A final possible example of vocal tract elongation is provided by the proboscises seen in a wide variety of mammalian species, including elephants,
elephant seals, elephant shrews, tapirs, male proboscis monkeys, and some
extinct but formerly common mammal groups, such as oreodonts. Such
elongations of the nasal cavity would inevitably lower the formant frequencies of vocalizations emitted through the nose (although, as mentioned
earlier, nasal vocalizations are likely to be considerably quieter than oral
vocalizations). Although there is little evidence suggesting that the primary
function of proboscises in most species is acoustic, such a hypothesis may
be reasonable in sexually dimorphic species such as proboscis monkeys,
Nasalis larvatus, in which only the male has an elongated nose.
2.2.5. Air Sacs
The final type of morphological modification of the vocal tract that we will
consider is vocal air sacs, which exist in a bewildering diversity among
tetrapods. We distinguish “vocal” sacs, which at least may have some
acoustic function and are typically attached to the larynx or vocal tract,
from the respiratory air sacs found in all birds and some reptiles (Lasiewski
1972), which are not likely to serve any acoustic function. There are many
types of vocal air sacs, for which we will offer a preliminary classification
into oral, nasal, laryngeal, tracheal, and “other,” depending on the location
of the air sac opening. However, even within the laryngeal sacs (the most
common type), there is great diversity of form, including soft-walled versus
hard-walled sacs, paired, midline, or asymmetric sacs, and a variety of possible opening locations relative to the glottis (sub-, supra-, or para-glottal).
We will review four plausible hypotheses that have been advanced for the
acoustic and/or respiratory function of air sacs and give possible examples
of each type (for more detail, see Negus 1949; Schneider 1964).
Although our survey of air sacs is organized by possible function for conceptual clarity, we do not mean to imply that air sacs serve a single function in any single species and certainly not across species. For example, in
frogs, the air sacs appear to serve both air-recycling and impedance-matching functions. Another good example of multipurpose air sacs is provided
by the walrus Odobenus rosmarus. Adult male walruses have large pharyngeal air sacs that are outgrowths of the pharyngeal wall, opening just
dorsal to the larynx (Sleptsov 1940; Fay 1960). These sacs appear to subserve production of the peculiar “bell” sound made by males during sexual
behavior (Schevill et al. 1966), although the mechanism for this is unknown.
However, the pouches can also be inflated as “life preservers,” allowing the
walrus to stay afloat during naps at sea. Fay (1960) reports that he observed
walruses sleeping at sea at least eight times, and the pharyngeal air sacs
were invariably inflated, holding the shoulders out of the water. Finally,
Sleptsov (1940) reported finding the sacs of two hunted walruses filled with
food (crustaceans and molluscs) and suggested a third function for the
3. Unpacking “Honesty”
93
elephant seals, elephant shrews, tapirs, male proboscis monkeys, and some
extinct but formerly common mammal groups, such as oreodonts. Such
elongations of the nasal cavity would inevitably lower the formant frequencies of vocalizations emitted through the nose (although, as mentioned
earlier, nasal vocalizations are likely to be considerably quieter than oral
vocalizations). Although there is little evidence suggesting that the primary
function of proboscises in most species is acoustic, such a hypothesis may
be reasonable in sexually dimorphic species such as proboscis monkeys,
Nasalis larvatus, in which only the male has an elongated nose.
2.2.5. Air Sacs
The final type of morphological modification of the vocal tract that we will
consider is vocal air sacs, which exist in a bewildering diversity among
tetrapods. We distinguish “vocal” sacs, which at least may have some
acoustic function and are typically attached to the larynx or vocal tract,
from the respiratory air sacs found in all birds and some reptiles (Lasiewski
1972), which are not likely to serve any acoustic function. There are many
types of vocal air sacs, for which we will offer a preliminary classification
into oral, nasal, laryngeal, tracheal, and “other,” depending on the location
of the air sac opening. However, even within the laryngeal sacs (the most
common type), there is great diversity of form, including soft-walled versus
hard-walled sacs, paired, midline, or asymmetric sacs, and a variety of possible opening locations relative to the glottis (sub-, supra-, or para-glottal).
We will review four plausible hypotheses that have been advanced for the
acoustic and/or respiratory function of air sacs and give possible examples
of each type (for more detail, see Negus 1949; Schneider 1964).
Although our survey of air sacs is organized by possible function for conceptual clarity, we do not mean to imply that air sacs serve a single function in any single species and certainly not across species. For example, in
frogs, the air sacs appear to serve both air-recycling and impedance-matching functions. Another good example of multipurpose air sacs is provided
by the walrus Odobenus rosmarus. Adult male walruses have large pharyngeal air sacs that are outgrowths of the pharyngeal wall, opening just
dorsal to the larynx (Sleptsov 1940; Fay 1960). These sacs appear to subserve production of the peculiar “bell” sound made by males during sexual
behavior (Schevill et al. 1966), although the mechanism for this is unknown.
However, the pouches can also be inflated as “life preservers,” allowing the
walrus to stay afloat during naps at sea. Fay (1960) reports that he observed
walruses sleeping at sea at least eight times, and the pharyngeal air sacs
were invariably inflated, holding the shoulders out of the water. Finally,
Sleptsov (1940) reported finding the sacs of two hunted walruses filled with
food (crustaceans and molluscs) and suggested a third function for the
3. Unpacking “Honesty”
93