reptile. More importantly, it suggests that the multiuse constraints described
above, which hinder change in the mammalian vocal tract, are not applicable to birds. Fitch (1999) suggested that this provides an explanation for the
phenomenon of tracheal elongation in birds.An overall correlation between
body size and vocal tract length was probably the primitive state for birds
and has indeed been documented across species by Hinds and Calder
(1971). Therefore, given an appropriately broadband source, formant
frequencies would provide an indication of the vocalizer’s body size. Once
perceivers had evolved to take advantage of this information, it provided an
opportunity for vocal subterfuge: a bird with an elongated trachea could
duplicate the formant dispersion of a larger conspecific and thus exaggerate
its own apparent size. Unlike the case in other vertebrates, where selection
for vocal tract elongation would face stiff opposing selection from multiuse
constraints, tracheal elongation in birds would be opposed only by a
decrease in respiratory efficiency due to increased tracheal dead space
(Hinds and Calder 1971; Clench 1978). However, due to the one-way, flowthrough nature of the avian respiratory tract (Lasiewski 1972; Liem 1985;
Schmidt-Nielsen 1997) and the small volume of the trachea relative to the
extensive respiratory air sacs system, this physiological effect may be
negligible (Prange et al. 1985). Thus, little stood in the way of the acoustic
exaggeration of size via the evolution of tracheal elongation in birds, which
would explain its repeated independent evolution in many orders of birds
and virtual absence in all other taxa; the only other example of tracheal
elongation of which we are aware is in the tortoise Geochelone pardalis
(Crumly 1984) and is of uncertain acoustic or behavioral significance. If this
hypothesis is correct, tracheal elongation in birds is a good example of size
exaggeration via vocal tract elongation (Fitch 1999).
There are a number of other possible examples of deceptive elongation
of the vocal tract. Weishampel (1981) suggested that the prominent crest
of many lambeosaurine dinosaurs, which contained an elongated nasal
passageway, functioned to lengthen the vocal tract and thus to decrease
formant frequencies. The proboscises found in many nonhuman mammals
(e.g., elephants, elephant shrews, various pinnipeds, and proboscis monkeys,
as well as oreodonts and other extinct taxa) have the inevitable result of
lengthening the nasal vocal tract and thus lowering the frequencies of nasal
formants. Whether this serves the function of exaggerating acoustically conveyed size remains an untested hypothesis but seems plausible in the case
of species such as elephant seals and proboscis monkeys where the proboscis is a sexually dimorphic trait. Finally, the vocal tract elongation resulting from the descent of the human larynx may have some size-exaggerating
effect. This hypothesis is supported both by the fact that formant dispersion
is known to be used as a cue to body size by human observers (Fitch 1994)
and that an additional descent of the larynx occurs at puberty in males
simultaneously with (but anatomically and functionally independent of) the
growth of the male larynx (Fitch and Giedd 1999).
3. Unpacking “Honesty”
105
above, which hinder change in the mammalian vocal tract, are not applicable to birds. Fitch (1999) suggested that this provides an explanation for the
phenomenon of tracheal elongation in birds.An overall correlation between
body size and vocal tract length was probably the primitive state for birds
and has indeed been documented across species by Hinds and Calder
(1971). Therefore, given an appropriately broadband source, formant
frequencies would provide an indication of the vocalizer’s body size. Once
perceivers had evolved to take advantage of this information, it provided an
opportunity for vocal subterfuge: a bird with an elongated trachea could
duplicate the formant dispersion of a larger conspecific and thus exaggerate
its own apparent size. Unlike the case in other vertebrates, where selection
for vocal tract elongation would face stiff opposing selection from multiuse
constraints, tracheal elongation in birds would be opposed only by a
decrease in respiratory efficiency due to increased tracheal dead space
(Hinds and Calder 1971; Clench 1978). However, due to the one-way, flowthrough nature of the avian respiratory tract (Lasiewski 1972; Liem 1985;
Schmidt-Nielsen 1997) and the small volume of the trachea relative to the
extensive respiratory air sacs system, this physiological effect may be
negligible (Prange et al. 1985). Thus, little stood in the way of the acoustic
exaggeration of size via the evolution of tracheal elongation in birds, which
would explain its repeated independent evolution in many orders of birds
and virtual absence in all other taxa; the only other example of tracheal
elongation of which we are aware is in the tortoise Geochelone pardalis
(Crumly 1984) and is of uncertain acoustic or behavioral significance. If this
hypothesis is correct, tracheal elongation in birds is a good example of size
exaggeration via vocal tract elongation (Fitch 1999).
There are a number of other possible examples of deceptive elongation
of the vocal tract. Weishampel (1981) suggested that the prominent crest
of many lambeosaurine dinosaurs, which contained an elongated nasal
passageway, functioned to lengthen the vocal tract and thus to decrease
formant frequencies. The proboscises found in many nonhuman mammals
(e.g., elephants, elephant shrews, various pinnipeds, and proboscis monkeys,
as well as oreodonts and other extinct taxa) have the inevitable result of
lengthening the nasal vocal tract and thus lowering the frequencies of nasal
formants. Whether this serves the function of exaggerating acoustically conveyed size remains an untested hypothesis but seems plausible in the case
of species such as elephant seals and proboscis monkeys where the proboscis is a sexually dimorphic trait. Finally, the vocal tract elongation resulting from the descent of the human larynx may have some size-exaggerating
effect. This hypothesis is supported both by the fact that formant dispersion
is known to be used as a cue to body size by human observers (Fitch 1994)
and that an additional descent of the larynx occurs at puberty in males
simultaneously with (but anatomically and functionally independent of) the
growth of the male larynx (Fitch and Giedd 1999).
3. Unpacking “Honesty”
105