Data from: Fission–fusion processes weaken dominance networks of female Asian elephants in a productive habitat

Dominance hierarchies are expected to form in response to socioecological pressures and competitive regimes. We assess dominance relationships among free-ranging female Asian elephants (Elephas maximus) and compare them with those of African savannah elephants (Loxodonta africana), which are known to exhibit age-based dominance hierarchies. Both species are generalist herbivores, however, the Asian population occupies a more productive and climatically stable environment relative to that of the African savannah population. We expected this would lower competition relative to the African taxon, relaxing the need for hierarchy. We tested whether 1) observed dominance interactions among individuals were transitive, 2) outcomes were structured either by age or by social unit according to 4 independent ranking methods, and 3) hierarchy steepness among classes was significant using David's score. Elephas maximus displayed less than a third the number of dominance interactions as observed in L. africana, with statistically insignificant transitivity among individuals. There was weak but significant order as well as steepness among age-classes but no clear order among social units. Loxodonta africana showed significant transitivity among individuals, with significant order and steepness among age-classes and social units. Elephas maximus had a greater proportion of age-reversed dominance outcomes than L. africana. When dominance hierarchies are weak and nonlinear, signals of dominance may have other functions, such as maintaining social exclusivity. We propose that resource dynamics reinforce differences via influence on fission–fusion processes, which we term "ecological release." We discuss implications of these findings for conservation and management when animals are spatially constrained.

优势等级（dominance hierarchies）的形成被认为是对社会生态压力与竞争体制的适应性响应。本研究针对自由活动的雌性亚洲象（Elephas maximus）的优势关系展开评估，并与已知存在年龄依赖型优势等级的非洲草原象（Loxodonta africana）进行对比。二者均为广食性植食动物，但亚洲象种群的栖息生境相较于非洲草原象种群更为高产且气候稳定。据此我们推测，亚洲象的种内竞争程度将低于非洲象类群，进而削弱了形成等级体系的选择压力。本研究通过三项预设开展验证：其一，个体间观测到的优势互动是否具备传递性；其二，基于4种独立的排序方法，优势地位的决出是否由年龄或社会单元主导；其三，利用戴维得分（David's score）检验不同类群间的等级陡峭度是否具有统计学显著性。观测数据显示，亚洲象的优势互动数量不足非洲草原象的三分之一，且个体间的传递性未达到统计学显著性水平。亚洲象的年龄类群间仅存在微弱但显著的等级秩序与陡峭度，而社会单元间并未形成明确的等级结构。与之相对，非洲草原象的个体间互动呈现显著传递性，其年龄类群与社会单元间均存在显著的等级秩序与陡峭度。亚洲象中年龄反转的优势地位结果占比显著高于非洲草原象。当优势等级体系微弱且呈非线性特征时，优势地位信号可能承担其他功能，例如维持社会排他性。我们提出，资源动态通过影响裂合-融合动态（fission–fusion processes）——我们将其命名为"生态释放（ecological release）"——来强化两个类群间的差异。最后，本文讨论了本研究结果在动物空间受限场景下，对物种保护与管理工作的启示。