sacs do seem to have longer maximum call durations than those without air
sacs. Interestingly, humans have the longest “calls” of all primates (our inordinately long single-expiration spoken sentences) but lack air sacs, unlike
all of our nearest relatives, the great apes. MacLarnon and Hewitt (1999)
suggest that this is due to an increase in breathing control in our species. If
this hypothesis is correct, it suggests that other species in which call length
plays an important selective role might also be expected to evolve enhanced
breath control.
The possible link between call length and body size, or body condition,
provides a nice example of an unexplored source of cheap, honest cues in
vertebrate acoustic communication. If only animals in good physical condition have larger, healthier lungs and can thus sustain longer calls, or longer
bouts of calling, we expect selection for discriminating perceivers who
attend to this unfakeable cue. For example, females might compare the
length of calls from two competing males in order to choose between them,
or males might avoid picking fights with rivals who can call longer than
they can.
2.3.5. Source-Related Cues to Body Size
The most frequently cited acoustic parameter that could provide a cue to
body size is mean and/or lowest fundamental frequency (Darwin 1871;
Morton 1977). In nonavian tetrapods, the lowest producible fundamental
frequency of phonation (f 0min ) is determined by the length of the vocal folds:
the longer the folds, the lower is f 0min (Titze 1994). Mass plays a role only if
it is unequally distributed over the fold, as in P. pustulosus discussed above.
If the length of the vocal folds is related to the vocalizer’s body size, f 0min
will thus provide an honest cue to body size (Morton 1977; Hauser 1993).
This indeed appears to be the case in some species, including some toads
and frogs (Martin 1972; Davies and Halliday 1978; Ryan 1988). However,
such a relationship between body size and vocal fold size does not seem to
be typical in other vertebrates. For instance, there is no correlation between
f 0 and body size in adult humans (Lass and Brown 1978; Cohen et al. 1980;
Künzel 1989; van Dommellen 1993), red deer (McComb 1991), and amphibian species (Sullivan 1984; Asquith and Altig 1990). This lack of correlation in adult humans may be particularly surprising given the widespread
assumption that a “deep” or low-pitched voice indicates large body size.
The lack of correlation between f 0 and size seems less surprising when
the anatomy of the vocal folds is considered. The folds are housed within
the flexible cartilaginous larynx, which itself floats at the top of a trachea
and is unconstrained in size by neighboring bony structures (the hyoid
bone, although ossified, grows as a unit with the larynx, Schneider et al.
1967; Schön 1971). Thus, the larynx and vocal folds can grow independently
of the rest of the head or body, as indeed occurs in human males at puberty
(Negus 1949; Goldstein 1980), where androgen receptors in the laryngeal
3. Unpacking “Honesty”
101
sacs. Interestingly, humans have the longest “calls” of all primates (our inordinately long single-expiration spoken sentences) but lack air sacs, unlike
all of our nearest relatives, the great apes. MacLarnon and Hewitt (1999)
suggest that this is due to an increase in breathing control in our species. If
this hypothesis is correct, it suggests that other species in which call length
plays an important selective role might also be expected to evolve enhanced
breath control.
The possible link between call length and body size, or body condition,
provides a nice example of an unexplored source of cheap, honest cues in
vertebrate acoustic communication. If only animals in good physical condition have larger, healthier lungs and can thus sustain longer calls, or longer
bouts of calling, we expect selection for discriminating perceivers who
attend to this unfakeable cue. For example, females might compare the
length of calls from two competing males in order to choose between them,
or males might avoid picking fights with rivals who can call longer than
they can.
2.3.5. Source-Related Cues to Body Size
The most frequently cited acoustic parameter that could provide a cue to
body size is mean and/or lowest fundamental frequency (Darwin 1871;
Morton 1977). In nonavian tetrapods, the lowest producible fundamental
frequency of phonation (f 0min ) is determined by the length of the vocal folds:
the longer the folds, the lower is f 0min (Titze 1994). Mass plays a role only if
it is unequally distributed over the fold, as in P. pustulosus discussed above.
If the length of the vocal folds is related to the vocalizer’s body size, f 0min
will thus provide an honest cue to body size (Morton 1977; Hauser 1993).
This indeed appears to be the case in some species, including some toads
and frogs (Martin 1972; Davies and Halliday 1978; Ryan 1988). However,
such a relationship between body size and vocal fold size does not seem to
be typical in other vertebrates. For instance, there is no correlation between
f 0 and body size in adult humans (Lass and Brown 1978; Cohen et al. 1980;
Künzel 1989; van Dommellen 1993), red deer (McComb 1991), and amphibian species (Sullivan 1984; Asquith and Altig 1990). This lack of correlation in adult humans may be particularly surprising given the widespread
assumption that a “deep” or low-pitched voice indicates large body size.
The lack of correlation between f 0 and size seems less surprising when
the anatomy of the vocal folds is considered. The folds are housed within
the flexible cartilaginous larynx, which itself floats at the top of a trachea
and is unconstrained in size by neighboring bony structures (the hyoid
bone, although ossified, grows as a unit with the larynx, Schneider et al.
1967; Schön 1971). Thus, the larynx and vocal folds can grow independently
of the rest of the head or body, as indeed occurs in human males at puberty
(Negus 1949; Goldstein 1980), where androgen receptors in the laryngeal
3. Unpacking “Honesty”
101