vocal tract morphologies, but the functional significance of this diversity is
only beginning to be explored. Advances in digital signal analysis, techniques for the visualization of the vocal tract in action, and an increasing
interest in the role of proximate mechanisms in evolution suggest that
progress in understanding this morphological diversity, and correlating it
with social behavior and evolutionary history, will be rapid in the coming
years.
2.3. Physical and Phylogenetic Constraints on
Vocal Production
In this section, we attempt to explicate some of the diversity documented
above by integrating the acoustic and anatomical data into a more comprehensive functional and evolutionary framework. In particular, we argue
that much of the anatomical diversity seen in tetrapod vocal tracts can be
understood from the point of view of ubiquitous selective pressures operating within a framework of physical and phylogenetic constraints together
with evasions of those constraints via “key innovations.”
Because this is a selective synthesis, there are two potentially relevant
topics that we will not cover: (1) adaptations of calls to the transmission
characteristics of the environment (Morton 1975; Wiley and Richards 1982;
Brown and Gomez 1992; Bradbury and Vehrencamp 1998; see also Ryan
and Kime, Chapter 5; and Bass and Clark, Chapter 2) and (2) adaptations
of alarm-call morphology that make localization difficult. This last topic was
initiated by Marler’s (1955) classic observation that the “seep” alarm calls
of passerine birds are difficult to spatially localize and has more recently
been reviewed by Catchpole and Slater (1995) and Hauser (1996).
2.3.1. Syringeal Diversity and Multiuse Constraints on
Laryngeal Function
The primary function of the tetrapod larynx, both functionally and in terms
of its history, is as a valve controlling access to and protecting the respiratory tree. Full of sensitive mucosa, the larynx will quickly close and exclude
any foreign bodies that near it. In mammals, the larynx also can engage into
the nasopharyngeal opening, forming a sealed respiratory passage from the
nostrils to the lungs. Nevertheless, during swallowing of large, solid food
items, and at all times in humans, food must pass over the opening of the
glottis during swallowing before entering the digestive tract. This situation,
as noted by Darwin, means that the “gatekeeper” role of the larynx is ever
present. Its role as a sound-producing organ must always coexist with this
gatekeeping role.
In contrast, the avian syrinx appears to serve only one function: sound
production. In birds, the larynx is devoted to the gatekeeping role exclusively, whereas the syrinx is free to create sound. We hypothesize that this
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W.T. Fitch and M.D. Hauser
only beginning to be explored. Advances in digital signal analysis, techniques for the visualization of the vocal tract in action, and an increasing
interest in the role of proximate mechanisms in evolution suggest that
progress in understanding this morphological diversity, and correlating it
with social behavior and evolutionary history, will be rapid in the coming
years.
2.3. Physical and Phylogenetic Constraints on
Vocal Production
In this section, we attempt to explicate some of the diversity documented
above by integrating the acoustic and anatomical data into a more comprehensive functional and evolutionary framework. In particular, we argue
that much of the anatomical diversity seen in tetrapod vocal tracts can be
understood from the point of view of ubiquitous selective pressures operating within a framework of physical and phylogenetic constraints together
with evasions of those constraints via “key innovations.”
Because this is a selective synthesis, there are two potentially relevant
topics that we will not cover: (1) adaptations of calls to the transmission
characteristics of the environment (Morton 1975; Wiley and Richards 1982;
Brown and Gomez 1992; Bradbury and Vehrencamp 1998; see also Ryan
and Kime, Chapter 5; and Bass and Clark, Chapter 2) and (2) adaptations
of alarm-call morphology that make localization difficult. This last topic was
initiated by Marler’s (1955) classic observation that the “seep” alarm calls
of passerine birds are difficult to spatially localize and has more recently
been reviewed by Catchpole and Slater (1995) and Hauser (1996).
2.3.1. Syringeal Diversity and Multiuse Constraints on
Laryngeal Function
The primary function of the tetrapod larynx, both functionally and in terms
of its history, is as a valve controlling access to and protecting the respiratory tree. Full of sensitive mucosa, the larynx will quickly close and exclude
any foreign bodies that near it. In mammals, the larynx also can engage into
the nasopharyngeal opening, forming a sealed respiratory passage from the
nostrils to the lungs. Nevertheless, during swallowing of large, solid food
items, and at all times in humans, food must pass over the opening of the
glottis during swallowing before entering the digestive tract. This situation,
as noted by Darwin, means that the “gatekeeper” role of the larynx is ever
present. Its role as a sound-producing organ must always coexist with this
gatekeeping role.
In contrast, the avian syrinx appears to serve only one function: sound
production. In birds, the larynx is devoted to the gatekeeping role exclusively, whereas the syrinx is free to create sound. We hypothesize that this
96
W.T. Fitch and M.D. Hauser