hammerheads is 1/3 its size in males. For the hammerhead bat, the selective pressures underlying laryngeal hypertrophy have been quantified by
the field work of Bradbury (1977), who studied mate selection in this
African species. In trees along riverbanks, male hammerheads form “leks”
(areas where males aggregate to attract mates), from which they emit an
extremely high-amplitude advertisement call. Females fly up and down the
riverine corridor and finally choose a male with which to mate. Males
provide no parental care or other resources, suggesting that female choice
might rely primarily on the vocal display. Bradbury found that five males
in his population of 85 males accounted for 79% of the matings observed.
These data suggest that sexual selection on the vocal-production apparatus
of this species, and perhaps other epomophorine species, may be extremely
intense.
These examples indicate that the mammalian larynx is not tightly constrained by body size. Although there is considerable interspecific variability in the size of the anuran larynx and avian syrinx, we are not aware of
any examples of hypertrophy of the voice source as extreme as those seen
in mammals. Some groups of birds that are known for having loud or lowpitched voices also have unusually large syringes (e.g., currassows and their
allies; Amadon 1969; Delacour and Amadon 1973). Many studies have
failed to find a correlation between body size and “dominant frequency” in
anurans, but it is unclear to what extent dominant frequency depends on
larynx size (see Section 2.1.3). Thus, current data are adequate only to
suggest a lack of constraint on source size, suggesting that even mild selection could disturb any primitive correlation between voice pitch and body
size (contra Morton 1977).
2.2.3. Diversity in Vocal Fold Morphology
Anuran vocal folds have a wide variety of cross-sectional shapes: they can
be T- or L-shaped, or rounded—more like the vocal folds in mammals (see
Schneider 1988 for examples). Anurans also often have additions to the
vocal folds, which modify their oscillatory characteristics. The best-studied
example is in the Túngara frog (Physaleamus pustulosus). This species has
two components in its advertisement call, the “whine” and “chuck.” The
high-pitched, frequency-modulated whine results from the oscillations of
the vocal folds alone. The lower-frequency chuck is hypothesized to result
from vibrations of two fibrous masses, coupled to the vocal folds, which are
introduced into the air stream late in the advertisement call (Drewry et al.
1982; see also Ryan and Kime, Chapter 5).
Another example of an anatomical modification of the voice source are
the vocal membranes found on the vocal folds of many mammalian species.
Vocal membranes, sometimes called “vocal lips” or “sharp-edged vocal
folds,” are thin, upward extensions of the glottal margin of the vocal folds.
They vary in thickness and in the details of their histology. They are
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W.T. Fitch and M.D. Hauser
the field work of Bradbury (1977), who studied mate selection in this
African species. In trees along riverbanks, male hammerheads form “leks”
(areas where males aggregate to attract mates), from which they emit an
extremely high-amplitude advertisement call. Females fly up and down the
riverine corridor and finally choose a male with which to mate. Males
provide no parental care or other resources, suggesting that female choice
might rely primarily on the vocal display. Bradbury found that five males
in his population of 85 males accounted for 79% of the matings observed.
These data suggest that sexual selection on the vocal-production apparatus
of this species, and perhaps other epomophorine species, may be extremely
intense.
These examples indicate that the mammalian larynx is not tightly constrained by body size. Although there is considerable interspecific variability in the size of the anuran larynx and avian syrinx, we are not aware of
any examples of hypertrophy of the voice source as extreme as those seen
in mammals. Some groups of birds that are known for having loud or lowpitched voices also have unusually large syringes (e.g., currassows and their
allies; Amadon 1969; Delacour and Amadon 1973). Many studies have
failed to find a correlation between body size and “dominant frequency” in
anurans, but it is unclear to what extent dominant frequency depends on
larynx size (see Section 2.1.3). Thus, current data are adequate only to
suggest a lack of constraint on source size, suggesting that even mild selection could disturb any primitive correlation between voice pitch and body
size (contra Morton 1977).
2.2.3. Diversity in Vocal Fold Morphology
Anuran vocal folds have a wide variety of cross-sectional shapes: they can
be T- or L-shaped, or rounded—more like the vocal folds in mammals (see
Schneider 1988 for examples). Anurans also often have additions to the
vocal folds, which modify their oscillatory characteristics. The best-studied
example is in the Túngara frog (Physaleamus pustulosus). This species has
two components in its advertisement call, the “whine” and “chuck.” The
high-pitched, frequency-modulated whine results from the oscillations of
the vocal folds alone. The lower-frequency chuck is hypothesized to result
from vibrations of two fibrous masses, coupled to the vocal folds, which are
introduced into the air stream late in the advertisement call (Drewry et al.
1982; see also Ryan and Kime, Chapter 5).
Another example of an anatomical modification of the voice source are
the vocal membranes found on the vocal folds of many mammalian species.
Vocal membranes, sometimes called “vocal lips” or “sharp-edged vocal
folds,” are thin, upward extensions of the glottal margin of the vocal folds.
They vary in thickness and in the details of their histology. They are
88
W.T. Fitch and M.D. Hauser