Heterogeneous social associations within a sperm whale, Physeter macrocephalus, unit reflect pairwise relatedness

Abstract

A good description of a social structure in which individuals live in stable social groups must also capture individual differences in social behaviour in order to understand the selective pressures behind formation and maintenance of those groups. Depending on the evolutionary mechanism acting and the nature of the benefits incurred by individuals, we might expect different patterns of intra-group associations. Female and immature sperm whales (Physeter macrocephalus, Linnaeus 1758) live in stable and partially matrilineal social units. Using photo identification and sloughed skin sampling for genetic determination of sex and relatedness, we investigated patterns of association within a social unit of sperm whales from the eastern Caribbean. Focus was given to how short-term spatio-temporal associations reflect relatedness among unit members. Contrary to previous findings, we found that the patterns of association among members of this unit were heterogeneous and that individuals had preferred associations or avoidances with specific individuals. Furthermore, these preferred associations correlate with relatedness between individuals, such that individuals associated more with their close relatives when variation in sociability of the individuals is controlled. These results are inconsistent with the simple equivalence model of homogeneous associations between members of a social unit. They indicate intra-unit social complexity, a basis for which seems to be genetic relatedness.