Data from: Sexual segregation in Indo-Pacific bottlenose dolphins is driven by female avoidance of males

Sexual segregation is widespread in mammals, although the proximate causes are poorly understood in monomorphic species. Indo-Pacific bottlenose dolphins (Tursiops aduncus), which exhibit a high degree of fission-fusion dynamics, offer a useful lens to examine the ecological and social drivers of sexual segregation. While ecological hypotheses suggest that sexual segregation is a by-product of sex-specific ecological preferences (e.g. related to habitat, foraging, or predator avoidance), the social hypothesis proffers that segregation results from same-sex preferences (e.g. due to cooperative benefits) and/or opposite-sex avoidance (e.g. due to competitive or exploitative interactions). Indo-Pacific bottlenose dolphin females range from nearly solitary to highly sociable. Males associate in alliances that cooperate to sequester individual females and exclude competing males. Given evidence for allied sexual coercion, our primary hypothesis was that sexual segregation is driven by female avoidance of aggressive males. However, given robust evidence for sex-biased foraging tactics, ecological factors likely also contribute. Using the Sexual Segregation and Aggregation Statistic with 17,468 sighting records spanning 31 years, we found strong sexual segregation. Unique to our work, we analyzed the direction of joins and leaves between males and females from focal observations (N=10,715 fission-fusion events, 87 females, 111 males) to determine which sex drives sexual segregation. Females drove segregation by rarely joining and often leaving males. Although ecological factors likely reinforce sexual segregation, social factors predominate. This study demonstrates a sex-bias in fission-fusion dynamics in a socially complex wild mammal population and offers strong empirical support to the social hypothesis of sexual segregation.

性分离现象在哺乳动物中广泛存在，但在雌雄外形单型的物种中，其直接成因（近因）仍未得到充分阐明。具有高度分裂-融合（fission-fusion）动态的印太瓶鼻海豚（Indo-Pacific bottlenose dolphins, Tursiops aduncus），为探究性分离的生态与社会驱动因素提供了极佳的研究视角。生态假说提出，性分离是性别特异性生态偏好的副产物（例如与栖息地选择、觅食行为或反捕食策略相关的偏好差异）；而社会假说则认为，性分离源于同性社交偏好（例如基于合作收益的同性聚集倾向）和/或异性回避行为（例如源于竞争或剥削性互动的异性排斥）。
印太瓶鼻海豚的雌性个体的社交模式从近乎独居到高度社会化跨度极大。雄性则结成联盟，通过合作控制单个雌性并排斥其他竞争雄性。鉴于已有雄性联合性胁迫的相关研究证据，我们的核心假设为：性分离是由雌性回避具有攻击性的雄性群体所驱动的。不过，鉴于存在大量关于性别偏向性觅食策略的可靠实证证据，生态因素同样可能对性分离起到推动作用。
我们采用覆盖31年的17468条目击记录，结合性分离与聚集统计量（Sexual Segregation and Aggregation Statistic）开展分析，结果证实了显著的性分离现象。本研究的独特之处在于，通过焦点观察数据（共10715次分裂-融合事件，涉及87头雌性个体、111头雄性个体），分析雌雄个体间的群体加入与脱离方向，以明确哪一性别主导了性分离过程。结果显示，雌性通过极少加入雄性群体、且频繁脱离雄性群体的行为，主导了性分离的形成。
尽管生态因素可能会强化性分离现象，但社会因素才是其主要驱动因子。本研究证实了具有复杂社会结构的野生哺乳动物种群中，分裂-融合动态存在性别偏向性，并为性分离的社会假说提供了强有力的实证支持。