Variation in sperm whale (Physeter macrocephalus) coda vocalizations and social structure in the North Atlantic Ocean

Abstract

This study aimed at complementing studies of sperm whale social and vocal behaviour that were restricted to the Pacific Ocean. The characteristic multi-pulsed structure of sperm whale clicks allows for estimation of whales' size from measurements of the inter-pulse intervals (IPI). I have developed two new automatic methods for IPI estimation from clicks recorded during foraging dives. When compared to other previously developed methods, the newly developed method that averages several clicks' autocorrelation function showed the best performance amongst the automatic methods. Previous studies did not support individual identity advertisement among social unit members as the function for the sperm whale communication signals called codas. I tested within coda type variation for individual specific patterns and found that, while some coda types do not allow for individual discrimination, one did so. This variation suggests that different coda types may have distinct functions. Analysis of social structure in the Azores found that, similar to the Eastern Tropical Pacific, sperm whales form long term social units of about 12 individuals. Unlike the Pacific Ocean, Azorean social units do not form temporary groups with other units, suggesting differences in the costs and benefits of group formation. I argue that these are due to differences in terms of predation pressure and intraspecific competition between the Azores and the Pacific study sites. The variation of coda repertoires in the Atlantic also showed a pattern dissimilar to that previously documented in the Eastern Tropical Pacific. In the North Atlantic, coda repertoire variation is mostly geographic, which is parsimoniously explained by random drift of culturally transmitted coda repertoires. No sympatric vocal clans with distinct dialects were found as has been noted in the Pacific. Drawing upon the differences found in social structure I argue that selection for maximization of differences between units with similar foraging strategies may have led to the Pacific vocal clans. The differences between oceans suggest that sperm whales may adaptively adjust their behaviour according to experienced ecological conditions.