In the last decade, increasing attention has been devoted to exploring some aspects of yawning in non-human animals. With their chin red mark, bony paranasal swellings, male large brains and long canines, drills (Mandrillus leucophaeus) offer a robust model for testing hypotheses on the phenomenon. We identified two yawn variants (covered, Y_CT and uncovered teeth, Y_UCT) which differ in terms of recruitment of muscular action units (AUs). We tested the effects of several variables (sex, dominance rank, context) on the duration of the yawn and the probability of Y_CT or Y_UCT occurrence. We found that males performed longer and more Y_UCT than females. These findings support the Brain Cooling Hypothesis suggesting that those species showing large brains tend to display larger and longer yawns. We also tested the State Changing Hypothesis predicting the presence of a temporal association of yawning and ongoing behavioral transitions. A sequential analysis revealed that after 30 s following a yawn, drills were significantly more likely to change their behavioral state. Through the observation of yawning, conspecifics might gain knowledge of impending state changes. Seeing other’s yawns increased the probability of a similar response in the observers, thus suggesting the presence of yawn contagion in drills. Although the dataset needs to be expanded, our findings indicate that yawning is variable in drills, it can be associated with subjects’ state changes, and the imminent shifts can be perceived/processed by conspecifics.

Graphical Abstract

中文翻译：

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不仅仅是简单的固定动作模式：练习中打哈欠

在过去的十年中，人们越来越关注探索非人类动物打哈欠的某些方面。钻头（ Mandrillus leucophaeus ）下巴呈红色，鼻旁骨肿胀，雄性大脑较大，犬齿较长，为检验这一现象的假设提供了一个强有力的模型。我们发现了两种哈欠变体（覆盖的，Y _CT和未覆盖的牙齿，Y _UCT），它们在肌肉动作单位（AU）的募集方面有所不同。我们测试了几个变量（性别、优势等级、背景）对打哈欠持续时间以及 Y _CT或 Y _UCT发生概率的影响。我们发现男性比女性执行的 Y _UCT更长、更多。这些发现支持大脑冷却假说，即那些大脑较大的物种往往会打更大、更长的哈欠。我们还测试了状态改变假设，预测打哈欠和持续行为转变之间存在时间关联。一项序贯分析显示，打哈欠 30 秒后，训练更有可能改变他们的行为状态。通过观察打哈欠，同种人可能会了解即将发生的状态变化。看到别人打哈欠会增加观察者做出类似反应的可能性，从而表明训练中存在哈欠传染现象。尽管数据集需要扩展，但我们的研究结果表明，打哈欠在训练中是可变的，它可以与受试者的状态变化相关，并且同种人可以感知/处理即将发生的变化。

图形概要