Abstract
Sperm whales have a multi-level social structure based upon long-term, cooperative social units. What role kinship plays in structuring this society is poorly understood. We combined extensive association data (518 days, during 2005–2016) and genetic data (18 microsatellites and 346 bp mitochondrial DNA (mtDNA) control region sequences) for 65 individuals from 12 social units from the Eastern Caribbean to examine patterns of kinship and social behaviour. Social units were clearly matrilineally based, evidenced by greater relatedness within social units (mean r = 0.14) than between them (mean r = 0.00) and uniform mtDNA haplotypes within social units. Additionally, most individuals (82.5%) had a first-degree relative in their social unit, while we found no first-degree relatives between social units. Generally and within social units, individuals associated more with their closer relatives (matrix correlations: 0.18–0.25). However, excepting a highly related pair of social units that merged over the study period, associations between social units were not correlated with kinship (p > 0.1). These results are the first to robustly demonstrate kinship's contribution to social unit composition and association preferences, though they also reveal variability in association preferences that is unexplained by kinship. Comparisons with other matrilineal species highlight the range of possible matrilineal societies and how they can vary between and even within species.
Highlights
Sperm whales have a multi-level social structure based upon long-term, cooperative social units
Kinship was clearly correlated with several measures of social association, social unit membership and intra-social unit association preferences within two well-sampled social units, suggesting that kin-selection may be a contributing driver of sperm whale social structure
We found a higher degree of relatedness and matrilineality in social units than has been reported in other regions [26,28,29]
Summary
Sperm whales have a multi-level social structure based upon long-term, cooperative social units. Cooperative behaviours between kin are typically explained in terms of kin selection [5,6], which predicts that individuals maximize their ‘inclusive fitness’ by helping relatives. This hypothesis, cannot explain cooperation between non-relatives, and often fails to explain observed variation in cooperation between relatives [1] In such cases, other mechanisms in lieu of kin selection, or in addition to it, are required to explain seemingly altruistic behaviours. To disentangle potential mechanisms driving cooperative behaviours, long-term studies of social relationships and behaviour are required, together with comprehensive genetic sampling for kinship. These types of datasets are rare among mammals, among marine mammals. Social units have only been observed to form groups with other social units that are members of the same ‘clan’ [18,19], with clan being a higher level of social structure composed of social units that share socially learned behaviours, including distinguishable vocal repertoires [20]
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