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P.L. Tyack and c.w. Clark
clicks because it still uses the full broadband source level of the click. The
observed intervals between clicks are quite a bit longer than would be
expected based upon these estimated detection ranges if sperm whales click
soon after twice the maximum expected round trip travel time. I have gone
into some detail on this point to emphasize how important knowledge
about auditory processes is for understanding echolocation in these
animals. We know next to nothing about hearing in sperm whales, especially
about auditory sonar processing. Studies of auditory processing in bats have
benefited from a synergy between field studies of how bats echolocate in
the wild, controlled experiments in the lab, and development of specific
acoustic and neural models of sonar processing. Echolocation has not been
tested experimentally because sperm whales have never been maintained
for long in captivity, but studies in cetaceans need this kind of synergy
between diverse approaches (Tyack 1997).
During the breeding season, large male sperm whales make especially
loud clicks at slow repetition rates. These slow clicks have low-frequency
spectral peaks and a complex structure of pulses within one click that may
result from reverberation or resonance within the sound production organ.
Weilgart and Whitehead (1988) suggest that these slow clicks may function
as a threat display when males compete for the opportunity to accompany
a breeding group of females. For example, a male may produce loud clicks
when approaching a female group. If a male is already accompanying the
group, he might click back. If some feature of these clicks such as loudness
or low frequencies correlates with a male's competitive ability, then this
information may help females to assess the male at a distance, and may help
a male to assess whether to challenge the other male or not. This dynamic
would select for each male making as extreme a version of the display as
possible. The interpulse intervals (IPIs) within the click may also provide a
reliable cue as to the size of the male. If males assess one another using the
IPI and if the IPI correlates with the length of the spermaceti organ, then
this may have created a selection pressure leading to the evolution of such
large heads in the sperm whale, with the most extreme development in adult
males. Figure 4.7 illustrates one of these loud slow clicks on the far left of
the figure, at 0.1 to 0.2s, showing how much longer the duration is than the
regular clicks.
While the behavioral contexts in which these slow clicks have been
recorded suggest a communicative function as a threat display, these slow
clicks also provide some of the best data on potential uses of sperm whale
sounds for echolocation. Most theoretical discussions of echolocation in
sperm whales have emphasized detection of prey, but the best evidence we
have of actual echoes from sperm whale clicks involve echoes from the
seafloor. A bottom echo from the slow click in Figure 4.7 is also visible on
the far right of the figure at about 1.5 s. If the bottom echo is so obvious to
a hydrophone at the sea surface, then it is likely that a sperm whale diving
near the bottom may be able to hear similar echoes even from fainter
P.L. Tyack and c.w. Clark
clicks because it still uses the full broadband source level of the click. The
observed intervals between clicks are quite a bit longer than would be
expected based upon these estimated detection ranges if sperm whales click
soon after twice the maximum expected round trip travel time. I have gone
into some detail on this point to emphasize how important knowledge
about auditory processes is for understanding echolocation in these
animals. We know next to nothing about hearing in sperm whales, especially
about auditory sonar processing. Studies of auditory processing in bats have
benefited from a synergy between field studies of how bats echolocate in
the wild, controlled experiments in the lab, and development of specific
acoustic and neural models of sonar processing. Echolocation has not been
tested experimentally because sperm whales have never been maintained
for long in captivity, but studies in cetaceans need this kind of synergy
between diverse approaches (Tyack 1997).
During the breeding season, large male sperm whales make especially
loud clicks at slow repetition rates. These slow clicks have low-frequency
spectral peaks and a complex structure of pulses within one click that may
result from reverberation or resonance within the sound production organ.
Weilgart and Whitehead (1988) suggest that these slow clicks may function
as a threat display when males compete for the opportunity to accompany
a breeding group of females. For example, a male may produce loud clicks
when approaching a female group. If a male is already accompanying the
group, he might click back. If some feature of these clicks such as loudness
or low frequencies correlates with a male's competitive ability, then this
information may help females to assess the male at a distance, and may help
a male to assess whether to challenge the other male or not. This dynamic
would select for each male making as extreme a version of the display as
possible. The interpulse intervals (IPIs) within the click may also provide a
reliable cue as to the size of the male. If males assess one another using the
IPI and if the IPI correlates with the length of the spermaceti organ, then
this may have created a selection pressure leading to the evolution of such
large heads in the sperm whale, with the most extreme development in adult
males. Figure 4.7 illustrates one of these loud slow clicks on the far left of
the figure, at 0.1 to 0.2s, showing how much longer the duration is than the
regular clicks.
While the behavioral contexts in which these slow clicks have been
recorded suggest a communicative function as a threat display, these slow
clicks also provide some of the best data on potential uses of sperm whale
sounds for echolocation. Most theoretical discussions of echolocation in
sperm whales have emphasized detection of prey, but the best evidence we
have of actual echoes from sperm whale clicks involve echoes from the
seafloor. A bottom echo from the slow click in Figure 4.7 is also visible on
the far right of the figure at about 1.5 s. If the bottom echo is so obvious to
a hydrophone at the sea surface, then it is likely that a sperm whale diving
near the bottom may be able to hear similar echoes even from fainter