Stone Handling (SH) is a solitary object play behaviour that can vary from simple exploratory actions to more complex manipulations. So far, among primates, this behaviour has been exclusively reported in macaques. We collected data on 62 geladas (Theropithecus gelada) housed at the NaturZoo (Rheine, Germany). We found that about 70% of subjects belonging to all age- and sex-classes engaged in SH. Due to their exceptional manipulative skills (the highest opposability index among nonhuman primates) and propensity to play, geladas are a good model species to test hypotheses on the function of this form of object play. While the frequency of SH tended to decrease with age of the player, the duration of each session and its complexity tended to increase in juveniles/subadults and adults compared to infants. This age-related variation in terms of frequency, duration and complexity suggests that, in agreement with the motor training hypothesis, SH could have a role in the neural-motor development of immature subjects and a basic function in stimulating neurogenesis and maintaining the psychological well-being of adults. In all age classes, the frequency of SH did not vary across pre-feeding, feeding and non-feeding conditions. Hence, our data do not support the misdirected foraging hypothesis, which predicts that animals engage in SH to anticipate food provisioning. In conclusion, our study reveals, for the first time, the presence of SH outside the genus Macaca and attempts to delineate possible functions of the behaviour in geladas. Since the hypotheses tested cannot be mutually exclusive, long-term studies of SH across individuals' lifetimes in both captive and wild groups of geladas are needed to clarify the proximate and ultimate functions of the behaviour in this species of Papionini. Finally, long-term studies could also provide some important indications about the cultural nature and social transmission of SH in a taxonomic group outside the Macaca genus.

中文翻译：

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在geladas中处理石头的第一个证据：从简单到更复杂的物体游戏形式

石头处理 (SH) 是一种单独的物体游戏行为，可以从简单的探索性动作到更复杂的操作。到目前为止，在灵长类动物中，这种行为只在猕猴中报道过。我们收集了饲养在 NaturZoo（德国莱茵河）的 62 只鸬鹚（Theropithecus gelada）的数据。我们发现大约 70% 的所有年龄和性别类别的受试者都参与了 SH。由于它们非凡的操纵技能（非人类灵长类动物中最高的对抗指数）和玩耍的倾向，geladas 是一个很好的模型物种，可以测试这种物体游戏形式的功能的假设。虽然 SH 的频率随着玩家年龄的增长而降低，但与婴儿相比，青少年/亚成人和成人每次会话的持续时间及其复杂性往往会增加。这种在频率、持续时间和复杂性方面与年龄相关的变化表明，与运动训练假设一致，SH 可能在未成熟受试者的神经运动发育中发挥作用，并在刺激神经发生和维持心理健康方面发挥基本功能- 成年人的存在。在所有年龄段中，SH 的频率在喂食前、喂食和非喂食条件下没有变化。因此，我们的数据不支持误导觅食假说，该假说预测动物参与 SH 以预期食物供应。总之，我们的研究首次揭示了猕猴属之外 SH 的存在，并试图描述鸬鹚行为的可能功能。由于测试的假设不能相互排斥，因此对个体的 SH 的长期研究 需要在圈养的和野生的鸬鹚群体中的寿命来阐明这种 Papionini 的行为的近端和最终功能。最后，长期研究还可以为猕猴属以外的分类群中 SH 的文化性质和社会传播提供一些重要指示。