Abstract Sexual coercion, in the form of forced copulations, is relatively frequently observed in orangutans and generally attributed to their semi-solitary lifestyle. High ecological costs of association for females may be responsible for this lifestyle and may have prevented the evolution of morphological fertility indicators (e.g., sexual swellings), which would attract (male) associates. Therefore, sexual conflict may arise not only about mating per se but also about associations, because males may benefit from associations with females to monitor their reproductive state and attempt to monopolize their sexual activities. Here, we evaluate association patterns and costs for females when associating with both males and females of two different orangutan species at two study sites: Suaq, Sumatra (Pongo abelii), and Tuanan, Borneo (Pongo pygmaeus wurmbii). Female association frequency with both males and females was higher in the Sumatran population, living in more productive habitat. Accordingly, we found that the cost of association, in terms of reduced feeding to moving ratio and increased time being active, is higher in the less sociable Bornean population. Males generally initiated and maintained such costly associations with females, and prolonged associations with males led to increased female fecal cortisol metabolite (FCM) levels at Tuanan, the Bornean population. We conclude that male-maintained associations are an expression of sexual conflict in orangutans, at least at Tuanan. For females, this cost of association may be responsible for the lack of sexual signaling, while needing to confuse paternity. Significance statement Socioecological theory predicts a trade-off between the benefits of sociality and the ecological costs of increased feeding competition. Orangutans’ semi-solitary lifestyle has been attributed to the combination of high association costs and low predation risk. Previous work revealed a positive correlation between association frequencies and habitat productivity, but did not measure the costs of association. In this comparative study, we show that females likely incur costs from involuntary, male-maintained associations, especially when they last for several days and particularly in the population characterized by lower association frequencies. Association maintenance therefore qualifies as another expression of sexual conflict in orangutans, and especially prolonged, male-maintained associations may qualify as an indirect form of sexual coercion. Supplementary Information The online version contains supplementary material available at 10.1007/s00265-020-02948-4.

中文翻译：

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婆罗洲和苏门答腊雌性猩猩与雄性交往的成本：一种隐藏的性冲突形式？

摘要 以强迫交配形式的性胁迫在红毛猩猩中比较常见，通常归因于它们的半独居生活方式。女性交往的高生态成本可能是这种生活方式的原因，并且可能阻止了形态生育指标的演变（例如，性肿胀），这将吸引（男性）同伴。因此，性冲突可能不仅与交配本身有关，也可能与交往有关，因为雄性可能会从与雌性的交往中受益，以监测其生殖状态并试图垄断其性活动。在这里，我们评估了在两个研究地点与两种不同猩猩物种的雄性和雌性相关联时雌性的关联模式和成本：Suaq、Sumatra (Pongo abelii) 和 Tuanan，婆罗洲 (Pongo pygmaeus wurmbii)。在苏门答腊人口中，女性与男性和女性的关联频率较高，生活在生产力更高的栖息地。因此，我们发现，在社交较少的婆罗洲人群中，就减少进食与移动比率和增加活动时间而言，关联成本更高。男性通常会发起并维持与女性的这种代价高昂的联系，而与男性的长期联系导致婆罗洲人口团南的女性粪便皮质醇代谢物 (FCM) 水平升高。我们得出结论，雄性维持的联系是猩猩性冲突的一种表现，至少在团南是这样。对于女性来说，这种关联成本可能是缺乏性信号的原因，同时需要混淆亲子关系。意义陈述 社会生态学理论预测社会性的好处和增加的饲养竞争的生态成本之间的权衡。红毛猩猩的半孤独生活方式被归因于高关联成本和低捕食风险的结合。以前的工作揭示了关联频率和栖息地生产力之间的正相关关系，但没有衡量关联的成本。在这项比较研究中，我们表明女性可能会因非自愿的、男性维持的关联而付出代价，尤其是当它们持续数天时，尤其是在关联频率较低的人群中。因此，协会维持有资格作为猩猩性冲突的另一种表现形式，尤其是长期的，男性维持的联系可能被视为一种间接形式的性胁迫。补充信息 在线版本包含补充材料，网址为 10.1007/s00265-020-02948-4。