all signals will eventually become so
degraded that potential receivers fail to recognize them altogether. The
temporal or spectral structure of signals, however, can influence the amount
of change they experience and thus the distance over which they can be
used. Selection can therefore act on the form of long-distance communication signals to decrease signal degradation and increase transmission distance. In this section, we will first review some of the basic properties of
signal transmission in the atmosphere (see Bass and Clark, Chapter 2,
for a discussion of underwater acoustics). We will then examine some of
the evidence that long-distance communication signals have evolved in
response to selection for increased propagation efficacy.
3.1. Signal Design for Maximum Range
The effects of transmission on acoustic signals have generally been partitioned into two main categories—loss of amplitude and loss of fidelity. Both
contribute to the degradation of a signal’s influence with distance. Because
of spherical spreading alone, signal amplitude will decrease, or attenuate,
by 6 dB for each doubling of distance, even in an ideal environment.
Absorption and scattering of sound waves by the air, ground, and vegetation cause additional, or “excess,” attenuation in most natural environments
(Wiley and Richards 1978). Attenuation of a long-distance communication
232
M.J. Ryan and N.M. Kime