We have focused in this section on acoustic cues to body size mainly
because body size is easily measured, is an extremely important variable in
many species, and has a direct and obvious effect on the production of
acoustic signals. However, we would like to stress that the approach outlined above is likely to be applicable to many other types of information
in animal signals as well. For example, individual differences in vocal tract
anatomy may provide robust cues to individual identity (Rendall et al.
1996), and differences in the use of nasal versus oral vocal tracts might serve
as a cue to group membership (Hauser 1992). Sex hormones can bind
preferentially to laryngeal tissues (Tuohima et al. 1981), suggesting that
certain aspects of the voice source may provide cues to sexual readiness or
other endocrinological information (Yamaguchi and Kelley, Chapter 6). A
possible example is oestrous-related calling in gelada baboons (MoosHeilen and Sossinka 1990): could steroid-related changes in tissue hydration over a female’s cycle lead to vocal cues to ovulation? Finally, there may
be vocal cues to age in some species. The histological composition of the
vocal folds changes with age (Titze 1994), potentially resulting in vocal cues
to a caller’s age and experience. Similarly, the vocal tract in male plain
chachalacas (Ortalis vetula) elongates with age (Marion 1977), presumably
lowering formant frequencies. Could male rivals use such cues to avoid
more experienced rivals, or might females use formants in mate choice? All
of these questions are highly relevant to the evolution and structure of
acoustic communication systems but demand advances in our knowledge
of proximate mechanisms before they can be adequately addressed. If the
preceding review spurs research along these lines, it will have achieved its
goal.
2.4. Conclusion
To summarize and conclude Section 2 of this chapter, we have seen that
physical and physiological constraints play a fundamental role in shaping
the signaling systems of terrestrial vertebrates, interacting with multiple
selective forces in various ways to produce an impressive variety of morphological adaptations in tetrapod vocal-production systems. Physical constraints, by creating nonarbitrary mappings between behaviorally relevant
parameters (such as body size) and aspects of acoustic signals (such as
frequency) can provide a starting point for the use of a certain parameter
in a species’ communication system. Physiological constraints (such as the
multiple functions of the mammalian larynx or the restriction of most
tetrapod vocal tracts to the skull) can play an important role in maintaining signal honesty in lieu of any specific selection “for” honesty. Knowledge
of these constraints can also provide a principled starting point for scientific analysis of a species’ vocal repertoire, allowing us to identify precisely
acoustic parameters that might play a role in signaling. Finally, “key innovations,” such as the syrinx in birds, can allow a species to evade such con106
W.T. Fitch and M.D. Hauser
because body size is easily measured, is an extremely important variable in
many species, and has a direct and obvious effect on the production of
acoustic signals. However, we would like to stress that the approach outlined above is likely to be applicable to many other types of information
in animal signals as well. For example, individual differences in vocal tract
anatomy may provide robust cues to individual identity (Rendall et al.
1996), and differences in the use of nasal versus oral vocal tracts might serve
as a cue to group membership (Hauser 1992). Sex hormones can bind
preferentially to laryngeal tissues (Tuohima et al. 1981), suggesting that
certain aspects of the voice source may provide cues to sexual readiness or
other endocrinological information (Yamaguchi and Kelley, Chapter 6). A
possible example is oestrous-related calling in gelada baboons (MoosHeilen and Sossinka 1990): could steroid-related changes in tissue hydration over a female’s cycle lead to vocal cues to ovulation? Finally, there may
be vocal cues to age in some species. The histological composition of the
vocal folds changes with age (Titze 1994), potentially resulting in vocal cues
to a caller’s age and experience. Similarly, the vocal tract in male plain
chachalacas (Ortalis vetula) elongates with age (Marion 1977), presumably
lowering formant frequencies. Could male rivals use such cues to avoid
more experienced rivals, or might females use formants in mate choice? All
of these questions are highly relevant to the evolution and structure of
acoustic communication systems but demand advances in our knowledge
of proximate mechanisms before they can be adequately addressed. If the
preceding review spurs research along these lines, it will have achieved its
goal.
2.4. Conclusion
To summarize and conclude Section 2 of this chapter, we have seen that
physical and physiological constraints play a fundamental role in shaping
the signaling systems of terrestrial vertebrates, interacting with multiple
selective forces in various ways to produce an impressive variety of morphological adaptations in tetrapod vocal-production systems. Physical constraints, by creating nonarbitrary mappings between behaviorally relevant
parameters (such as body size) and aspects of acoustic signals (such as
frequency) can provide a starting point for the use of a certain parameter
in a species’ communication system. Physiological constraints (such as the
multiple functions of the mammalian larynx or the restriction of most
tetrapod vocal tracts to the skull) can play an important role in maintaining signal honesty in lieu of any specific selection “for” honesty. Knowledge
of these constraints can also provide a principled starting point for scientific analysis of a species’ vocal repertoire, allowing us to identify precisely
acoustic parameters that might play a role in signaling. Finally, “key innovations,” such as the syrinx in birds, can allow a species to evade such con106
W.T. Fitch and M.D. Hauser