years (the first published record, for the European crane Grus grus, is from
Emperor Friederich II in 1250 A.D.; Niemeier 1979), a wide variety of
hypotheses have been offered to explain its function (reviewed in Fitch
1999). Briefly, these can be classified into physiological hypotheses and
acoustic hypotheses. “Physiological” hypotheses hold that TE serves some
nonacoustic function and include the idea that TE is a respiratory adaptation to retain CO 2 , to increase nonvascular respiratory area for cooling, or
to retain water, to give a few examples. Because only one sex exhibits TE
in many species, all of the physiological hypotheses run into immediate
problems explaining the absence of a presumed physiological adaptation in
one sex (typically males) and not the other. Even in nondimorphic species,
each of the various physiological hypotheses has problems explaining the
presence of TE in the wide variety of species and habitats in which it is
seen. For example, Schmidt-Nielsen’s (1972) hypothesis that TE represents
a respiratory adaptation to long, high-altitude migratory flights, while applicable to trumpeter swans and many cranes, cannot explain TE in sedentary
rainforest birds such as cracids and manucodes. Although TE may serve
some physiological function in some species, it is unlikely to play the same
role in all (or many) of them. Considerations similar to these, combined
with the problem of explaining sexual dimorphism, have led most workers
to dismiss physiological functions as a general explanation of TE. Acoustic
hypotheses suggest that the function of TE has to do with modifying vocal
output; recall that because the vocal source in birds is the syrinx, at the base
of the trachea, the trachea is an integral part of the avian vocal tract. Thus,
elongation of the trachea is also vocal tract elongation and may allow an
individual to deceptively mimic the vocalizations of a larger bird that lacks
this feature. This acoustic hypothesis, which explicitly links production
mechanisms to honest signaling, will be taken up below (Section 2.3.6); see
Fitch (1999) for a more complete review.
A second relatively common type of vocal tract modification is widespread among geese and ducks. Many of these species show bony enlargements of the syrinx or trachea, which are often confined to males and
probably play some as yet undetermined acoustic role. A good review of
the anatomy and its relevance in taxonomy is given by Johnsgard (1961,
1971), but like so many of the morphological phenomena reviewed here,
there has been no further work elucidating their role in sound production
or the evolution of anatid communication systems. Similarly, there are a
number of possible vocal tract modifications of unknown significance
among reptiles. These include an elongated trachea and bronchi in some
tortoises (Siebenrock 1899; Crumly 1984), tracheal diverticula in snakes
(Young 1992), the narial excresence of gharials (Martin and Bellairs 1977),
and the unusual hollow bony crests of many lambeosaurine dinosaurs
(Weishampel 1981).
In mammals, the supralaryngeal vocal tract spans from the larynx to the
lips or nostrils. Consequently, the vocal tract can be elongated in three ways:
3. Unpacking “Honesty”
91
Emperor Friederich II in 1250 A.D.; Niemeier 1979), a wide variety of
hypotheses have been offered to explain its function (reviewed in Fitch
1999). Briefly, these can be classified into physiological hypotheses and
acoustic hypotheses. “Physiological” hypotheses hold that TE serves some
nonacoustic function and include the idea that TE is a respiratory adaptation to retain CO 2 , to increase nonvascular respiratory area for cooling, or
to retain water, to give a few examples. Because only one sex exhibits TE
in many species, all of the physiological hypotheses run into immediate
problems explaining the absence of a presumed physiological adaptation in
one sex (typically males) and not the other. Even in nondimorphic species,
each of the various physiological hypotheses has problems explaining the
presence of TE in the wide variety of species and habitats in which it is
seen. For example, Schmidt-Nielsen’s (1972) hypothesis that TE represents
a respiratory adaptation to long, high-altitude migratory flights, while applicable to trumpeter swans and many cranes, cannot explain TE in sedentary
rainforest birds such as cracids and manucodes. Although TE may serve
some physiological function in some species, it is unlikely to play the same
role in all (or many) of them. Considerations similar to these, combined
with the problem of explaining sexual dimorphism, have led most workers
to dismiss physiological functions as a general explanation of TE. Acoustic
hypotheses suggest that the function of TE has to do with modifying vocal
output; recall that because the vocal source in birds is the syrinx, at the base
of the trachea, the trachea is an integral part of the avian vocal tract. Thus,
elongation of the trachea is also vocal tract elongation and may allow an
individual to deceptively mimic the vocalizations of a larger bird that lacks
this feature. This acoustic hypothesis, which explicitly links production
mechanisms to honest signaling, will be taken up below (Section 2.3.6); see
Fitch (1999) for a more complete review.
A second relatively common type of vocal tract modification is widespread among geese and ducks. Many of these species show bony enlargements of the syrinx or trachea, which are often confined to males and
probably play some as yet undetermined acoustic role. A good review of
the anatomy and its relevance in taxonomy is given by Johnsgard (1961,
1971), but like so many of the morphological phenomena reviewed here,
there has been no further work elucidating their role in sound production
or the evolution of anatid communication systems. Similarly, there are a
number of possible vocal tract modifications of unknown significance
among reptiles. These include an elongated trachea and bronchi in some
tortoises (Siebenrock 1899; Crumly 1984), tracheal diverticula in snakes
(Young 1992), the narial excresence of gharials (Martin and Bellairs 1977),
and the unusual hollow bony crests of many lambeosaurine dinosaurs
(Weishampel 1981).
In mammals, the supralaryngeal vocal tract spans from the larynx to the
lips or nostrils. Consequently, the vocal tract can be elongated in three ways:
3. Unpacking “Honesty”
91