fundamentally different from the acoustics of wind instruments. There is
little evidence for anything but weak coupling either in the human voice or
in other vertebrates that have been studied thus far. Thus, to a first approximation, the frequencies produced by the vocal source (typically a fundamental and its harmonics) are independent of the filtering frequencies of
the vocal tract (Miller 1934; Sutherland and McChesney 1965; Hersch 1966;
Pye 1967; Greenewalt 1968; Capranica and Moffat 1983; Gaunt et al. 1987;
Nowicki 1987; Hartley and Suthers 1988; Rand and Dudley 1993; Westneat
et al. 1993; Brittan-Powell et al. 1997).
The best evidence for source/tract independence comes from experiments with animals vocalizing in light gases.Typically, researchers have used
heliox, a mixture of helium and oxygen with nearly double the speed of
sound in air. Because formants are dependent on the transit time of sound
waves up and down the vocal tract, raising the speed of sound shortens
transit time and thus nearly doubles formant frequencies. In a coupled
system such as a wind instrument, doubling the air column resonances also
doubles the fundamental frequency at which the source vibrates. However,
in the human voice (e.g., Beil 1962) and in those animals tested (birds:
Hersch 1966; Gaunt et al. 1987; Nowicki 1987; Brittan-Powell et al. 1997;
anurans: Capranica and Moffat 1983; Rand and Dudley 1993; bats: Pye 1967;
Hartley and Suthers 1988), the fundamental frequency does not shift appreciably in heliox. Where formant frequencies are present (mammals and
birds), they shift upward. In humans, this leads to the peculiar “Donald
Duck” quality of helium speech, with a normal, low fundamental frequency
and high formants (Beil 1962). In the case of birds and bats, the formant
shift often “unmasks” harmonics that are present in the source signal but
are normally filtered out as the signal passes through the vocal tract (Pye
1967; Nowicki 1987; Hartley and Suthers 1988; Nowicki and Marler 1988).
The only case of which we are aware where the perceptual relevance of
heliox-shifted vocalizations has been examined is the work by Strote and
Nowicki (1996), who found in a two-speaker choice experiment that song
sparrows respond slightly more strongly to normal calls than to heliumshifted calls. There appears to be no consistent, significant effect of helium
on vocalizations in the anuran species tested to date (Rand and Dudley
1993).
Despite the consistency of these heliox data in these species, there is little
information relevant to source/tract independence for the vast majority of
tetrapod species. Less direct analyses suggest independence simply because
the relatively short vocal tract of nonavian tetrapods would result in
formant frequencies that are high relative to the fundamental frequency in
most mammals and anurans. Thus, in macaque and baboon grunts (Andrew
1976; Rendall 1996; Owren et al. 1997) and dog growls (Riede and Fitch
1999), the fundamental frequency falls far below that of the lowest formant.
Although Bauer (1987) found a correlation between fundamental frequency and mouth opening in an adult male chimpanzee, there was no
3. Unpacking “Honesty”
83
little evidence for anything but weak coupling either in the human voice or
in other vertebrates that have been studied thus far. Thus, to a first approximation, the frequencies produced by the vocal source (typically a fundamental and its harmonics) are independent of the filtering frequencies of
the vocal tract (Miller 1934; Sutherland and McChesney 1965; Hersch 1966;
Pye 1967; Greenewalt 1968; Capranica and Moffat 1983; Gaunt et al. 1987;
Nowicki 1987; Hartley and Suthers 1988; Rand and Dudley 1993; Westneat
et al. 1993; Brittan-Powell et al. 1997).
The best evidence for source/tract independence comes from experiments with animals vocalizing in light gases.Typically, researchers have used
heliox, a mixture of helium and oxygen with nearly double the speed of
sound in air. Because formants are dependent on the transit time of sound
waves up and down the vocal tract, raising the speed of sound shortens
transit time and thus nearly doubles formant frequencies. In a coupled
system such as a wind instrument, doubling the air column resonances also
doubles the fundamental frequency at which the source vibrates. However,
in the human voice (e.g., Beil 1962) and in those animals tested (birds:
Hersch 1966; Gaunt et al. 1987; Nowicki 1987; Brittan-Powell et al. 1997;
anurans: Capranica and Moffat 1983; Rand and Dudley 1993; bats: Pye 1967;
Hartley and Suthers 1988), the fundamental frequency does not shift appreciably in heliox. Where formant frequencies are present (mammals and
birds), they shift upward. In humans, this leads to the peculiar “Donald
Duck” quality of helium speech, with a normal, low fundamental frequency
and high formants (Beil 1962). In the case of birds and bats, the formant
shift often “unmasks” harmonics that are present in the source signal but
are normally filtered out as the signal passes through the vocal tract (Pye
1967; Nowicki 1987; Hartley and Suthers 1988; Nowicki and Marler 1988).
The only case of which we are aware where the perceptual relevance of
heliox-shifted vocalizations has been examined is the work by Strote and
Nowicki (1996), who found in a two-speaker choice experiment that song
sparrows respond slightly more strongly to normal calls than to heliumshifted calls. There appears to be no consistent, significant effect of helium
on vocalizations in the anuran species tested to date (Rand and Dudley
1993).
Despite the consistency of these heliox data in these species, there is little
information relevant to source/tract independence for the vast majority of
tetrapod species. Less direct analyses suggest independence simply because
the relatively short vocal tract of nonavian tetrapods would result in
formant frequencies that are high relative to the fundamental frequency in
most mammals and anurans. Thus, in macaque and baboon grunts (Andrew
1976; Rendall 1996; Owren et al. 1997) and dog growls (Riede and Fitch
1999), the fundamental frequency falls far below that of the lowest formant.
Although Bauer (1987) found a correlation between fundamental frequency and mouth opening in an adult male chimpanzee, there was no
3. Unpacking “Honesty”
83