Kinship influences social bonds among male southern Australian bottlenose dolphins (Tursiops cf. australis)

Abstract

Male mammals employ a wide variety of mating strategies in order to increase their reproductive success, which in turn influence their social behavior. In some populations of bottlenose dolphins (Tursiops spp.), males cooperate in small groups or alliances to gain access to females for mating. However, the occurrence of these male cooperative groups has been predicted to occur only under certain social and ecological conditions, driven by factors such as differences in population density, operational sex ratio, and sexual size dimorphism. Here, we used generalized affiliation indices, social network techniques, and maternally and bi-parentally inherited genetic markers to investigate the affiliation patterns and genetic relatedness among male southern Australian bottlenose dolphins (Tursiops cf. australis) in a small embayment in South Australia. Photo-identification data and biopsy samples were collected in Coffin Bay from 2013 to 2015 through systematic boat-based surveys. We found that highly sighted male dolphins formed 12 social clusters composed of two to five individuals. Genetic analyses revealed that general male affiliation patterns were significantly correlated with mtDNA haplotype sharing. In addition, preferred affiliates showed significantly higher levels of genetic relatedness compared to casual and avoided male pairs. Our results corroborate theoretical expectations for the formation of social bonds in small delphinids and suggest that a high density of dolphins, with an expected skewed operational sex ratio and no apparent sexual size dimorphism, is likely to have favored the formation of strong male affiliations. In addition, the availability of genetic relatives within the population may have favored male affiliations based on kinship relationships. Male bottlenose dolphins use different strategies to gain access to females, from single roving individuals, pairs or trios in alliances, to stable mixed-sex groups. Moreover, the role of kinship in the formation of male social bonds also varies within and among populations. This variability has been attributed to differences in ecological and intrinsic factors. Here, we studied the affiliation patterns and genetic relatedness in southern Australian bottlenose dolphins and demonstrated that kinship plays an important role in the formation of close male social ties. Furthermore, the results corroborate previous findings about the factors that contribute to the formation of male social bonds in these animals.