Does yawning increase arousal through mechanical stimulation of the carotid body?

More than a simple fixed action pattern: Yawning in drills

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, YCT and uncovered teeth, YUCT) 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 YCT or YUCT occurrence. We found that males performed longer and more YUCT 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.

How video calls affect mimicry and trust during interactions

Many social species, humans included, mimic emotional expressions, with important consequences for social bonding. Although humans increasingly interact via video calls, little is known about the effect of these online interactions on the mimicry of scratching and yawning, and their linkage with trust. The current study investigated whether mimicry and trust are affected by these new communication media. Using participant-confederate dyads (n = 27), we tested the mimicry of four behaviours across three different conditions: watching a pre-recorded video, online video call, and face-to-face. We measured mimicry of target behaviours frequently observed in emotional situations, yawn and scratch and control behaviours, lip-bite and face-touch. In addition, trust in the confederate was assessed via a trust game. Our study revealed that (i) mimicry and trust did not differ between face-to-face and video calls, but were significantly lower in the pre-recorded condition; and (ii) target behaviours were significantly more mimicked than the control behaviours. This negative relationship can possibly be explained by the negative connotation usually associated with the behaviours included in this study. Overall, this study showed that video calls might provide enough interaction cues for mimicry to occur in our student population and during interactions between strangers. This article is part of a discussion meeting issue 'Face2face: advancing the science of social interaction'.

Earlier than previously thought: Yawn contagion in preschool children

Yawning is a primitive and stereotyped motor action involving orofacial, laryngeal, pharyngeal, thoracic and abdominal muscles. Contagious yawning, an involuntarily action induced by viewing or listening to others' yawns, has been demonstrated in human and several non-human species. Previous studies with humans showed that infants and preschool children, socially separated during video experiments, were not infected by others' yawns. Here, we tested the occurrence of yawn contagion in 129 preschool children (ranging from 2.5 to 5.5 years) belonging to five different classes by video recording them in their classrooms during the ordinary school activities. As it occurs in adult humans, children of all ages were infected by others' yawns within the 2 min after the perception of the stimulus. The yawn contagion occurred earlier than previously thought. For children, it appears that the natural social setting is more conducive to yawn contagion than the inherently artificial experimental approach. Moreover, children's gender did not affect the level of contagious yawning. The neural, emotional and behavioural traits of preschool children are probably not sufficiently mature to express variability between boys and girls; nevertheless, children appeared to be already well equipped with the 'neural toolkit' necessary for expressing yawn contagion.

Yawning: a cue and a signal

Yawning behaviour has been associated with a variety of physiological and social events and a number of corresponding functions have been attributed to it. Non-directed (self-directed behaviour) and directed yawning (display behaviour) might nonetheless encompass all expressions of yawning, although it is difficult to differentiate one type from the other in a social context. Here we analysed more fully the data from a study in which four combinations of sensory cues were presented to pairs of either cage mate or stranger rats. The aim of the re-analysis was to demonstrate that non-directed and directed yawning might be identified by their distinctive functions. All pairs of rats used olfactory cues to recognise each other as stranger or cage mate companions, but only stranger rats used auditory cues to detect and respond to each other’s yawning. Increasing defecation rates (i.e. an index of emotional reactivity) inhibited yawning in cage mate rats such that yawning frequency reflected each rat’s physiological state. These results suggest that non-directed yawning functions as a cue in cage mate rats and directed yawning as a signal in stranger rats. We hypothesize that cue yawning might be a regulatory act that animals perform to adjust muscle tone for a coordinated change of state. Signal yawning might indicate the physiological capacity of rats in male-male conflicts.

Testing yawning hypotheses in wild populations of two strepsirrhine species: Propithecus verreauxi and Lemur catta

Yawning, although easily recognized, is difficult to explain. Traditional explanations stressed physiological mechanisms, but more recently, behavioral processes have received increasing attention. This is the first study to test a range of hypotheses on yawning in wild primate populations. We studied two sympatric strepsirrhine species, Lemur catta, and Propithecus verreauxi, of the Ankoba forest (24.99°S, 46.29°E, Berenty reserve) in southern Madagascar. Sexual dimorphism is lacking in both species. However, their differences in ecological and behavioral characteristics facilitate comparative tests of hypotheses on yawning. Our results show that within each species males and females yawned with similar frequencies supporting the Dimorphism Hypothesis, which predicts that low sexual dimorphism leads to little inter-sexual differences in yawning. In support of the State Changing Hypothesis yawning frequencies was linked to the sleep-wake cycle and punctuated transitions from one behavior to another. Accordingly, yawning frequencies were significantly higher in L. catta than in P. verreauxi, because L. catta has a higher basal level of activity and consequently a higher number of behavioral transitions. In agreement with the Anxiety Hypothesis, yawning increased significantly in the 10 min following predatory attacks or aggression. Our findings provide the first empirical evidence of a direct connection between anxiety and yawning in lemurs. Our results show that yawning in these two strepsirrhines occurs in different contexts, but more research will be necessary to determine if yawns are a single, unitary behavior.

Yawning and its physiological significance

Although yawning is a commonly witnessed human behavior, yet it has not been taught in much detail in medical schools because, until the date, no particular physiological significance has been associated with it. It is characterized by opening up of mouth which is accompanied by a long inspiration, with a brief interruption of ventilation and followed by a short expiration. Since time immemorial, yawning has been associated with drowsiness and boredom. However, this age old belief is all set to change as the results of some newer studies have pointed out that yawning might be a way by which our body is trying to accomplish some more meaningful goals. In this review, we have tried to put together some of the important functions that have been proposed by a few authors, with the hope that this article will stimulate the interest of newer researchers in this hitherto unexplored field.

In bonobos yawn contagion is higher among kin and friends

In humans, the distribution of yawn contagion is shaped by social closeness with strongly bonded pairs showing higher levels of contagion than weakly bonded pairs. This ethological finding led the authors to hypothesize that the phenomenon of yawn contagion may be the result of certain empathic abilities, although in their most basal form. Here, for the first time, we show the capacity of bonobos (Pan paniscus) to respond to yawns of conspecifics. Bonobos spontaneously yawned more frequently during resting/relaxing compared to social tension periods. The results show that yawn contagion was context independent suggesting that the probability of yawning after observing others' yawns is not affected by the propensity to engage in spontaneous yawns. As it occurs in humans, in bonobos the yawing response mostly occurred within the first minute after the perception of the stimulus. Finally, via a Linear Mixed Model we tested the effect of different variables (e.g., sex, rank, relationship quality) on yawn contagion, which increased when subjects were strongly bonded and when the triggering subject was a female. The importance of social bonding in shaping yawn contagion in bonobos, as it occurs in humans, is consistent with the hypothesis that empathy may play a role in the modulation of this phenomenon in both species. The higher frequency of yawn contagion in presence of a female as a triggering subject supports the hypothesis that adult females not only represent the relational and decisional nucleus of the bonobo society, but also that they play a key role in affecting the emotional states of others.

Changes in Physiology before, during, and after Yawning

The ultimate function of yawning continues to be debated. Here, we examine physiological measurements taken before, during, and after yawns in humans, in an attempt to identify key proximate mechanisms associated with this behavior. In two separate studies we measured changes in heart rate, lung volume, eye closure, skin conductance, ear pulse, respiratory sinus arrhythmia, and respiratory rate. Data were depicted from 75 s before and after yawns, and analyzed at baseline, during, and immediately following yawns. Increases in heart rate, lung volume, and eye muscle tension were observed during or immediately following yawning. Patterns of physiological changes during yawning were then compared to data from non-yawning deep inhalations. In one study, respiration period increased following the execution of a yawn. Much of the variance in physiology surrounding yawning was specific to the yawning event. This was not the case for deep inhalation. We consider our findings in light of various hypotheses about the function of yawning and conclude that they are most consistent with the brain cooling hypothesis.

CHANGES IN AMBIENT TEMPERATURE TRIGGER YAWNING BUT NOT STRETCHING IN RATS

Yawning appears to be involved in arousal, state change, and activity across vertebrates. Recent research suggests that yawning may support effective changes in mental state or vigilance through cerebral cooling. To further investigate the relationship between yawning, state change, and thermoregulation, 12 Sprague-Dawley rats (Rattus norvegicus) were exposed to a total of two hours of ambient temperature manipulation over a period of 48 hours. Using a repeated measures design, each rat experienced a range of increasing (22→32°C), decreasing (32→22°C), and constant temperatures (22°C; 32°C). Yawning and locomotor activity occurred most frequently during initial changes in temperature, irrespective of direction, compared to more extended periods of temperature manipulation. The rate of yawning also diminished during constant high temperatures (32°C) compared to low temperatures (22°C). Unlike yawning, however, stretching was unaffected by ambient temperature variation. These findings are compared to recent work on budgerigars (Melopsittacus undulatus), and the ecological selective pressures for yawning in challenging thermal environments are discussed. The results support previous comparative research connecting yawning with arousal and state change, and contribute to refining the predictions of the thermoregulatory hypothesis across vertebrates.

Yawning and stretching predict brain temperature changes in rats: support for the thermoregulatory hypothesis

Recent research suggests that yawning is an adaptive behavior that functions to promote brain thermoregulation among homeotherms. To explore the relationship between brain temperature and yawning we implanted thermocoupled probes in the frontal cortex of rats to measure brain temperature before, during and after yawning. Temperature recordings indicate that yawns and stretches occurred during increases in brain temperature, with brain temperatures being restored to baseline following the execution of each of these behaviors. The circulatory changes that accompany yawning and stretching may explain some of the thermal similarities surrounding these events. These results suggest that yawning and stretching may serve to maintain brain thermal homeostasis.

The dawn of the yawn: is yawning a warning? Linking neurological disorders

Yawning continues to pose as a scientist's conundrum. Evidence is presented of yawning and contagious yawning in a number of different neurological disorders. Explanations are discussed in the context of disparate neurological disorders together with proposals for how theses findings may be linked. Thus, greater understanding of yawning and of neurological disorders may be achieved by exploring common neuro-chemical pathways and the involvement of neurotransmitters that are implicated in these different disorders. Finally, contagious yawning is discussed in the context of the susceptibility of persons and the similarity this presents with our understanding of the mechanisms involved in hypnosis.