2.1.2. Cetaceans—Bottlenose Dolphin Whistles
Bottlenose dolphin (Tursiops truncatus) social structure is characterized as
fluid, meaning individuals interact with many others during their long lives.
However, two types of long-term associations occur: mother–calf pairs and
male coalitions (Wells et al. 1987; Smolker et al. 1992). Presumably, identifying individuals in these long-term associations is important, although little
data are available to test this hypothesis. Calves are dependent on their
mothers for three to five years and may remain with mothers for up to ten
years, and females of reproductive age associate with other females in their
matriline (Wells et al. 1987). Benefits of long-term associations vary. Female
matrilines may receive aid in caring for and protecting calves against
predators and may gain feeding benefits from associating with conspecifics
(Hoese 1971). Males form long-term coalitions (Smolker et al. 1992; Connor
and Smolker 1995) that aggressively herd and consort with females in
reproductive condition (Connor et al. 1992b) and compete with other males
and coalitions for access to these females. Long-term and large coalitions
tend to be more successful in these competitions (Connor et al. 1992b).
Bottlenose dolphins produce a variety of social calls in addition to
echolocation calls, including pulsed, tonal, and whistled sounds. The specific
function of pulsed and tonal sounds has not been thoroughly investigated,
but whistles have been studied in some depth. Adults produce a repertoire
of three to nine FM whistles (McCowan and Reiss 1995a). Usually one of
these (but up to three) is unique to an individual and is termed that individual’s signature whistle (Caldwell and Caldwell 1965). Signature whistles
can make up from 5% (McCowan and Reiss 1995a) to 95% (Caldwell and
Caldwell 1965) of an individual’s call production. The primary function of
signature whistles is thought to be individual identification (Caldwell and
Caldwell 1965; Caldwell et al. 1990).The combination of small, stable groups
such as mother–calf pairs and male coalitions, and the fluid and repeated
association of these groups with other individuals over many years is
thought to favor individual identification (Janik 1999). Whistles are somewhat variable within individuals (Sayigh et al. 1990), but tend to have a
greater amount of variation between individuals (P.L. Tyack unpublished
data), so their structure is appropriate to serve as individual signatures.
Statistical analysis can classify calls to the correct individual (Buck and
Tyack 1993), and dolphins have sufficiently good discrimination abilities to
differentiate among whistle variants, although no study has tested for recognition of individuals by whistles (Tyack 1997). This is a case where data on
signal structure have been taken as proof of signal function. Calves who are
separated from their mothers whistle as they initiate reunions (McBride
and Kritzler 1951; Smolker et al. 1993). The exact function of whistling in
this context is unclear. It seems doubtful that whistles facilitate reunions
in a manner similar to isolation calls given by bat pups because the calf
approaches the mother and mothers rarely call in response to calves. Also,
calves have usually oriented to the mother prior to calling.
4. Comparative Vocal Learning
159
Bottlenose dolphin (Tursiops truncatus) social structure is characterized as
fluid, meaning individuals interact with many others during their long lives.
However, two types of long-term associations occur: mother–calf pairs and
male coalitions (Wells et al. 1987; Smolker et al. 1992). Presumably, identifying individuals in these long-term associations is important, although little
data are available to test this hypothesis. Calves are dependent on their
mothers for three to five years and may remain with mothers for up to ten
years, and females of reproductive age associate with other females in their
matriline (Wells et al. 1987). Benefits of long-term associations vary. Female
matrilines may receive aid in caring for and protecting calves against
predators and may gain feeding benefits from associating with conspecifics
(Hoese 1971). Males form long-term coalitions (Smolker et al. 1992; Connor
and Smolker 1995) that aggressively herd and consort with females in
reproductive condition (Connor et al. 1992b) and compete with other males
and coalitions for access to these females. Long-term and large coalitions
tend to be more successful in these competitions (Connor et al. 1992b).
Bottlenose dolphins produce a variety of social calls in addition to
echolocation calls, including pulsed, tonal, and whistled sounds. The specific
function of pulsed and tonal sounds has not been thoroughly investigated,
but whistles have been studied in some depth. Adults produce a repertoire
of three to nine FM whistles (McCowan and Reiss 1995a). Usually one of
these (but up to three) is unique to an individual and is termed that individual’s signature whistle (Caldwell and Caldwell 1965). Signature whistles
can make up from 5% (McCowan and Reiss 1995a) to 95% (Caldwell and
Caldwell 1965) of an individual’s call production. The primary function of
signature whistles is thought to be individual identification (Caldwell and
Caldwell 1965; Caldwell et al. 1990).The combination of small, stable groups
such as mother–calf pairs and male coalitions, and the fluid and repeated
association of these groups with other individuals over many years is
thought to favor individual identification (Janik 1999). Whistles are somewhat variable within individuals (Sayigh et al. 1990), but tend to have a
greater amount of variation between individuals (P.L. Tyack unpublished
data), so their structure is appropriate to serve as individual signatures.
Statistical analysis can classify calls to the correct individual (Buck and
Tyack 1993), and dolphins have sufficiently good discrimination abilities to
differentiate among whistle variants, although no study has tested for recognition of individuals by whistles (Tyack 1997). This is a case where data on
signal structure have been taken as proof of signal function. Calves who are
separated from their mothers whistle as they initiate reunions (McBride
and Kritzler 1951; Smolker et al. 1993). The exact function of whistling in
this context is unclear. It seems doubtful that whistles facilitate reunions
in a manner similar to isolation calls given by bat pups because the calf
approaches the mother and mothers rarely call in response to calves. Also,
calves have usually oriented to the mother prior to calling.
4. Comparative Vocal Learning
159