instrument, with the coiled trachea serving a function analogous to the
bridge. The main problem with this hypothesis is a different impedance mismatch, that between the vibrations in the tracheal air column and the walls
of the trachea. In a stringed instrument, mechanical vibrations in the strings
are efficiently conveyed to the instrument body and sounding board, where
they are then converted to acoustic energy. In contrast, the vibrations in
the vocal tract start out as acoustic pressure waves and suffer a large
impedance mismatch that prevents these acoustic vibrations from being
converted to mechanical vibrations in the bony tracheal walls and sternum.
In fact, Gaunt et al. (1987) report that virtually all acoustic energy radiates
from the mouth in cranes, not from the chest; see Fitch (1999) for further
discussion.
Although impedance-matching systems such as air sacs have been shown
experimentally to be effective, and appear to have evolved independently
multiple times, the extension of the low-frequency range for a given size air
sac is limited. The fully inflated air sac of a 3-cm spring peeper substantially
increases the efficiency with which its 3-kHz call is radiated to the environment, but it would have no effect on a 300-Hz call with a wavelength greater
than 1 m. Thus, impedance-matching air sacs ameliorate the situation
without actually evading the physical constraint relating low frequencies to
large bodies. We still expect this constraint to play a significant role over
the large range of body sizes seen in terrestrial vertebrates.
2.3.4. Lung Volume and Acoustic Cues to Size
The lungs (along with air sacs in birds) occupy most of the thorax in
mammals, reptiles, and birds.Thus, it is unsurprising that the size of the lungs
is closely related to body size (Scammon 1927; Krogman 1941; Hinds and
Calder 1971). If an acoustic variable directly depended on lung volume, it
would also be correlated with body size. The most obvious example is the
maximum length of a single call, where one would expect longer calls to
indicate larger callers. However, because a quiet call requires less air flow
than a loud one, the relevant acoustic parameter might be more complex
(e.g., the integral of call amplitude over an entire call). Such details aside,
it is reasonable to hypothesize that the production of long, loud calls might
be restricted to large individuals and thus provide a cue to body size. We
currently lack data relevant to this prediction. The nearest example comes
from the classic study on red deer vocalization by Clutton-Brock and Albon
(1979) discussed in the introduction to this chapter.
As mentioned earlier (Section 2.2.5), a possible function of the elastic air
sacs found in many primate species, including most apes, might be as “accessory lungs” (Fitch and Hauser 1995), either prolonging vocalizations or
increasing the intensity of calls relative to those produced solely by lung
deflation. There has been no experimental test of this hypothesis to date,
although MacLarnon and Hewitt (1999) found that those primates with air
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W.T. Fitch and M.D. Hauser
bridge. The main problem with this hypothesis is a different impedance mismatch, that between the vibrations in the tracheal air column and the walls
of the trachea. In a stringed instrument, mechanical vibrations in the strings
are efficiently conveyed to the instrument body and sounding board, where
they are then converted to acoustic energy. In contrast, the vibrations in
the vocal tract start out as acoustic pressure waves and suffer a large
impedance mismatch that prevents these acoustic vibrations from being
converted to mechanical vibrations in the bony tracheal walls and sternum.
In fact, Gaunt et al. (1987) report that virtually all acoustic energy radiates
from the mouth in cranes, not from the chest; see Fitch (1999) for further
discussion.
Although impedance-matching systems such as air sacs have been shown
experimentally to be effective, and appear to have evolved independently
multiple times, the extension of the low-frequency range for a given size air
sac is limited. The fully inflated air sac of a 3-cm spring peeper substantially
increases the efficiency with which its 3-kHz call is radiated to the environment, but it would have no effect on a 300-Hz call with a wavelength greater
than 1 m. Thus, impedance-matching air sacs ameliorate the situation
without actually evading the physical constraint relating low frequencies to
large bodies. We still expect this constraint to play a significant role over
the large range of body sizes seen in terrestrial vertebrates.
2.3.4. Lung Volume and Acoustic Cues to Size
The lungs (along with air sacs in birds) occupy most of the thorax in
mammals, reptiles, and birds.Thus, it is unsurprising that the size of the lungs
is closely related to body size (Scammon 1927; Krogman 1941; Hinds and
Calder 1971). If an acoustic variable directly depended on lung volume, it
would also be correlated with body size. The most obvious example is the
maximum length of a single call, where one would expect longer calls to
indicate larger callers. However, because a quiet call requires less air flow
than a loud one, the relevant acoustic parameter might be more complex
(e.g., the integral of call amplitude over an entire call). Such details aside,
it is reasonable to hypothesize that the production of long, loud calls might
be restricted to large individuals and thus provide a cue to body size. We
currently lack data relevant to this prediction. The nearest example comes
from the classic study on red deer vocalization by Clutton-Brock and Albon
(1979) discussed in the introduction to this chapter.
As mentioned earlier (Section 2.2.5), a possible function of the elastic air
sacs found in many primate species, including most apes, might be as “accessory lungs” (Fitch and Hauser 1995), either prolonging vocalizations or
increasing the intensity of calls relative to those produced solely by lung
deflation. There has been no experimental test of this hypothesis to date,
although MacLarnon and Hewitt (1999) found that those primates with air
100
W.T. Fitch and M.D. Hauser