Abstract
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.
Highlights
Spontaneous yawning is a widespread behaviour in vertebrates
We found a single best model which included only the variable “context” and explained 99% of the distribution (AICc = −56.70; F = 60.74, df1 = 3, df2 = 48, p = 0.0001) (Fig. 2)
The Linear Mixed Model (LMM) revealed that the variable ‘context’ strongly affected the distribution of yawning frequency but did not have a strong effect on its mean duration
Summary
Spontaneous yawning is a widespread behaviour in vertebrates. data on marine mammals are scarce. These hypotheses can be classified into two groups, the physiological and social hypotheses In the former group, yawning is posited to act as a homeostatic restoring mechanism, and, with the intensification of the studies, some of these hypotheses such as brain cooling, anxiety and drowsiness hypothesis have found an increasing support[3,4,8,9,10,11,12,13,14]. Goodall[24] observed that chimpanzees yawned more frequently in the presence of human observers and, in captivity, the presence of humans in front of animal facilities produced an increase of yawning in monkeys (lion-tailed macaques, Macaca silenus[23]) For this reason, along with self-directed behaviours such as self-grooming and self-scratching, yawning can be considered an indicator of anxiety[25,26]. Recent findings suggest a possible effect of yawning as a stress-releaser, which helps restoring physiological/emotional homeostasis[4,28,29]
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