4. Acoustic Communication in Whales and Dolphins
165
2.4 Detection or Incapacitation of Prey by
Sound Production
2.4.1 Can Odontocetes Produce Sounds Loud Enough to Stun or Injure
Prey or Conspecifics?
One of the most popular and original theories of how dolphins may use
sound is the acoustic stunning hypothesis of Ken Norris and BertI MlZIhl
(1983). They review evidence that odontocetes can produce high pressure
levels of sound. For example, bottlenose dolphins (Tursiops truncatus)
trained to echolocate on distant targets in open waters can produce clicks
with peak-to-peak sound pressure levels over 220dB re IIlPa at 1 m (Au,
Chapter 9). However, these are high-frequency clicks with a very rapid rise
time. The actual duration during which these sounds achieve such high pressure levels is only several microseconds, so the actual energy delivered
by the signal is comparable to the less intense, longer duration sounds
produced by many cetaceans (Au 1993). Sperm whales and dolphins also
produce intense lower-frequency clicks, with durations of tens of milliseconds. There are few reliable calibrated estimates of the source level of these
sounds, so it is difficult to analyze the possible acoustic effects of these
signals.
Norris and MlZIhl (1983) review observations that fish being preyed upon
by dolphins may appear disoriented or incapacitated. They propose that this
debilitation is caused by exposure to the intense pulsed sounds of odontocetes. Testing this hypothesis requires exposing fish to sounds of odontocetes where the received level at the fish is measured with a calibrated
system, coupled with subsequent behavioral and anatomical testing of the
fish subjects. However, in the 15 years since this paper was published, no
carefully controlled experiments of this sort have been conducted.
Johnson and Norris (1994) also suggest that dolphins might also be able
to produce sounds loud enough to deafen a nearby schoolmate. Some of
the loud low frequency pulses that they associate with debilitation of prey
in bottlenose dolphins are similar if not identical to jaw claps, which are
thought to function as threats in aggressive interactions (Overstrom 1983).
However, while louder sounds may create a more intense threat, there is
no evidence that these sounds are painful or damaging to conspecifics. Since
dolphins can create sounds with high sound pressure levels, it is likely that
they have mechanisms to protect their own inner ears from exposure when
they make a sound themselves. The details of such mechanisms are not currently known, nor whether animals can protect themselves as well from
sounds produced by others as from self-generated sounds. The general issue
of whether animals have evolved mechanisms to protect hearing from exposure to the loudest sound levels produced by conspecifics nearby is not just
of academic interest. This assumption that cetacean ears have protective
mechanisms has been used to suggest that intense human-made noises may
165
2.4 Detection or Incapacitation of Prey by
Sound Production
2.4.1 Can Odontocetes Produce Sounds Loud Enough to Stun or Injure
Prey or Conspecifics?
One of the most popular and original theories of how dolphins may use
sound is the acoustic stunning hypothesis of Ken Norris and BertI MlZIhl
(1983). They review evidence that odontocetes can produce high pressure
levels of sound. For example, bottlenose dolphins (Tursiops truncatus)
trained to echolocate on distant targets in open waters can produce clicks
with peak-to-peak sound pressure levels over 220dB re IIlPa at 1 m (Au,
Chapter 9). However, these are high-frequency clicks with a very rapid rise
time. The actual duration during which these sounds achieve such high pressure levels is only several microseconds, so the actual energy delivered
by the signal is comparable to the less intense, longer duration sounds
produced by many cetaceans (Au 1993). Sperm whales and dolphins also
produce intense lower-frequency clicks, with durations of tens of milliseconds. There are few reliable calibrated estimates of the source level of these
sounds, so it is difficult to analyze the possible acoustic effects of these
signals.
Norris and MlZIhl (1983) review observations that fish being preyed upon
by dolphins may appear disoriented or incapacitated. They propose that this
debilitation is caused by exposure to the intense pulsed sounds of odontocetes. Testing this hypothesis requires exposing fish to sounds of odontocetes where the received level at the fish is measured with a calibrated
system, coupled with subsequent behavioral and anatomical testing of the
fish subjects. However, in the 15 years since this paper was published, no
carefully controlled experiments of this sort have been conducted.
Johnson and Norris (1994) also suggest that dolphins might also be able
to produce sounds loud enough to deafen a nearby schoolmate. Some of
the loud low frequency pulses that they associate with debilitation of prey
in bottlenose dolphins are similar if not identical to jaw claps, which are
thought to function as threats in aggressive interactions (Overstrom 1983).
However, while louder sounds may create a more intense threat, there is
no evidence that these sounds are painful or damaging to conspecifics. Since
dolphins can create sounds with high sound pressure levels, it is likely that
they have mechanisms to protect their own inner ears from exposure when
they make a sound themselves. The details of such mechanisms are not currently known, nor whether animals can protect themselves as well from
sounds produced by others as from self-generated sounds. The general issue
of whether animals have evolved mechanisms to protect hearing from exposure to the loudest sound levels produced by conspecifics nearby is not just
of academic interest. This assumption that cetacean ears have protective
mechanisms has been used to suggest that intense human-made noises may