basic concepts and analysis tools for the rest of bioacoustics, we follow tradition (e.g., Greenewalt 1968; Lieberman 1968; Hartley and Suthers 1988)
in using data from human vocal production to ground our discussion of call
production in other vertebrates. Where appropriate, we point out the many
significant differences between human speech and animal calls and call
attention to the importance of developing new methodological and theoretical tools to cope with these differences.
2.1.1. Generating Power: The Lungs
Voice production is the conversion of air flow into acoustic energy (that is,
longitudinal pressure waves in the audible frequency range). Typically, this
air flow emanates from the lungs. Tetrapod lungs are filled during normal
respiration by various means, including diaphragmatic contraction in
mammals, buccal pumping in some reptiles and amphibians, intercostal contraction in birds, and even a piston-like retraction of the liver in crocodilians (Liem 1985). Due to the lung’s elastic recoil resulting from alveolar
elasticity and surface tension, optionally combined with muscular compression from intercostal or abdominal muscles, this air can be pressurized,
resulting in a flow outward through the glottis. It is this air flow that typically provides the energy for vocalization, either directly or indirectly by
filling air sacs. The diversity of systems for moving air in and out of the lungs
is of less relevance in understanding vocal diversity than diversity in the
vertebrate voice source. Broad comparative treatments of diversity and
function in the vertebrate respiratory system can be found in Gans (1970),
Liem (1985), or Perry (1989), or Lasiewski (1972) for birds.
Although most tetrapod vocalizations (e.g., human speech) are apparently generated upon expiration, inspiration also plays an important role in
vocalization in some anurans, mammals, and birds. In anuran advertisement
calls, for example, air typically flows outward from the lungs into a distensible submandibular air sac, which can inflate in some cases to the size of
the animal itself (Dudley and Rand 1991). It is this outward flow that fuels
vocal cord vibration and vocalization. The air captured in the sac is then
returned, via deflation, to the lungs, where it can then fuel another vocalization (Gans 1973). This conservation of air serves at least two functions,
which are discussed in more detail below. First, it enables anurans to vocalize at higher rates and for longer than would otherwise be possible (due to
the inefficiency of lung inflation in this group; Rand and Dudley 1993). The
relevance of call duration to both energy expenditure and to female choice
has been documented in anurans (Klump and Gerhardt 1987; Welch et al.
1998; see also Ryan and Kime, Chapter 5) and provides a good example of
a nonarbitrary signal parameter. Second, the inflated air sac may serve as
an impedance-matching device, more efficiently transferring acoustic
energy to the environment (Watkins et al. 1970). A similar mechanism may
operate in nonhuman primates with distensible air sacs (Gautier 1971).
3. Unpacking “Honesty”
73
in using data from human vocal production to ground our discussion of call
production in other vertebrates. Where appropriate, we point out the many
significant differences between human speech and animal calls and call
attention to the importance of developing new methodological and theoretical tools to cope with these differences.
2.1.1. Generating Power: The Lungs
Voice production is the conversion of air flow into acoustic energy (that is,
longitudinal pressure waves in the audible frequency range). Typically, this
air flow emanates from the lungs. Tetrapod lungs are filled during normal
respiration by various means, including diaphragmatic contraction in
mammals, buccal pumping in some reptiles and amphibians, intercostal contraction in birds, and even a piston-like retraction of the liver in crocodilians (Liem 1985). Due to the lung’s elastic recoil resulting from alveolar
elasticity and surface tension, optionally combined with muscular compression from intercostal or abdominal muscles, this air can be pressurized,
resulting in a flow outward through the glottis. It is this air flow that typically provides the energy for vocalization, either directly or indirectly by
filling air sacs. The diversity of systems for moving air in and out of the lungs
is of less relevance in understanding vocal diversity than diversity in the
vertebrate voice source. Broad comparative treatments of diversity and
function in the vertebrate respiratory system can be found in Gans (1970),
Liem (1985), or Perry (1989), or Lasiewski (1972) for birds.
Although most tetrapod vocalizations (e.g., human speech) are apparently generated upon expiration, inspiration also plays an important role in
vocalization in some anurans, mammals, and birds. In anuran advertisement
calls, for example, air typically flows outward from the lungs into a distensible submandibular air sac, which can inflate in some cases to the size of
the animal itself (Dudley and Rand 1991). It is this outward flow that fuels
vocal cord vibration and vocalization. The air captured in the sac is then
returned, via deflation, to the lungs, where it can then fuel another vocalization (Gans 1973). This conservation of air serves at least two functions,
which are discussed in more detail below. First, it enables anurans to vocalize at higher rates and for longer than would otherwise be possible (due to
the inefficiency of lung inflation in this group; Rand and Dudley 1993). The
relevance of call duration to both energy expenditure and to female choice
has been documented in anurans (Klump and Gerhardt 1987; Welch et al.
1998; see also Ryan and Kime, Chapter 5) and provides a good example of
a nonarbitrary signal parameter. Second, the inflated air sac may serve as
an impedance-matching device, more efficiently transferring acoustic
energy to the environment (Watkins et al. 1970). A similar mechanism may
operate in nonhuman primates with distensible air sacs (Gautier 1971).
3. Unpacking “Honesty”
73