ON THE DYNAMICS OF SOCIAL RELATIONSHIPS AND VOCAL COMMUNICATION BETWEEN INDIVIDUALS AND SOCIAL UNITS OF SPERM WHALES
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Within-population behavioural variation can greatly affect the ecology of a species and the outcome of evolutionary processes. This study aimed to determine how variable sperm whale social and vocal behaviour is between both individuals and their social units. The population of whales off Dominica is small and isolated from communities in neighbouring waters. Female and immature whales live together in social units containing about 7 animals. I analysed their social relationships and their ‘coda’ communication signals using an unparalleled dataset of social and vocal interactions at the level of the individual. Within units, calves were significant nodes in their social unit’s network, and thus I provide quantitative support for the hypothesis that communal calf care acts as the primary evolutionary driver for group formation in this species. Social relationships within and between units were diverse, while the spatial spread of individuals within units and their travel speeds were similar among all of the units. I identified long-term patterns of association between units consistent over decadal time scales. Social units had characteristic vocal repertoires, but all were dominated by the ‘1+1+3’ and ‘5R’ coda types. Differences between units resulted from some units using specific 4-click coda types. Units and individuals used different accents on their ‘5R’ codas, but the ‘1+1+3’ coda was stereotyped across all individuals and units studied. The repertoires of different units were as similar as units within vocal clans in the Pacific. My results support the hypothesis that the ‘5R’ coda may function in individual identification. The stability of the ‘1+1+3’ coda may be the result of selection for a marker of clan membership. Individual repertoires differed consistently across years; and contrary to an existing hypothesis, new mothers did not vary their repertoire to be more distinct after giving birth. However, calves did use a class-specific ‘3+1’ coda. In summary, sperm whale social and vocal behaviour vary between individuals and among units. Variation in the social and vocal behaviour of female sperm whales results from a trade-off between individuality and conformity within units and clans.