forming a significant portion of the consonantal repertoire. Much less is
known about the use of turbulence in animal communication, although
hissing is obviously a widespread type of vocalization among tetrapods.
Examples include llamas, cats, viverrids, and sloths among mammals, many
snakes, turtles, and crocodilians among reptiles, and oxpeckers (genus
Buphagus), vultures, geese, swans, and ostriches among birds.
To give a concrete example of the value of mechanistic understanding in
addressing evolutionary questions, we will briefly consider the role of fluctuating asymmetry in mate choice. Fluctuating asymmetry (FA), individual
deviations from physical symmetry that are hypothesized to provide an
indication of developmental stability (see Møller and Thornhill 1998 for a
recent review, and Houle 1998 for a critique), has been shown to play an
important role in mate choice in an impressive variety of species. FA is an
intrinsically unfakeable cue: the simple fact of bilateral symmetry means
that most animals have paired structures, and small differences in the developmental environment of these structures can potentially have perceptible
effects on the adult that can be used to evaluate FA and provide an indication of developmental stability. The use of FA in mate choice has been
documented in a great diversity of species, but in all of these examples
visual cues were used to evaluate FA. However, because asymmetries in the
vocal folds exist (Hirano et al. 1989) and can have a perceptible effect on
the vocal signals (Isshiki et al. 1977; Steinecke and Herzel 1995; Tigges et
al. 1997), it is plausible that mistuned vocal folds could provide an acoustic
indicator of FA. Similarly, the different lengths of bronchi in oilbirds give
acoustic cues to vocal tract asymmetries in the species (Suthers 1994). Such
acoustic indicators of FA could theoretically play a role in mate choice in
addition to, or instead of, the well-known visual indicators. Such a supposition could be tested via tests of animals that vary naturally in FA, with calls
synthesized with vocal tract models possessing varying degrees of asymmetry, or via experimental manipulation of vocal fold asymmetry (e.g., via
unilateral vocal fold injections).
2.1.3. The Vocal Tract
The acoustic energy generated at the source must pass through the remainder of the respiratory tract before it can emanate out into the environment.
In birds, this portion of the respiratory system is called the suprasyringeal
vocal tract, whereas in other terrestrial vertebrates it is the supralaryngeal
vocal tract. Although the entire vocal-production system, including lungs,
source, and supralaryngeal respiratory passages, is sometimes called the
vocal tract, it is convenient when discussing tetrapod vocal acoustics to
restrict use of the term “vocal tract” to the suprasyringeal or supralaryngeal air passages and their associated articulators, using the term “vocalproduction system” to refer to the entire system. As a broad generalization,
there is much more diversity in vocal tract morphology than in the voice
3. Unpacking “Honesty”
79
known about the use of turbulence in animal communication, although
hissing is obviously a widespread type of vocalization among tetrapods.
Examples include llamas, cats, viverrids, and sloths among mammals, many
snakes, turtles, and crocodilians among reptiles, and oxpeckers (genus
Buphagus), vultures, geese, swans, and ostriches among birds.
To give a concrete example of the value of mechanistic understanding in
addressing evolutionary questions, we will briefly consider the role of fluctuating asymmetry in mate choice. Fluctuating asymmetry (FA), individual
deviations from physical symmetry that are hypothesized to provide an
indication of developmental stability (see Møller and Thornhill 1998 for a
recent review, and Houle 1998 for a critique), has been shown to play an
important role in mate choice in an impressive variety of species. FA is an
intrinsically unfakeable cue: the simple fact of bilateral symmetry means
that most animals have paired structures, and small differences in the developmental environment of these structures can potentially have perceptible
effects on the adult that can be used to evaluate FA and provide an indication of developmental stability. The use of FA in mate choice has been
documented in a great diversity of species, but in all of these examples
visual cues were used to evaluate FA. However, because asymmetries in the
vocal folds exist (Hirano et al. 1989) and can have a perceptible effect on
the vocal signals (Isshiki et al. 1977; Steinecke and Herzel 1995; Tigges et
al. 1997), it is plausible that mistuned vocal folds could provide an acoustic
indicator of FA. Similarly, the different lengths of bronchi in oilbirds give
acoustic cues to vocal tract asymmetries in the species (Suthers 1994). Such
acoustic indicators of FA could theoretically play a role in mate choice in
addition to, or instead of, the well-known visual indicators. Such a supposition could be tested via tests of animals that vary naturally in FA, with calls
synthesized with vocal tract models possessing varying degrees of asymmetry, or via experimental manipulation of vocal fold asymmetry (e.g., via
unilateral vocal fold injections).
2.1.3. The Vocal Tract
The acoustic energy generated at the source must pass through the remainder of the respiratory tract before it can emanate out into the environment.
In birds, this portion of the respiratory system is called the suprasyringeal
vocal tract, whereas in other terrestrial vertebrates it is the supralaryngeal
vocal tract. Although the entire vocal-production system, including lungs,
source, and supralaryngeal respiratory passages, is sometimes called the
vocal tract, it is convenient when discussing tetrapod vocal acoustics to
restrict use of the term “vocal tract” to the suprasyringeal or supralaryngeal air passages and their associated articulators, using the term “vocalproduction system” to refer to the entire system. As a broad generalization,
there is much more diversity in vocal tract morphology than in the voice
3. Unpacking “Honesty”
79