4. Acoustic Communication in Whales and Dolphins
173
The mother-young bond is not the only setting that may create selection
pressures to maintain contact and recognize partners. There are a variety
of social bonds and social systems among cetaceans that create different
requirements for recognition systems. Sperm whales and killer whales, for
example, live in very stable groups. Fish-eating killer whales from the Pacific
Northwest of the United States have the most stable groups known; neither
sex disperses from its natal group, and the group composition only changes
when a new animal is born, an old one dies, or by rare fissions of large
groups (Bigg et al. 1990). Yet animals within these killer whale and sperm
whale groups frequently disperse kilometers away from one another, and
may be out of sight for hours. Two groups may meet and intermingle for
hours to days (Whitehead and Kahn 1992), but then resegregate into the
original groups. Both killer and sperm whales have group distinctive vocal
repertoires that are thought to function "to maintain the integrity of
the pod" (Ford 1989; Weilgart and Whitehead 1997). Some other cetacean
species such as bottlenose dolphins (T. truncatus) do not have groups as
stable as those of fish-eating killer whales, but rather they live in a fissionfusion society in which group composition changes from hour-to-hour
or even minute-by-minute (Wells et al. 1987). While dolphin groups are
remarkably fluid, there may be strong and stable bonds between particular
individuals. Some wild individual bottlenose dolphins show stable patterns
of association for many years (Wells et al. 1987). Not only do a mother and
her young calf tend to be sighted together for five years or more, but groups
of two to three adult males may be sighted together over periods of years.
These males appear to form coalitions with other individual males, in which
they rely on each other for improving their chances of mating and in fights
with other males (Connor et al. 1992). These male coalitions are often
sighted within larger groups, but they seldom remain with the larger group
for long. This combination of highly structured patterns of association
between individuals, coupled with fluid patterns of social grouping, argues
that individual-specific social relationships are an important element of
bottlenose dolphin societies (Tyack 1986). This kind of social system
would appear to select for individually distinctive recognition signals.
Caldwell and Caldwell (1965) demonstrated that each dolphin within a
captive group produced an individually distinctive whistle, which they
called signature whistles. Caldwell and Caldwell (1965) postulated that
signature whistles function to broadcast individual identity. Caldwell et al.
(1990) suggest that a majority of the whistles produced by isolated captive
dolphins are signature whistles. Janik et al. (1994) report that signature
whistles are more common when a captive dolphin is in an isolation
condition than when it is being trained. Janik and Slater (1998) suggest
that the increased tendency of dolphins to produce signature whistles in
isolation supports the hypothesis that dolphins use signature whistles to
maintain contact with individuals from which they have been separated.
Signature whistles were initially discovered in captive dolphins, but similar
173
The mother-young bond is not the only setting that may create selection
pressures to maintain contact and recognize partners. There are a variety
of social bonds and social systems among cetaceans that create different
requirements for recognition systems. Sperm whales and killer whales, for
example, live in very stable groups. Fish-eating killer whales from the Pacific
Northwest of the United States have the most stable groups known; neither
sex disperses from its natal group, and the group composition only changes
when a new animal is born, an old one dies, or by rare fissions of large
groups (Bigg et al. 1990). Yet animals within these killer whale and sperm
whale groups frequently disperse kilometers away from one another, and
may be out of sight for hours. Two groups may meet and intermingle for
hours to days (Whitehead and Kahn 1992), but then resegregate into the
original groups. Both killer and sperm whales have group distinctive vocal
repertoires that are thought to function "to maintain the integrity of
the pod" (Ford 1989; Weilgart and Whitehead 1997). Some other cetacean
species such as bottlenose dolphins (T. truncatus) do not have groups as
stable as those of fish-eating killer whales, but rather they live in a fissionfusion society in which group composition changes from hour-to-hour
or even minute-by-minute (Wells et al. 1987). While dolphin groups are
remarkably fluid, there may be strong and stable bonds between particular
individuals. Some wild individual bottlenose dolphins show stable patterns
of association for many years (Wells et al. 1987). Not only do a mother and
her young calf tend to be sighted together for five years or more, but groups
of two to three adult males may be sighted together over periods of years.
These males appear to form coalitions with other individual males, in which
they rely on each other for improving their chances of mating and in fights
with other males (Connor et al. 1992). These male coalitions are often
sighted within larger groups, but they seldom remain with the larger group
for long. This combination of highly structured patterns of association
between individuals, coupled with fluid patterns of social grouping, argues
that individual-specific social relationships are an important element of
bottlenose dolphin societies (Tyack 1986). This kind of social system
would appear to select for individually distinctive recognition signals.
Caldwell and Caldwell (1965) demonstrated that each dolphin within a
captive group produced an individually distinctive whistle, which they
called signature whistles. Caldwell and Caldwell (1965) postulated that
signature whistles function to broadcast individual identity. Caldwell et al.
(1990) suggest that a majority of the whistles produced by isolated captive
dolphins are signature whistles. Janik et al. (1994) report that signature
whistles are more common when a captive dolphin is in an isolation
condition than when it is being trained. Janik and Slater (1998) suggest
that the increased tendency of dolphins to produce signature whistles in
isolation supports the hypothesis that dolphins use signature whistles to
maintain contact with individuals from which they have been separated.
Signature whistles were initially discovered in captive dolphins, but similar