Data from: A multilevel society of herring-eating killer whales indicates adaptation to prey characteristics

Non-social factors can influence animal social structure. In killer whales (Orcinus orca), fish- vs. mammal-eating ecological differences are regarded as key ecological drivers of their multilevel society, including group size, but the potential importance of specific target prey remains unclear. Here, we investigate the social structure of herring-eating killer whales in Iceland and compare it to the described social structures of primarily salmon- and seal-eating populations in the Northeast Pacific, which form stable coherent basic units nested within a hierarchical multilevel society. Using 29023 photographs collected over 6 years, we examined the association patterns of 198 individuals combining clustering, social network structure and temporal patterns of association analysis. The Icelandic population had largely weak but non-random associations, which were not completely assorted by known ranging patterns. A fission-fusion dynamic of constant and temporary associations was observed but this was not due to permanent units joining. The population-level society was significantly structured but not in a clear hierarchical tier system. Social clusters were highly diverse in complexity and there were indications of subsclusters. There was no indication of dispersal nor strong sex differences in associations. These results indicate that the Icelandic herring-eating killer whale population has a multilevel social structure without clear hierarchical tiers or nested coherent social units, different from other populations of killer whales. We suggest that local ecological context, such as the characteristics of the specific target prey (e.g. predictability, biomass and density) and subsequent foraging strategies may strongly influence killer whale social association patterns.

非社会因素可对动物的社会结构产生影响。在虎鲸（Orcinus orca）中，食鱼与食哺乳动物的生态分化被认为是其多层级社会（multilevel society）的关键生态驱动因子，包括群体规模在内，但特定目标猎物的潜在重要性仍不明确。本研究以冰岛海域的食鲱虎鲸为研究对象，探究其社会结构，并将其与东北太平洋主要以鲑鱼和海豹为食的虎鲸种群的已报道社会结构进行对比——后者形成嵌套于层级化多层级社会中的稳定连贯基本单元。本研究依托6年间采集的29023张照片，结合聚类分析、社会网络结构（social network structure）分析与关联模式时序分析，对198头虎鲸个体的关联模式展开了考察。冰岛虎鲸种群的个体关联整体较弱，但并非随机发生，且未完全依照已知的活动范围模式进行分组。研究观察到存在恒定关联与临时关联的裂变-融合动态（fission-fusion dynamic），但该动态并非由永久性社会单元的聚合所引发。该种群的社会层面存在显著结构，但未形成清晰的层级体系。社会集群的复杂度差异显著，且存在亚集群的相关迹象。未发现个体扩散的迹象，关联模式也未呈现显著的性别差异。上述结果表明，冰岛食鲱虎鲸种群拥有多层级社会结构，但不存在清晰的层级划分或嵌套式连贯社会单元，这与其他虎鲸种群存在显著差异。本研究认为，当地生态背景——如特定目标猎物的特性（例如可预测性、生物量与种群密度）以及由此衍生的觅食策略——或可对虎鲸的社会关联模式产生强烈影响。