freedom is at least partly responsible for the considerable variability of the
syrinx as an organ (Wunderlich 1886; Warner 1972a, 1972b), which can have
from zero to nine pairs of muscles and is variously located tracheally,
bronchially, or tracheobronchially. Raikow (1986) observed a correlation
between syringeal complexity and the number of species in various taxa of
passerine birds and suggested that morphological changes in syringeal form
might facilitate reproductive isolation and thus speciation. In contrast, the
mammalian larynx is always made up of the same basic cartilages and
muscles, and although the shapes and sizes of these may vary somewhat,
the larynx is overall quite a conservative organ. In anurans, the situation
appears to be intermediate: there is considerable variability in laryngeal
structure, although still minor compared with that seen in the syrinx.
The larynx of mammals and reptiles is under what can be described as a
“multiuse constraint”: the same structure serves multiple functions with
incompatible design requirements, and its function is thus an unhappy compromise between these functions. We suggest that one of the virtues of the
avian syrinx was as an evolutionary “key innovation” (Liem 1973) that
allowed birds to escape from this constraint. The evolution of a specialized
sound-producing organ allowed birds to evade the conservative restrictions
imposed on laryngeal anatomy and nervous control by its critical role in
swallowing. This constraint was presumably in effect in the ancestors of
birds; the closest extant group, the crocodilians, possess a surprisingly
mammal-like larynx, including a nonhomologous “epiglottis” and soft
palate that allow them to form a sealed nostril-to-lung respiratory pathway,
and some crocodilians use the larynx in vocalization. Although little is
known about the evolutionary origins of the syrinx, we argue that its
freedom from the role of gatekeeper to the trachea has been significant in
the evolution of the impressive morphological diversity of the syrinx relative to the anuran or mammalian larynx. In turn, it seems plausible that
morphological diversity is tied to repertoire diversity and perhaps has implications for the rapid diversification and speciation of the passerine birds,
which have the most complex syrinx (but see Raikow 1986). In contrast to
the syrinx, the avian larynx shows almost no functionally significant variation throughout the entire class (McLelland 1989), consonant with its
primary and unchanging role as protector of the airway.
2.3.2. Physical Constraints and the Communication of Body Size
A fundamental fact differentiating the physics of sound from the physics of
light is that sound waves are about the same size as organisms. For example,
an average human female’s speaking voice has a fundamental frequency
around 220 Hz, with a wavelength of 1.6 m, on the order of her height. In
contrast, a spring peeper’s (Hyla crucifer) 3-kHz call has a wavelength
about four times its 3-cm body length. This simple fact has enormous consequences for the production and propagation of sound at an immediate
3. Unpacking “Honesty”
97
syrinx as an organ (Wunderlich 1886; Warner 1972a, 1972b), which can have
from zero to nine pairs of muscles and is variously located tracheally,
bronchially, or tracheobronchially. Raikow (1986) observed a correlation
between syringeal complexity and the number of species in various taxa of
passerine birds and suggested that morphological changes in syringeal form
might facilitate reproductive isolation and thus speciation. In contrast, the
mammalian larynx is always made up of the same basic cartilages and
muscles, and although the shapes and sizes of these may vary somewhat,
the larynx is overall quite a conservative organ. In anurans, the situation
appears to be intermediate: there is considerable variability in laryngeal
structure, although still minor compared with that seen in the syrinx.
The larynx of mammals and reptiles is under what can be described as a
“multiuse constraint”: the same structure serves multiple functions with
incompatible design requirements, and its function is thus an unhappy compromise between these functions. We suggest that one of the virtues of the
avian syrinx was as an evolutionary “key innovation” (Liem 1973) that
allowed birds to escape from this constraint. The evolution of a specialized
sound-producing organ allowed birds to evade the conservative restrictions
imposed on laryngeal anatomy and nervous control by its critical role in
swallowing. This constraint was presumably in effect in the ancestors of
birds; the closest extant group, the crocodilians, possess a surprisingly
mammal-like larynx, including a nonhomologous “epiglottis” and soft
palate that allow them to form a sealed nostril-to-lung respiratory pathway,
and some crocodilians use the larynx in vocalization. Although little is
known about the evolutionary origins of the syrinx, we argue that its
freedom from the role of gatekeeper to the trachea has been significant in
the evolution of the impressive morphological diversity of the syrinx relative to the anuran or mammalian larynx. In turn, it seems plausible that
morphological diversity is tied to repertoire diversity and perhaps has implications for the rapid diversification and speciation of the passerine birds,
which have the most complex syrinx (but see Raikow 1986). In contrast to
the syrinx, the avian larynx shows almost no functionally significant variation throughout the entire class (McLelland 1989), consonant with its
primary and unchanging role as protector of the airway.
2.3.2. Physical Constraints and the Communication of Body Size
A fundamental fact differentiating the physics of sound from the physics of
light is that sound waves are about the same size as organisms. For example,
an average human female’s speaking voice has a fundamental frequency
around 220 Hz, with a wavelength of 1.6 m, on the order of her height. In
contrast, a spring peeper’s (Hyla crucifer) 3-kHz call has a wavelength
about four times its 3-cm body length. This simple fact has enormous consequences for the production and propagation of sound at an immediate
3. Unpacking “Honesty”
97