5. Acoustics and Social Behavior
247
sensitivity. Sound has an advantage as a medium for two-way, modifiable
information (Norris and Evans 1988). It is unclear if dolphins use passive
listening for predator detection, if they use their active echolocation system,
or both. In the Bahamas, audible vocal activity ceases in murky water, counterintuitive to expectations during such conditions. Dolphins echolocate
sporadically on or at objects on the bottom after apparently passive detection of information. The lack of echolocation clicks produced during traveling formations, low-light, and nocturnal feeding conditions also supports
the importance of listening, and using echolocation to "follow-up" on
suspect sounds. Echolocation may be a secondary, or an additional proactive, searching technique after other primary signal detection systems are
employed. The questions then become: When do dolphins listen and when
do they actively search? When is it safe to broadcast your signature whistle
and other social sounds, and when is it smarter to listen and decode instead?
These questions have been relevant in the recent studies on acoustic alarms
and porpoise mortality reduction (Kraus et al. 1997) and in recent results
of the significance of silence in wild bottlenose dolphins (dos Santos and
Alamada 1998).
The costs of vigilance, usually by active scanning, have been calculated
for other species (Illius and Fitzgibbon 1994; Shennan et al. 1994), but
passive vigilance systems that listening might provide have not been
addressed. Individual passive acoustic vigilance and acoustic "eavesdropping" of the vigilance activities, or flight activities of neighbors (in dolphins,
sharp clicks or cracks that precede rapid fleeing) have distinct advantages
over a visual or active acoustic vigilance system. Such conspicuous acoustic
signals insure that information on predators passes freely from one individual to another.
The need for stealth could be critical for both a prey species and a predator. An interesting example is the recent work by Barrett-Lennard et al.
(1996) on echolocation strategies by fish-eating versus mammal-eating 0.
orca. This study documents that 0. orca use passive listening as a primary
means of locating prey and use different echolocation patterns for different hunting strategies (i.e., they mask their clicks and encode their signals
in background noise-acoustic crypticity-when hunting other cetaceans,
prey that can hear their high-frequency clicks). In contrast, 0. orca hunting
fish do not mask their high-frequency signals for fish that do not hear in
high-frequency ranges. They also found no correlation between either group
size or clarity of water and increased echolocation rates, which suggests that
echolocation does not directly kick in when vision fails. This suggests that
passive listening may be primary and echolocation, as an active and supplementary system, is used for final targeting of prey or clarification of
information that has been already detected.
Clupeid fish have recently been reported to respond to ultrasound (Mann
et al. 1997) and to simulated dolphin echolocation (Mann et al. 1998),
suggesting even more need for stealth hunting skills for predators of such
fish. Mann and his colleagues report that all extant clupeids share this audi-
247
sensitivity. Sound has an advantage as a medium for two-way, modifiable
information (Norris and Evans 1988). It is unclear if dolphins use passive
listening for predator detection, if they use their active echolocation system,
or both. In the Bahamas, audible vocal activity ceases in murky water, counterintuitive to expectations during such conditions. Dolphins echolocate
sporadically on or at objects on the bottom after apparently passive detection of information. The lack of echolocation clicks produced during traveling formations, low-light, and nocturnal feeding conditions also supports
the importance of listening, and using echolocation to "follow-up" on
suspect sounds. Echolocation may be a secondary, or an additional proactive, searching technique after other primary signal detection systems are
employed. The questions then become: When do dolphins listen and when
do they actively search? When is it safe to broadcast your signature whistle
and other social sounds, and when is it smarter to listen and decode instead?
These questions have been relevant in the recent studies on acoustic alarms
and porpoise mortality reduction (Kraus et al. 1997) and in recent results
of the significance of silence in wild bottlenose dolphins (dos Santos and
Alamada 1998).
The costs of vigilance, usually by active scanning, have been calculated
for other species (Illius and Fitzgibbon 1994; Shennan et al. 1994), but
passive vigilance systems that listening might provide have not been
addressed. Individual passive acoustic vigilance and acoustic "eavesdropping" of the vigilance activities, or flight activities of neighbors (in dolphins,
sharp clicks or cracks that precede rapid fleeing) have distinct advantages
over a visual or active acoustic vigilance system. Such conspicuous acoustic
signals insure that information on predators passes freely from one individual to another.
The need for stealth could be critical for both a prey species and a predator. An interesting example is the recent work by Barrett-Lennard et al.
(1996) on echolocation strategies by fish-eating versus mammal-eating 0.
orca. This study documents that 0. orca use passive listening as a primary
means of locating prey and use different echolocation patterns for different hunting strategies (i.e., they mask their clicks and encode their signals
in background noise-acoustic crypticity-when hunting other cetaceans,
prey that can hear their high-frequency clicks). In contrast, 0. orca hunting
fish do not mask their high-frequency signals for fish that do not hear in
high-frequency ranges. They also found no correlation between either group
size or clarity of water and increased echolocation rates, which suggests that
echolocation does not directly kick in when vision fails. This suggests that
passive listening may be primary and echolocation, as an active and supplementary system, is used for final targeting of prey or clarification of
information that has been already detected.
Clupeid fish have recently been reported to respond to ultrasound (Mann
et al. 1997) and to simulated dolphin echolocation (Mann et al. 1998),
suggesting even more need for stealth hunting skills for predators of such
fish. Mann and his colleagues report that all extant clupeids share this audi-