Unit of Ethology, Department of Biology, University of Pisa, Via A. Volta 6, 560126, Pisa, Italy.
Natural History Museum, University of Pisa, Via Roma 79, 56011, Calci, Pisa, Italy.
Sci Rep. 2019 Nov 21;9(1):17226. doi: 10.1038/s41598-019-53613-4.
Spontaneous yawning is a widespread behaviour in vertebrates. However, data on marine mammals are scarce. In this study, we tested some hypotheses on the functions of yawning in a captive group of South American sea lions (Otaria flavescens). According to the Dimorphism Hypothesis, species showing low levels of sexual dimorphism in canine size do not show sex differences in yawning distribution; this was supported by our findings, since yawning did not differ between the sexes. Yawning was more frequently performed during resting/sleeping contexts, thus supporting the Drowsiness Hypothesis. Yawning and self-scratching are considered reliable indicators of short-term anxiety in sea lions, since they immediately increased after conflicts both in aggressors and victims (Social Distress Hypothesis supported). In the long-term, yawning was not correlated with individuals' dominance status, thus showing that anxiety is similarly experienced by dominants and subordinates. The last two findings can be explained by the social competition of this species, that involves individuals independently from their sex, age or ranking status. Therefore, the exposure to frequent stressful events can induce similar levels of anxiety in all the subjects (Resource Inequity Hypothesis supported). In conclusion, spontaneous yawning in sea lions seems to share similar functions with other social mammals, suggesting that this behaviour is a possible plesiomorphic trait.
自发性打哈欠是脊椎动物中广泛存在的行为。然而，关于海洋哺乳动物的相关数据却很少。在这项研究中，我们针对在一个南美的海狮（Otaria flavescens）圈养群体中打哈欠的一些功能提出了一些假设。根据二态性假说，如果犬齿大小的性二态性水平较低的物种，在打哈欠的分布上没有性别差异；这一假说得到了支持，因为在性别之间打哈欠没有差异。打哈欠更频繁地发生在休息/睡眠环境中，因此支持瞌睡假说。打哈欠和自我抓挠被认为是海狮短期焦虑的可靠指标，因为在冲突中无论是攻击者还是受害者，打哈欠和自我抓挠都会立即增加（支持社会压力假说）。从长远来看，打哈欠与个体的支配地位没有相关性，因此表明优势个体和从属个体都会经历类似的焦虑。最后两个发现可以用该物种的社会竞争来解释，因为社会竞争涉及到独立于其性别、年龄或排名的个体。因此，频繁的压力事件可能会导致所有个体产生类似程度的焦虑（支持资源不均假说）。总之，海狮的自发性打哈欠似乎与其他社会哺乳动物具有相似的功能，这表明这种行为可能是一种古老的特征。
