3. Impulse Sound Sources
113
Our current understanding of aquatic echolocation by odontocetes, especially in free-ranging animals, is in its infancy. Still, a bountiful harvest of
information on odontocete echolocation has been gathered in the past four
decades, primarily from studies with trained animals (see review in Au 1993).
The majority of these studies have been designed to avoid superfluous movements and eliminate some of the recording complexity by holding the animal
and any targets stationary. While it is clear that these studies have provided
a solid foundation for our current understanding of odontocete biosonar,
we should guard against the temptation to limit our view of aquatic echolocation based upon the results of these studies. For example, technical
advancements have only recently allowed us to collect high-fidelity, highbandwidth recordings from dolphins that are free to swim during long-range
search and detection tasks (Sigurdson 1997a,b). This should allow us to
consider the possible effects of kinematics and rhythmicity that probably
accompany echolocation in free-swimming dolphins. By comparison,
echolocation studies with bats have long considered locomotor dynamics
and morphological influences on echolocation (Schnitzler and Grinnell
1977; Kick and Simmons 1984; Simmons 1989; Lancaster et al. 1992;
Lancanster et al. 1995). Fortunately, for the present discussions of odontocete sound generation, it is likely that all brief pulses are produced by a
similar mechanism, using homologous structures, regardless of the pattern
of repetition rates, the frequency composition, or the species in question.
Repetition rates for echolocation clicks are generally less than 400 Hz,
but there are a few reports of animals using rates upwards of 800 Hz to
inspect objects at close range. We generally attribute these high repetition
rates to a function other than echolocation. This, however, is not necessarily the case, since much of what we now know has been gleaned from
psychoacoustic experiments conducted with trained animals under precisely designed and strictly controlled situational paradigms. Recently,
Verfuss et al. (1999) conducted an enlightening set of experiments with
harbor porpoises trained to pursue and capture free-swimming prey fish
within an enclosure. They found, among other parameters, that two distinct
stages could be defined by differences in click repetition rates. In both
Far and Near stages the click interval is larger than the two way transit
time. In the Near Stage the lag time decreases to a minimum (from
perhaps 50msec in the Far Stage, to approximately 1.5 msec in the Near
Stage), giving the porpoise no time to process the returning echo before
sending out the next click. This suggests that the animal may process
acoustic information differently in the two stages. When compared to
decades of sonar research with bats, the work of Verfuss and her colleagues
accentuates the gap in our knowledge of odontocete sonar behavior. Bat
sonar research has focussed for decades on prey detection, localization,
tracking, recognition, pursuit, and capture. The high repetition rate of the
porpoise's near stage approach has an obvious analog in the terminal buzz
of microchiropterans during the capture of flying insects.
113
Our current understanding of aquatic echolocation by odontocetes, especially in free-ranging animals, is in its infancy. Still, a bountiful harvest of
information on odontocete echolocation has been gathered in the past four
decades, primarily from studies with trained animals (see review in Au 1993).
The majority of these studies have been designed to avoid superfluous movements and eliminate some of the recording complexity by holding the animal
and any targets stationary. While it is clear that these studies have provided
a solid foundation for our current understanding of odontocete biosonar,
we should guard against the temptation to limit our view of aquatic echolocation based upon the results of these studies. For example, technical
advancements have only recently allowed us to collect high-fidelity, highbandwidth recordings from dolphins that are free to swim during long-range
search and detection tasks (Sigurdson 1997a,b). This should allow us to
consider the possible effects of kinematics and rhythmicity that probably
accompany echolocation in free-swimming dolphins. By comparison,
echolocation studies with bats have long considered locomotor dynamics
and morphological influences on echolocation (Schnitzler and Grinnell
1977; Kick and Simmons 1984; Simmons 1989; Lancaster et al. 1992;
Lancanster et al. 1995). Fortunately, for the present discussions of odontocete sound generation, it is likely that all brief pulses are produced by a
similar mechanism, using homologous structures, regardless of the pattern
of repetition rates, the frequency composition, or the species in question.
Repetition rates for echolocation clicks are generally less than 400 Hz,
but there are a few reports of animals using rates upwards of 800 Hz to
inspect objects at close range. We generally attribute these high repetition
rates to a function other than echolocation. This, however, is not necessarily the case, since much of what we now know has been gleaned from
psychoacoustic experiments conducted with trained animals under precisely designed and strictly controlled situational paradigms. Recently,
Verfuss et al. (1999) conducted an enlightening set of experiments with
harbor porpoises trained to pursue and capture free-swimming prey fish
within an enclosure. They found, among other parameters, that two distinct
stages could be defined by differences in click repetition rates. In both
Far and Near stages the click interval is larger than the two way transit
time. In the Near Stage the lag time decreases to a minimum (from
perhaps 50msec in the Far Stage, to approximately 1.5 msec in the Near
Stage), giving the porpoise no time to process the returning echo before
sending out the next click. This suggests that the animal may process
acoustic information differently in the two stages. When compared to
decades of sonar research with bats, the work of Verfuss and her colleagues
accentuates the gap in our knowledge of odontocete sonar behavior. Bat
sonar research has focussed for decades on prey detection, localization,
tracking, recognition, pursuit, and capture. The high repetition rate of the
porpoise's near stage approach has an obvious analog in the terminal buzz
of microchiropterans during the capture of flying insects.