118
T.W. Cranford
If this assessment of the situation has not further muddied the intellectual waters, it may cast some order on what, at times, may seem a dismaying collection of reports. In summary then, all odontocetes can be expected
to generate at least two acoustic components when producing pulses. The
airborne (LF) component will always be present, but its detectability
depends heavily upon the particular recording situation. The tissue-borne
(HF) component can have one or two (perhaps more) peaks, depending
upon the species in question and the recording situation. The number of
tissue-borne HF peaks and the energy distribution across the spectrum is
apparently dependent upon several factors, primary among them are the
anatomic configuration of the bilateral sound generation complexes and
their respective activity.
2. Early Work
The experiments of Schevill and Lawrence (1956) were the first to offer
strong evidence that dolphins used echolocation. In one test, a bottlenose
dolphin (T. truncatus) emitted click trains and was able to locate and swim
to a food reward presented in murky pond water. In a second test, the
animal was able to choose between alternate feeding locations (separated
by a net in murky water) from a distance of at least 2.5 m. Both tests were
carried out on dark nights with a dolphin blind in one eye.
Schevill and Lawrence (1956) also reported that their dolphin was
successful better than two-thirds of the time and emitted "impulsive creaking" sounds; the type of sounds we currently recognize as characteristic of
odontocete echolocation. In their concluding remarks about these sounds
they note,
At close range the creaks were timed to a horizontal sweeping of the head (nodding
when on his side). These observations may be interpreted as indicating directionality, presumably in his sound production. We have not investigated this arresting possibility further, except to consider that perhaps the pneumatic cephalic sinuses may
modify the radiation of sound from the larynx.
Thus, their experiments provided: (1) the first solid evidence that odontocetes could echolocate, (2) the suggestion that dolphins produced a directed
sound beam from the head, at least partially as a result of reflective air sinus
elements, and (3) a statement of the presupposition that sounds were generated in the larynx. Their paper also set forth a series of intriguing questions that engaged workers for the next thirty years. These questions
concerned: (1) dolphin echolocation proficiency, (2) biosonar beam formation and shape, and (3) the source(s) of odontocete sonar signals. Since then,
most of the research papers in this field have addressed one or more of
these issues.
Although convincing evidence existed (Kellogg et a1.1953; McBride 1956;
Kellogg 1958,1961), it was not until Norris and his colleagues fashioned a
T.W. Cranford
If this assessment of the situation has not further muddied the intellectual waters, it may cast some order on what, at times, may seem a dismaying collection of reports. In summary then, all odontocetes can be expected
to generate at least two acoustic components when producing pulses. The
airborne (LF) component will always be present, but its detectability
depends heavily upon the particular recording situation. The tissue-borne
(HF) component can have one or two (perhaps more) peaks, depending
upon the species in question and the recording situation. The number of
tissue-borne HF peaks and the energy distribution across the spectrum is
apparently dependent upon several factors, primary among them are the
anatomic configuration of the bilateral sound generation complexes and
their respective activity.
2. Early Work
The experiments of Schevill and Lawrence (1956) were the first to offer
strong evidence that dolphins used echolocation. In one test, a bottlenose
dolphin (T. truncatus) emitted click trains and was able to locate and swim
to a food reward presented in murky pond water. In a second test, the
animal was able to choose between alternate feeding locations (separated
by a net in murky water) from a distance of at least 2.5 m. Both tests were
carried out on dark nights with a dolphin blind in one eye.
Schevill and Lawrence (1956) also reported that their dolphin was
successful better than two-thirds of the time and emitted "impulsive creaking" sounds; the type of sounds we currently recognize as characteristic of
odontocete echolocation. In their concluding remarks about these sounds
they note,
At close range the creaks were timed to a horizontal sweeping of the head (nodding
when on his side). These observations may be interpreted as indicating directionality, presumably in his sound production. We have not investigated this arresting possibility further, except to consider that perhaps the pneumatic cephalic sinuses may
modify the radiation of sound from the larynx.
Thus, their experiments provided: (1) the first solid evidence that odontocetes could echolocate, (2) the suggestion that dolphins produced a directed
sound beam from the head, at least partially as a result of reflective air sinus
elements, and (3) a statement of the presupposition that sounds were generated in the larynx. Their paper also set forth a series of intriguing questions that engaged workers for the next thirty years. These questions
concerned: (1) dolphin echolocation proficiency, (2) biosonar beam formation and shape, and (3) the source(s) of odontocete sonar signals. Since then,
most of the research papers in this field have addressed one or more of
these issues.
Although convincing evidence existed (Kellogg et a1.1953; McBride 1956;
Kellogg 1958,1961), it was not until Norris and his colleagues fashioned a