Data from: Cape buffalo (Syncerus caffer caffer) social dynamics in a flood-pulsed environment

Fission-fusion social dynamics allow animals to respond to short-term environmental changes by temporarily adjusting group size. The drivers of such complex social dynamics are thought to relate to resource availability, density effects and social interactions. During 2008-09, we collared 15 Cape buffalo (Syncerus caffer caffer) cows in different groups in the Okavango Delta, Botswana, to study social dynamics in a flood-pulsed ecosystem. Hierarchical clustering identified 2 subpopulations, 1 migratory and 1 resident. We calculated Utilization Distribution Overlap Index (UDOI) and number and duration of fusion events between buffalo dyads and related them to environmental variables. Number of fusion events and duration of fusion periods did not vary seasonally, total fusion time varied seasonally and annually, and UDOI varied with year and subpopulation. Fission events were more likely in cluttered habitats, but only in the late flood season. There was more open habitat in home range overlap (HRO) areas than home ranges (HR) in most seasons. Pan density in rainy season HROs was lower and higher than in HRs in 2008 and 2009, respectively, and in all flood seasons HRO areas were closer to permanent water than HRs, suggesting that fusion occurred when buffalo congregated on resources. Whereas previous studies described large herds that sometimes split, we identified numerous smaller groups that occasionally fused, indicating a very fluid social system. Our results highlight the need to understand social system flexibility to ensure appropriate management and understand the varying impacts of environmental and anthropogenic effects on subpopulations within the same geographic area.

裂变-融合社会动态（Fission-fusion social dynamics）允许动物通过临时调整群体规模来响应短期环境变化。这类复杂社会动态的驱动因素被认为与资源可获得性、密度效应以及社会互动相关。2008至2009年间，我们在博茨瓦纳奥卡万戈三角洲的不同种群中，为15头开普水牛（Cape buffalo, Syncerus caffer caffer）母牛佩戴了项圈，以研究洪水脉冲生态系统中的社会动态。层级聚类（Hierarchical clustering）识别出2个亚种群，分别为迁徙型亚种群与定居型亚种群。我们计算了成对水牛间的利用分布重叠指数（Utilization Distribution Overlap Index, UDOI）以及融合事件的数量与持续时长，并将其与环境变量进行关联分析。融合事件的数量与融合时段的持续时长均无季节性变化，但总融合时长存在季节性与年度差异，且UDOI随年份与亚种群的不同而产生变化。分裂事件更易发生在密集生境中，但仅在洪水后期出现。多数季节中，家域重叠区（Home Range Overlap, HRO）的开阔生境占比高于家域（Home Range, HR）。雨季家域重叠区的浅塘密度在2008年低于家域，2009年则高于家域；且所有洪水季节中，家域重叠区均比家域更靠近永久性水源，这表明水牛会在聚集于资源点时发生融合行为。既往研究多描述为有时会分裂的大种群，而本研究则识别出众多偶尔会发生融合的小型群体，这表明该种群拥有极为灵活的社会系统。本研究结果强调，理解社会系统的灵活性对于制定合理的管理策略、厘清同一地理区域内亚种群所受环境与人为活动影响的差异化效应至关重要。