A related possibility, the “accessory lung” hypothesis, is proposed here
for the laryngeal air sacs of the great apes. Chimpanzees, orangutans, and
gorillas all have voluminous air sacs (6 liters in orangutans, Schön-Ybarra
1995) that can be inflated with air from the lungs. The air sacs connect to
the larynx via a long, thin-walled channel that opens directly above the
vocal membranes and vocal folds. The air sacs extend into the subdermal
space in the pectoral region and are overlaid by the sheet-like platysma
muscle. Thus, an ape could inflate the air sacs via lung pressure and then
forcibly deflate them by tensing the platysma and other pectoral muscles
(or by pounding the chest, as in Gorilla). This anatomy suggests that great
ape air sacs may act as “accessory lungs,” providing an additional source of
expiratory air flow and thus of energy into the source.This hypothesis seems
more plausible than that offered by Negus (1949), who suggested that ape
air sacs act as storage sites for oxygen during vigorous activity. Because the
sacs are inflated with exhaled air that has already been in the lungs, and
thus will be low in oxygen and high in CO 2 , such an air reserve would be
of dubious respiratory value (Fitch and Hauser 1995). Air sacs are also
found in some pinnipeds, where a gas-storage function would be of clear
value during diving (Sleptsov 1940), but Fay (1960) doubted this possibility because the additional oxygen stored even in large sacs would be trivial
relative to dissolved blood O 2 in a diving pinniped.
A final class of laryngeal air sacs, found in many nonhuman primate
species, are subhyoid air sacs. This type of thin-walled sac opens into the
glottis and extends into an enlarged hollow bulla in the hyoid bone. Such a
hard-walled laryngeal sac is typical of cercopithecids (Old World monkeys)
and is developed to the extreme in New World howler monkeys (Alouatta
spp.). Because these sacs are surrounded by bone, they would be of little
value in radiating sound out to the environment and no value as an accessory lung. We speculate that they could act as Helmholtz resonators and
that the small plug of air that vibrates in and out of the narrow neck of the
sac would support vocalization at the Helmholtz resonance frequency. If
true, this would constitute a form of source tract coupling. Although the
opening of these sacs directly at the glottis is consistent with this hypothesis, there are currently no empirical data (e.g., using light gases) available
to further evaluate this hypothesis.
2.2.6. Morphological Diversity: Summary
As this brief review makes clear, there is considerable variability in the
anatomy of the tetrapod vocal-production system. Unfortunately, little of
this impressive morphological diversity has received enough concentrated
empirical attention for any firm conclusions to be reached about its proximate, much less ultimate, function. This is particularly true regarding the
significant morphological diversity in the vocal tract. Compared with the
relatively conservative tetrapod larynx, there is a bewildering diversity of
3. Unpacking “Honesty”
95
for the laryngeal air sacs of the great apes. Chimpanzees, orangutans, and
gorillas all have voluminous air sacs (6 liters in orangutans, Schön-Ybarra
1995) that can be inflated with air from the lungs. The air sacs connect to
the larynx via a long, thin-walled channel that opens directly above the
vocal membranes and vocal folds. The air sacs extend into the subdermal
space in the pectoral region and are overlaid by the sheet-like platysma
muscle. Thus, an ape could inflate the air sacs via lung pressure and then
forcibly deflate them by tensing the platysma and other pectoral muscles
(or by pounding the chest, as in Gorilla). This anatomy suggests that great
ape air sacs may act as “accessory lungs,” providing an additional source of
expiratory air flow and thus of energy into the source.This hypothesis seems
more plausible than that offered by Negus (1949), who suggested that ape
air sacs act as storage sites for oxygen during vigorous activity. Because the
sacs are inflated with exhaled air that has already been in the lungs, and
thus will be low in oxygen and high in CO 2 , such an air reserve would be
of dubious respiratory value (Fitch and Hauser 1995). Air sacs are also
found in some pinnipeds, where a gas-storage function would be of clear
value during diving (Sleptsov 1940), but Fay (1960) doubted this possibility because the additional oxygen stored even in large sacs would be trivial
relative to dissolved blood O 2 in a diving pinniped.
A final class of laryngeal air sacs, found in many nonhuman primate
species, are subhyoid air sacs. This type of thin-walled sac opens into the
glottis and extends into an enlarged hollow bulla in the hyoid bone. Such a
hard-walled laryngeal sac is typical of cercopithecids (Old World monkeys)
and is developed to the extreme in New World howler monkeys (Alouatta
spp.). Because these sacs are surrounded by bone, they would be of little
value in radiating sound out to the environment and no value as an accessory lung. We speculate that they could act as Helmholtz resonators and
that the small plug of air that vibrates in and out of the narrow neck of the
sac would support vocalization at the Helmholtz resonance frequency. If
true, this would constitute a form of source tract coupling. Although the
opening of these sacs directly at the glottis is consistent with this hypothesis, there are currently no empirical data (e.g., using light gases) available
to further evaluate this hypothesis.
2.2.6. Morphological Diversity: Summary
As this brief review makes clear, there is considerable variability in the
anatomy of the tetrapod vocal-production system. Unfortunately, little of
this impressive morphological diversity has received enough concentrated
empirical attention for any firm conclusions to be reached about its proximate, much less ultimate, function. This is particularly true regarding the
significant morphological diversity in the vocal tract. Compared with the
relatively conservative tetrapod larynx, there is a bewildering diversity of
3. Unpacking “Honesty”
95