Abstract:
Male dolphins form stable, long-term alliances comparable to long-term
relationships formed by terrestrial species. The goal of this thesis was to determine the
effect of the formation of these alliances on vocal development. Comparing whistles
produced in isolation revealed that alliance partners have similar whistles, while non-partners
do not. Whistle similarity seen in alliance partners mirrors group-specific vocal
convergence in stable groups of birds and bats.
Males produce more variable whistles than females, and females have more stable
whistle repertoires. Unlike males, females do not maintain the strong, stable
relationships seen in male alliances. Increased vocal plasticity in males may be related to
modifying whistle production while forming alliances. Females produced whistles that
were less similar to other females than to males. Females may rely on whistle
distinctiveness for mother-offspring recognition, while males may rely on whistle
convergence to maintain specific social bonds.
The whistles produced by an isolated individual may not represent its complete
repertoire. A hydrophone array was used to record whistles of free-swimming,
socializing individuals to compare to the whistles produced by those animals in isolation.
There was no significant difference in the whistle repertoires of restrained vs. free-swimming
dolphins for over 60% of the animals, and most produced at least one whistle
type in both contexts. Therefore, animals use similar whistles in isolated and free-swimming
conditions.
Recordings of different social groups were examined to test if signature whistles
function as contact calls. An allied male produced signature whistles most often when
separated from his partner and least often when with his partner. Signature whistles were
also highly individually distinctive, and therefore well suited as contact calls, while
variant whistles were not. Separations and reunions between alliance partners were
examined to determine if whistles are used to maintain contact between preferred
associates. Most whistles recorded from separated males were signature whistles. The
timing of whistle production was correlated with the timing of the maximum partner
separation and the initiation of a reunion. Few whistles were produced as the partners
separated. Therefore, whistles may initiate reunions between partners. This thesis
demonstrates that free-ranging male dolphins use signature whistles in the same way as
females and captive dolphins.
