ing two independent fundamental frequencies to be produced by one bird.
Greenewalt based his theory on observation of spectrograms, but more
direct evidence was provided by Nottebohm (1971), who sectioned the right
or left hypoglossal nerves in several songbird species. He found that disabling the right nerve had little effect on canary or chaffinch song, whereas
sectioning the left nerve produced dramatic effects, with most syllables disappearing entirely from the song. Thus, these birds are lateralized for song
production, with one side being dominant. Such asymmetries have also been
discovered in other species, but in some species, such as zebra finches, the
asymmetry is reversed (Williams et al. 1992). Conclusive evidence for the
two-voice theory was provided by Suthers’ (1990) elegant experiments with
mimic thrushes, which produce elaborate songs including imitations of
other species’ songs. By implanting pressure and flow sensors in living birds,
Suthers was able to definitively observe the two voice sources creating independent portions of the awake, singing birds’ final song. Interestingly, mimic
thrushes (at least the catbirds and thrashers that Suthers studied) appear
to utilize both sides of the syrinx relatively equally, although each side
appears to habitually produce certain syllables and not others.
Although full use of a two-voice system appears limited to birds by virtue
of the anatomy of the syrinx, it should be noted that the two vocal folds of
the mammalian larynx can also vibrate independently in certain cases. Normally, the vocal folds collide with every vibratory cycle, which forces them
into the same frequency and phase. However, during breathy voice, or in
pathological cases of unilateral laryngeal paralysis, the vocal folds do not
collide and have been shown to be capable of vibrating at two independent fundamental frequencies (Tigges et al. 1997). However, mammals
appear to lack the fully independent anatomy and nervous control that
would allow each vocal fold to generate rapidly varying and independent
pitches as in many birds. Thus, true two-voice phonation appears to be
limited to the class Aves.
2.2.2. Hypertrophy of the Voice Source
In many mammalian species, the male larynx is enlarged relative to that of
female conspecifics. Male-specific enlargement of the larynx is probably
common, but we know of no systematic review of this topic.The best-known
example is provided by our own species. At puberty, the cartilages of the
human male larynx increase rapidly in size (to about 150% of female laryngeal dimensions), and the length of the vibrating portion of the vocal folds
increases even more, to nearly twice the female size (Hollien 1960; Titze
1994). This change in vocal fold length leads to a precipitous drop in f 0 at
puberty that is one component of the pubertal voice change in males (f 0
about 50% of prepubertal values, Hollien et al. 1994). That this laryngeal
enlargement is triggered by androsteroid hormones such as testosterone
has been known for centuries, leading to the widespread practice in
86
W.T. Fitch and M.D. Hauser
Greenewalt based his theory on observation of spectrograms, but more
direct evidence was provided by Nottebohm (1971), who sectioned the right
or left hypoglossal nerves in several songbird species. He found that disabling the right nerve had little effect on canary or chaffinch song, whereas
sectioning the left nerve produced dramatic effects, with most syllables disappearing entirely from the song. Thus, these birds are lateralized for song
production, with one side being dominant. Such asymmetries have also been
discovered in other species, but in some species, such as zebra finches, the
asymmetry is reversed (Williams et al. 1992). Conclusive evidence for the
two-voice theory was provided by Suthers’ (1990) elegant experiments with
mimic thrushes, which produce elaborate songs including imitations of
other species’ songs. By implanting pressure and flow sensors in living birds,
Suthers was able to definitively observe the two voice sources creating independent portions of the awake, singing birds’ final song. Interestingly, mimic
thrushes (at least the catbirds and thrashers that Suthers studied) appear
to utilize both sides of the syrinx relatively equally, although each side
appears to habitually produce certain syllables and not others.
Although full use of a two-voice system appears limited to birds by virtue
of the anatomy of the syrinx, it should be noted that the two vocal folds of
the mammalian larynx can also vibrate independently in certain cases. Normally, the vocal folds collide with every vibratory cycle, which forces them
into the same frequency and phase. However, during breathy voice, or in
pathological cases of unilateral laryngeal paralysis, the vocal folds do not
collide and have been shown to be capable of vibrating at two independent fundamental frequencies (Tigges et al. 1997). However, mammals
appear to lack the fully independent anatomy and nervous control that
would allow each vocal fold to generate rapidly varying and independent
pitches as in many birds. Thus, true two-voice phonation appears to be
limited to the class Aves.
2.2.2. Hypertrophy of the Voice Source
In many mammalian species, the male larynx is enlarged relative to that of
female conspecifics. Male-specific enlargement of the larynx is probably
common, but we know of no systematic review of this topic.The best-known
example is provided by our own species. At puberty, the cartilages of the
human male larynx increase rapidly in size (to about 150% of female laryngeal dimensions), and the length of the vibrating portion of the vocal folds
increases even more, to nearly twice the female size (Hollien 1960; Titze
1994). This change in vocal fold length leads to a precipitous drop in f 0 at
puberty that is one component of the pubertal voice change in males (f 0
about 50% of prepubertal values, Hollien et al. 1994). That this laryngeal
enlargement is triggered by androsteroid hormones such as testosterone
has been known for centuries, leading to the widespread practice in
86
W.T. Fitch and M.D. Hauser