surface and bottom would have on the temporal properties of these signals
(see Bradbury and Vehrencamp 1998).
13.2. Behavioral–Structural Mechanisms
Another explanation for the existence of low-frequency acoustic signaling
(behavioral characters) among teleosts is the neuromuscular systems that
lead to sound production (structural characters). Teleosts generate sounds
using diverse neuromuscular mechanisms, including pectoral fin stridulation, pectoral girdle vibration, and swim bladder vibration (Demski et al.
1973; Ladich and Bass 1998). The most extensively studied among these is
the sonic swim bladder, which includes a pair of skeletal, sonic muscles that
are either attached to the walls of the swim bladder or to a bony element
that vibrates against the bladder (the swim bladder is considered an efficient mechanism for generating sound in a large volume of water and, as
mentioned earlier, is generally modeled as a pulsating monopole; see Harris
1964).
The sonic swim bladder muscles of batrachoidids (midshipman and toadfish) are considered the fastest-contracting vertebrate muscles (Skoglund
1961; Rome et al. 1999). They show a number of structural adaptations at
both light and electron-microscopic levels that are consistent with high contraction rates that may last as long as 1 h, as in the midshipman (Fawcett
and Revel 1961; Ibara et al. 1983; Bass and Marchaterre 1989; Walsh et al.
1995; Bass et al. 1999; Rome et al. 1999). Each muscle contraction generates
a single sound pulse so that the contraction rate can be directly translated
into a sound’s fundamental frequency. An advantage of high contraction
rates is the production of a sound with a maximal fundamental frequency
that is separated as far as possible from the cutoff frequency at any
one depth. However, as others point out (e.g., Bradbury and Vehrencamp
1998), a limitation to such a muscle—based mechanism is an upper limit
to the contraction mechanics of striated muscle and, in turn, a sound’s
fundamental frequency.
The production of highly stereotyped acoustic signals depends on the
operation of a rhythmically active vocal neuron network. This vocal control
circuitry, again best studied among batrachoidids, extends along the entire
axis of the brain and shares a number of traits with the vocal circuitry of
other vertebrates (Demski and Gerald 1972; Bass et al. 1994; Bass and
Baker 1997; Goodson and Bass 2000a, 2000b; Fig. 2.14). Neurophysiological studies show a pacemaker-motoneuron circuit positioned in the hindbrain that establishes the rhythmic firing frequency of vocal motoneurons
that determine the contraction rate of sonic muscles, which, in turn, sets the
fundamental frequency of emitted sounds (Fig. 2.14). This hindbrain circuitry provides for the extensive coupling among vocal neurons that leads
to the simultaneous contraction of the paired sonic muscles and maximal
output (Bass and Baker 1990, 1991; Bass et al. 1994).
2. Physical Acoustics of Underwater Sound Communication
55
(see Bradbury and Vehrencamp 1998).
13.2. Behavioral–Structural Mechanisms
Another explanation for the existence of low-frequency acoustic signaling
(behavioral characters) among teleosts is the neuromuscular systems that
lead to sound production (structural characters). Teleosts generate sounds
using diverse neuromuscular mechanisms, including pectoral fin stridulation, pectoral girdle vibration, and swim bladder vibration (Demski et al.
1973; Ladich and Bass 1998). The most extensively studied among these is
the sonic swim bladder, which includes a pair of skeletal, sonic muscles that
are either attached to the walls of the swim bladder or to a bony element
that vibrates against the bladder (the swim bladder is considered an efficient mechanism for generating sound in a large volume of water and, as
mentioned earlier, is generally modeled as a pulsating monopole; see Harris
1964).
The sonic swim bladder muscles of batrachoidids (midshipman and toadfish) are considered the fastest-contracting vertebrate muscles (Skoglund
1961; Rome et al. 1999). They show a number of structural adaptations at
both light and electron-microscopic levels that are consistent with high contraction rates that may last as long as 1 h, as in the midshipman (Fawcett
and Revel 1961; Ibara et al. 1983; Bass and Marchaterre 1989; Walsh et al.
1995; Bass et al. 1999; Rome et al. 1999). Each muscle contraction generates
a single sound pulse so that the contraction rate can be directly translated
into a sound’s fundamental frequency. An advantage of high contraction
rates is the production of a sound with a maximal fundamental frequency
that is separated as far as possible from the cutoff frequency at any
one depth. However, as others point out (e.g., Bradbury and Vehrencamp
1998), a limitation to such a muscle—based mechanism is an upper limit
to the contraction mechanics of striated muscle and, in turn, a sound’s
fundamental frequency.
The production of highly stereotyped acoustic signals depends on the
operation of a rhythmically active vocal neuron network. This vocal control
circuitry, again best studied among batrachoidids, extends along the entire
axis of the brain and shares a number of traits with the vocal circuitry of
other vertebrates (Demski and Gerald 1972; Bass et al. 1994; Bass and
Baker 1997; Goodson and Bass 2000a, 2000b; Fig. 2.14). Neurophysiological studies show a pacemaker-motoneuron circuit positioned in the hindbrain that establishes the rhythmic firing frequency of vocal motoneurons
that determine the contraction rate of sonic muscles, which, in turn, sets the
fundamental frequency of emitted sounds (Fig. 2.14). This hindbrain circuitry provides for the extensive coupling among vocal neurons that leads
to the simultaneous contraction of the paired sonic muscles and maximal
output (Bass and Baker 1990, 1991; Bass et al. 1994).
2. Physical Acoustics of Underwater Sound Communication
55