common in microchiropteran bats, where they appear to subserve the production of ultrasonic echolocation pulses (Griffin 1958). Vocal membranes
are also common in primates, where they have been hypothesized to allow
individuals to generate calls with very high fundamental frequencies
(Schön-Ybarra 1995) or two simultaneous frequencies (“biphonation”:
Brown and Cannito 1995), and perhaps to create instabilities in the fundamental frequency (Lieberman 1968; Schön-Ybarra 1995; but see Hauser and
Fowler 1991). The hypothesis of higher-frequency calling follows directly
from the fact that, if the lightweight vocal membranes are free to vibrate
independently of the much larger vocal folds, they will do so with a much
higher frequency. Similarly, Brown and Cannito’s “biphonation” hypothesis follows from the possibility that the vocal folds could vibrate simultaneously with the membranes, leading to two independent frequencies in the
vocal output. This effect was inferred from electroglottographs of the vocalizations of the Syke’s monkey (Cercopithecus albogularis).
A recent modeling study sought to understand the acoustic role of the
vocal membranes by simulating their effects in a nonlinear dynamical computer model (Mergell et al. 1999). Mergell and colleagues modeled the
membranes as fixed upward extensions of the upper mass in a well-studied
two-mass model of the vocal folds (Ishizaka and Flanagan 1972); thus, the
membrane was not simply treated as an independent oscillator but as an
integral portion of the vocal fold as a whole. This relatively minor geometrical change had significant effects on the dynamics of vocalization. In particular, the addition of vocal membranes enabled the model to support
louder and higher-pitched vocalizations. Because echolocating bats need
to produce extremely loud and high-pitched calls in order to provide a
detailed, long-range “picture” of their surroundings, the functional utility of
this enhancement is clear. Mergell and colleagues also found that the addition of membranes increased the possibility of source–tract coupling in the
model, resulting in an increased possibility of nonlinear effects and irregularities in vocalizations of species with vocal membranes, as predicted
by Lieberman (1968) and Schön-Ybarra (1995). More empirical work is
needed to further test these predictions, including simple anatomical measurements of vocal membranes in different species, in vivo observations of
vocal membranes during vocalization, and investigations of the social consequences of sounds acoustically manipulated to possess, or to lack, the
characteristics caused by vocal membranes.
A second, and less common, modification of the mammalian vocal folds
is essentially the opposite of vocal membranes: the addition of thick, fleshy
pads to the vocal folds rather than thin membranes. Such “vocal pads” are
seen in lions and other cats of the genus Panthera (Hast 1989; Harrison
1995). No detailed physiological or observational data are available on their
function. However, it seems quite likely that, due to their large mass, they
play a critical role in the production of the low-pitched roars made by all
of these “roaring cat” species (Hast 1989). Finally, there are significant
3. Unpacking “Honesty”
89
are also common in primates, where they have been hypothesized to allow
individuals to generate calls with very high fundamental frequencies
(Schön-Ybarra 1995) or two simultaneous frequencies (“biphonation”:
Brown and Cannito 1995), and perhaps to create instabilities in the fundamental frequency (Lieberman 1968; Schön-Ybarra 1995; but see Hauser and
Fowler 1991). The hypothesis of higher-frequency calling follows directly
from the fact that, if the lightweight vocal membranes are free to vibrate
independently of the much larger vocal folds, they will do so with a much
higher frequency. Similarly, Brown and Cannito’s “biphonation” hypothesis follows from the possibility that the vocal folds could vibrate simultaneously with the membranes, leading to two independent frequencies in the
vocal output. This effect was inferred from electroglottographs of the vocalizations of the Syke’s monkey (Cercopithecus albogularis).
A recent modeling study sought to understand the acoustic role of the
vocal membranes by simulating their effects in a nonlinear dynamical computer model (Mergell et al. 1999). Mergell and colleagues modeled the
membranes as fixed upward extensions of the upper mass in a well-studied
two-mass model of the vocal folds (Ishizaka and Flanagan 1972); thus, the
membrane was not simply treated as an independent oscillator but as an
integral portion of the vocal fold as a whole. This relatively minor geometrical change had significant effects on the dynamics of vocalization. In particular, the addition of vocal membranes enabled the model to support
louder and higher-pitched vocalizations. Because echolocating bats need
to produce extremely loud and high-pitched calls in order to provide a
detailed, long-range “picture” of their surroundings, the functional utility of
this enhancement is clear. Mergell and colleagues also found that the addition of membranes increased the possibility of source–tract coupling in the
model, resulting in an increased possibility of nonlinear effects and irregularities in vocalizations of species with vocal membranes, as predicted
by Lieberman (1968) and Schön-Ybarra (1995). More empirical work is
needed to further test these predictions, including simple anatomical measurements of vocal membranes in different species, in vivo observations of
vocal membranes during vocalization, and investigations of the social consequences of sounds acoustically manipulated to possess, or to lack, the
characteristics caused by vocal membranes.
A second, and less common, modification of the mammalian vocal folds
is essentially the opposite of vocal membranes: the addition of thick, fleshy
pads to the vocal folds rather than thin membranes. Such “vocal pads” are
seen in lions and other cats of the genus Panthera (Hast 1989; Harrison
1995). No detailed physiological or observational data are available on their
function. However, it seems quite likely that, due to their large mass, they
play a critical role in the production of the low-pitched roars made by all
of these “roaring cat” species (Hast 1989). Finally, there are significant
3. Unpacking “Honesty”
89