and filter components and independently modify one specific parameter of
interest (Markel and Gray 1976; see Owren and Bernacki 1998 for a bioacoustically oriented review). Such analysis/synthesis techniques provide an
extremely powerful way to isolate the relevant acoustic parameters in bioacoustic communication systems; these are only starting to be explored
(Owren and Bernacki 1988, 1998; Fitch and Kelley 2000).
2.1.5. Vertebrate Vocal Production: Summary
The vocal-production apparatus of terrestrial vertebrates is made up of two
components. The source—typically the syrinx in birds and the larynx in
mammals, reptiles, and amphibians—creates the sound. When this sourcegenerated signal is periodic, its fundamental frequency (and in some cases
the spacing between harmonics) determines the perceived pitch of the
signal. The term “pitch” should be used with care and only to refer to
source-related acoustic parameters (e.g., the fundamental frequency). This
source-generated energy then passes through and is filtered by the vocal
tract, which includes the mouth, nasal cavities, and pharynx in all vertebrates, and additionally the trachea in birds. The filtering frequencies of the
vocal tract are called formants. There is no colloquial term to refer to their
perceptual correlate of formant frequencies, which is nonetheless perceptually salient to humans and to all vertebrates that have been tested.
Broadly speaking, formants are one correlate of the percept of “timbre” in
animal sounds; in no case are formants correlates of pitch. The frequencies
of the source signal appear, in normal situations, to be completely independent of formant frequencies. This is in contrast to the situation in wind
instruments, where source and filter are strongly coupled.
2.2. Morphological Diversity in the Vertebrate
Vocal-Production System
Although all of the vocal-production systems considered here work in
roughly the same fashion and are governed by the same physical principles
described above, there is an impressive diversity of form in the vertebrate
vocal tract. Below, we will provide a selective overview of this morphological diversity and of some of the hypotheses that have been put forward to
account for this variation.
2.2.1. Dual Sources: The Two-Voice Phenomenon in Birds
An important distinction between the syrinx and the larynx is that the
typical syrinx contains two independent sets of vibrating membranes, one
in each bronchus, which are also under independent nervous control (via
left and right branches of the twelfth cranial nerve, the hypoglossal). Based
on this anatomy, Greenewalt (1968) proposed the “two-voice” theory, which
holds that the two sides of the syrinx in many birds are independent, allow3. Unpacking “Honesty”
85
interest (Markel and Gray 1976; see Owren and Bernacki 1998 for a bioacoustically oriented review). Such analysis/synthesis techniques provide an
extremely powerful way to isolate the relevant acoustic parameters in bioacoustic communication systems; these are only starting to be explored
(Owren and Bernacki 1988, 1998; Fitch and Kelley 2000).
2.1.5. Vertebrate Vocal Production: Summary
The vocal-production apparatus of terrestrial vertebrates is made up of two
components. The source—typically the syrinx in birds and the larynx in
mammals, reptiles, and amphibians—creates the sound. When this sourcegenerated signal is periodic, its fundamental frequency (and in some cases
the spacing between harmonics) determines the perceived pitch of the
signal. The term “pitch” should be used with care and only to refer to
source-related acoustic parameters (e.g., the fundamental frequency). This
source-generated energy then passes through and is filtered by the vocal
tract, which includes the mouth, nasal cavities, and pharynx in all vertebrates, and additionally the trachea in birds. The filtering frequencies of the
vocal tract are called formants. There is no colloquial term to refer to their
perceptual correlate of formant frequencies, which is nonetheless perceptually salient to humans and to all vertebrates that have been tested.
Broadly speaking, formants are one correlate of the percept of “timbre” in
animal sounds; in no case are formants correlates of pitch. The frequencies
of the source signal appear, in normal situations, to be completely independent of formant frequencies. This is in contrast to the situation in wind
instruments, where source and filter are strongly coupled.
2.2. Morphological Diversity in the Vertebrate
Vocal-Production System
Although all of the vocal-production systems considered here work in
roughly the same fashion and are governed by the same physical principles
described above, there is an impressive diversity of form in the vertebrate
vocal tract. Below, we will provide a selective overview of this morphological diversity and of some of the hypotheses that have been put forward to
account for this variation.
2.2.1. Dual Sources: The Two-Voice Phenomenon in Birds
An important distinction between the syrinx and the larynx is that the
typical syrinx contains two independent sets of vibrating membranes, one
in each bronchus, which are also under independent nervous control (via
left and right branches of the twelfth cranial nerve, the hypoglossal). Based
on this anatomy, Greenewalt (1968) proposed the “two-voice” theory, which
holds that the two sides of the syrinx in many birds are independent, allow3. Unpacking “Honesty”
85