248
D.L. Herzing
tory specialization, preceding the evolution of marine mammal hearing and
sound production. Such convergent evolution, like the specialization of
moths to detect ultrasonic signals of bats, may also suggest that the marine
environment contains enough biological and possibly nonbiological ultrasonic signals to warrant the specialization of ultrasonic detection, possibly
preceeding the evolution of ultrasonic production specializations (also see
Richardson et al. 1995, p. 92 for ambient ultrasonic biologics, bubble, and
thermal noise).
Dolphins are both prey and predator; prey for sharks and orcas and
predators on fish and squid. Whether dolphins systematically detect ultrasonic information and low-frequency vibrations from the movement of their
prey or predators, in addition to from their environment, is unmeasured but
is an intriguing possibility. Although most species of dolphin are sensitive
to the middle range of frequencies, many produce signals with a bimodal
frequency band, emphasizing, for example, in S. frontalis, the 40 to 50kHz
range and another 130 to 140kHz area (Au et al. 1998). If dolphin hearing
and communication has evolved under the above pressures, then silent
areas in their bandwidth production might indicate frequency bands that
may be most sensitive to hearing salient cues in the environment, bandwidths detectable by prey (such as 80 kHz detection of simulated bottlenose
nose clicks by shad [Mann et al. 1997]), or bands most available for active
signal transmission between conspecifics.
Another interesting aspect for the detection of signals is the transmission
pathway of sound through the lower jaw and body. Open-mouth behavior
during agonistic encounters is certainly a postural and visual signal for many
delphinids, but it has potential for the manipulation of sound or chemical
reception. At first glance, this activity can look simply like the venting of
an aroused dolphin, but upon further contextual observations we see openmouth orientation when no receiver of such a visual signal is within range.
Could it be that dolphins scan the water with their tongue for possible
chemical cues of a conspecific? Could they be orienting their lower jaw to
"tune in" and localize sounds from other dolphins in the area? (see Ketten,
Chapter 2, for discussion on acoustic "windows"). "Feeling sound," or the
ability for mechanoreceptors on the body to receive acoustic information,
are potential senses used during the detection and decoding of environmental and conspecific sounds. Critical information exists in the low-frequency
bands and includs environmental sounds, water pressure, currents, seismic
activity, predator movement,and components ofconspecificsounds.Although
most vocalizations studied are above 2 kHz, many conspecific sounds emphasize low frequencies and most environmental noise is less than 2 kHz.
4.1.2 Decoding
Decoding signatures in the environment is critical to the initial processing
of sensory information by all animals. Bats determine not only distance,
D.L. Herzing
tory specialization, preceding the evolution of marine mammal hearing and
sound production. Such convergent evolution, like the specialization of
moths to detect ultrasonic signals of bats, may also suggest that the marine
environment contains enough biological and possibly nonbiological ultrasonic signals to warrant the specialization of ultrasonic detection, possibly
preceeding the evolution of ultrasonic production specializations (also see
Richardson et al. 1995, p. 92 for ambient ultrasonic biologics, bubble, and
thermal noise).
Dolphins are both prey and predator; prey for sharks and orcas and
predators on fish and squid. Whether dolphins systematically detect ultrasonic information and low-frequency vibrations from the movement of their
prey or predators, in addition to from their environment, is unmeasured but
is an intriguing possibility. Although most species of dolphin are sensitive
to the middle range of frequencies, many produce signals with a bimodal
frequency band, emphasizing, for example, in S. frontalis, the 40 to 50kHz
range and another 130 to 140kHz area (Au et al. 1998). If dolphin hearing
and communication has evolved under the above pressures, then silent
areas in their bandwidth production might indicate frequency bands that
may be most sensitive to hearing salient cues in the environment, bandwidths detectable by prey (such as 80 kHz detection of simulated bottlenose
nose clicks by shad [Mann et al. 1997]), or bands most available for active
signal transmission between conspecifics.
Another interesting aspect for the detection of signals is the transmission
pathway of sound through the lower jaw and body. Open-mouth behavior
during agonistic encounters is certainly a postural and visual signal for many
delphinids, but it has potential for the manipulation of sound or chemical
reception. At first glance, this activity can look simply like the venting of
an aroused dolphin, but upon further contextual observations we see openmouth orientation when no receiver of such a visual signal is within range.
Could it be that dolphins scan the water with their tongue for possible
chemical cues of a conspecific? Could they be orienting their lower jaw to
"tune in" and localize sounds from other dolphins in the area? (see Ketten,
Chapter 2, for discussion on acoustic "windows"). "Feeling sound," or the
ability for mechanoreceptors on the body to receive acoustic information,
are potential senses used during the detection and decoding of environmental and conspecific sounds. Critical information exists in the low-frequency
bands and includs environmental sounds, water pressure, currents, seismic
activity, predator movement,and components ofconspecificsounds.Although
most vocalizations studied are above 2 kHz, many conspecific sounds emphasize low frequencies and most environmental noise is less than 2 kHz.
4.1.2 Decoding
Decoding signatures in the environment is critical to the initial processing
of sensory information by all animals. Bats determine not only distance,