Socially segregated, sympatric sperm whale clans in the Atlantic Ocean.

Shane Gero,Hal Whitehead,Anne Bøttcher,Peter Teglberg Madsen

doi:10.1098/rsos.160061

Abstract

Sperm whales (Physeter macrocephalus) are unusual in that there is good evidence for sympatric populations with distinct culturally determined behaviour, including potential acoustic markers of the population division. In the Pacific, socially segregated, vocal clans with distinct dialects coexist; by contrast, geographical variation in vocal repertoire in the Atlantic has been attributed to drift. We examine networks of acoustic repertoire similarity and social interactions for 11 social units in the Eastern Caribbean. We find the presence of two socially segregated, sympatric vocal clans whose dialects differ significantly both in terms of categorical coda types produced by each clan (Mantel test between clans: matrix correlation = 0.256; p ≤ 0.001) and when using classification-free similarity which ignores defined types (Mantel test between clans: matrix correlation = 0.180; p ≤ 0.001). The more common of the two clans makes a characteristic 1 + 1 + 3 coda, while the other less often sighted clan makes predominantly regular codas. Units were only observed associating with other units within their vocal clan. This study demonstrates that sympatric vocal clans do exist in the Atlantic, that they define a higher order level of social organization as they do in the Pacific, and suggests that cultural identity at the clan level is probably important in this species worldwide.

Highlights

The formation of social boundaries based on culture was critically important to the evolution of humans
This study supports the hypothesis generated by the social and cultural patterns observed in the Pacific: in the North Atlantic, as in the Pacific, there are sympatric clans of social units of female sperm whales with distinctive repertoires, who tend to appear in a study area in temporal waves
It would appear that sperm whales in the Eastern Caribbean live in a much more individualized society than their counterparts in the Eastern Tropical Pacific

Summary

Introduction

The formation of social boundaries based on culture was critically important to the evolution of humans. Cultural boundaries are often the limits of cooperative and altruistic exchanges; among humans, language helped to identify these social boundaries in order to solve the dilemma of with whom to cooperate [1,2,3,4] These variations in vocal repertoire marked cultural structures. If vocal learning does occur and cultural segregation mirrors geographical separation, vocal repertoires can diverge over time through spatial or demographic factors with copying errors and random cultural drift creating variation between isolated communities [14,15,16] Once either of these occurs, the behavioural differences between the two communities can itself lead to genetic distinctiveness and reproductive isolation [17]

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Discussion

Conclusion