False killer whales are highly social, known for maintaining strong, long-term bonds and engaging in cultural behaviors including prey-sharing and mass stranding that may make them vulnerable to anthropogenic impacts. An endangered main Hawaiian Islands population includes three recognized social clusters (C1, C2, C3). Although they appear longitudinally stable, all three are interconnected and some individuals are regularly associated with multiple clusters. Such individuals may have a strong impact on the health of the population by disseminating information between clusters or acting as a vector for diseases. To understand their influence on fine-scale social structure, as well as assess peripheral individuals that do not link clusters, we examined sighting histories (2000-2016) and associations of socially ambiguous individuals using photo-identification and social network analysis. We then simulated targeted removals of each ambiguous individual, imposed restrictions on association strength and recorded the impact on the social network. Depending on the restrictions used, either four or six peripheral clusters were identified using eigenvector-based modularity, two or three of which were mostly younger individuals seen 1-2 times and considered sample size artifacts. One of the remaining peripheral clusters was robust to change: at association strengths ≥0.3, all links between it and other clusters dissolved, warranting designation of a new cluster (C4). Links among established clusters were extensive but weak: almost half of all association links within the network disappeared when restricted to ≥0.3, leaving one individual linking C1-C2 and three individuals linking C2-C3. Links between C1 and C3 either disappeared entirely, with the two clusters linked instead through two remaining peripheral clusters, or were joined by a small number of individuals that clustered with either C1 or C3. These individuals may provide valuable lines of communication among clusters and help minimize inbreeding, although additional encounters may be required to elucidate their cluster affiliation.
Mahaffy, S., R. Baird, A. Gorgone, T. Cullins, D. McSweeney, D. Webster. 2017. Group dynamics of the endangered insular population of false killer whales in Hawai‘i. Abstract (Proceedings) 22nd Biennial on the Biology of Marine Mammals, Halifax, Nova Scotia, October 22-27, 2017.Download PDF
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