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mise à jour du
16 mai 2010
Neurosci Biobehav Rev
Why do we Yawn ?
Adrian G. Guggisberg, Johannes Mathis,
Armin Schnider, Christian W. Hess

University of Geneva Department of Clinical Neurosciences Switzerland


Andrew C. Gallup. Yawning and the thermoregulatory hypothesis 
-Guggisberg AG, Mathis J, Herrmann US, Hess CW. The functional relationship between yawning and vigilance. Behav Brain Res 2007;179(1):159-166
-Guggisberg AG, Mathis J, Schnider A, Hess CW. Why do we yawn ? Neurosci Biobehav Rev 2010;34:1267-1276
-Guggisberg AG, Mathis J, Schnider A, Hess CW Why do we yawn? The importance of evidence for specific yawn-induced effects. Neurosci Biobehav Rev. 2011;35(5):1302-4.
-Guggisberg AG, Mathis J, Hess CW. Interplay between yawning and vigilance: a review of the experimental evidence. Front Neurol Neurosci. 2010;28:47-54.
-Guggisberg A, Hess Ch. Clinical significance of yawning in disorders of consciousness and vigilance. Epileptologie. 2014;31:82-86.

Yawning is a phylogenetically old behaviour that can be observed in most vertebrate species from foetal stages to old age. The origin and function of this conspicuous phenomenon have been subject to speculations for centuries. Here, we review the experimental evidence for each of these hypotheses. It is found that theories ascribing a physiological role to yawning (such as the respiratory, arousal, or thermoregulation hypotheses) lack evidence. Conversely, the notion that yawning has a communicative function involved in the transmission of drowsiness, boredom, or mild psychological stress receives increasing support from research in different fields. In humans and some other mammals, yawning is part of the action repertoire of advanced empathic and social skills.
1 Introduction Yawning can be observed in most vertebrate species from foetal stages to old age. In mammals, it consists of an involuntary sequence of mouth opening, deep inspiration, brief apnea, and slow expiration (Walusinski and Deputte, 2004). It can be accompanied by other facultative motor acts such as stretching (Provine et al., 1987a). In humans, yawns last on average about 6 seconds, and the individual yawn duration and frequency remains remarkably stable over weeks (Provine, 1986). In birds and fish species, a mouth gaping similar to yawning can be observed, and yawning as opposed to other forms of mouth openings has been defined as a slow opening of the mouth, maintenance of the open position for more than 3 s, followed by a more rapid closure of the mouth (Baenninger, 1987). The homology of yawning between different species is controversial, but at least similar movement sequences and similar conditions of occurrence can be observed (Baenninger, 1987; Deputte, 1994). Since yawning seems to be a phylogenically old and frequent phenomenon, one would expect that it provides some evolutionary advantage, i.e., that is has a certain useful function.
Indeed, numerous hypotheses on the function of yawing have been posited throughout the centuries. They were usually derived from behavioural observations of yawns. In mammals, it has been observed that more than 90% of yawns occur at rest whereas the remaining yawns seem to be triggered by social or emotional stimuli. These contextual differences have motivated a classification of yawning into "physiological" and "social" yawns, although the phenomenology of yawns does not depend on the context (Deputte, 1994; Walusinski and Deputte, 2004). In accordance with the distinction of physiological and social yawn contexts, the hypotheses on the function of yawning have emphasised either a physiological or a social role of yawning.
In contrast to the abundance of theoretical considerations, experimental data is relatively scarce. Yet, in the last few decades, an increasing number of studies have shed some light on its conditions and effects. Although the available data is still far from providing a complete or generally accepted account of the mechanisms and consequences of yawning, it does allow confronting some of the theoretical models with empirical observations. In this review, we will try to classify existing hypotheses according to their current experimental evidence. All hypotheses postulating a physiological role of yawning share the common assumption that yawning regulates a particular body function, e.g., the blood oxygen level or the brain arousal level. Thus, the mechanisms of yawning are characterised as a homeostatic system with negative feedback regulation. Accordingly, physiological models necessarily make at least two different predictions that can be empirically tested: i) yawning is triggered by up- or downturns of a given body state and, ii) yawning acts on the corresponding body function. We will therefore review the evidence of each physiological hypothesis based on its predictions with regards to triggers and effects of yawning. In the case of social models of yawning, the postulated regulating function of yawning would not concern body functions of individuals but rather the communication within social groups. The predictions of this model as well as the corresponding evidence will also be reviewed. This article will focus on normal yawning; a recent review on pathological yawns can be found elsewhere (Walusinski, 2009).
2 Anatomy and Pharmacology ....
3 Physiological Hypotheses
3.1 Respiratory and Circulatory Hypotheses For several centuries, at least since Hippocrates in the 4th century BC, scholars have thought that yawning might remove "bad air" from the lungs and increase oxygen circulation in the brain (Trautmann, 1901; Schiller, 2002; Matikainen and Elo, 2008).
3.1.1 Oxygen Need and Hypercapnia Do Not Induce Yawning This hypothesis predicts that yawning is triggered when blood or brain oxygenation is insufficient, i.e., when oxygen (O2) levels drop and the CO2 concentration rises. However, from self-observation most people will confirm that they do not yawn more frequently when they do exercise and need more oxygen than when they are at rest (Provine et al., 1987b). In accordance with this notion, experiments by Provine et al. (Provine et al., 1987b) demonstrated that healthy subjects who are exposed to gas mixtures with high levels of CO2 or physical exercise, do not yawn more frequently. .........
Taken together, the occurrence of yawning during periods with too much blood oxygenation but not during periods with oxygen need is exactly the opposite of what would have been predicted by the respiration hypothesis and thus casts severe doubts on its correctness.
3.1.2 Yawning Does Probably Not Increase Brain Oxygenation There are, to our knowledge, no studies that measured the change in blood oxygenation induced by yawning. However, yawning would be a much less effective way of increasing oxygen intake than rapid breathing, especially since the deep inspiration during yawning is followed by a period of relative apnoea (Baenninger, 1997). ........
3.1.3 Conclusions The predictions of the respiratory hypothesis are not supported by current experimental data. Additional research is needed to test the effects of hypoxia on the yawning rate under more controlled conditions. Studies investigating the effects of yawning on blood and brain ygenation are also missing. Given current evidence, it seems unlikely that yawning has respiratory or circulatory functions.
3.2 The Arousal Hypothesis The idea that yawning might play an important role in regulating physiological brain processes has remained in the literature also after the appearance of evidence against the respiratory hypotheses. A widely expressed proposition now speculated that yawning might be responsible for the homeostatic regulation of vigilance and brain arousal level (Baenninger, 1997; Giganti et al., 2002; Walusinski and Deputte, 2004; Matikainen and Elo, 2008; Vick and Paukner, 2010).
3.2.1 Drowsiness Induces Yawning Yawning occurs preferentially during periods of drowsiness, as it is predicted by the arousal hypothesis. Behavioural studies consistently reported that yawns occur most frequently before and after sleep, i.e., during periods with lower levels of alertness (Greco et al., 1993; Provine et al., 1987a). The circadian distribution of yawns precisely reflects the individual sleep-wake rhythm (Giganti et al., 2007; Zilli et al., 2007; Zilli et al., 2008).
Thus, sleep pressure and drowsiness proved significantly greater when subjects yawned than when they moved only.
3.2.2 Yawning Does Not Produce an Arousal Arousals are defined as a global activation of brain activity that progresses from brain stem structures to centres of the autonomic nervous system and to distributed cortical areas (Moruzzi and Magoun, 1949; Sforza et al., 2000). They are accompanied by a typical acceleration of EEG activity. Several studies have therefore analyzed spectral EEG changes after yawns in humans to test the hypothesis that yawning has an arousing effect. However, the results were negative. Two studies looking at 30 s samples of EEG before and after yawns were unable to find significant and lasting changes in EEG activity related to yawns (Laing and Ogilvie, 1988; Regehr et al., 1992). One of these studies reported transient increases in theta, spindle, and beta activity, but they only reached significance when the analysis was a priori limited to data segments between 10 and 20 s before and after yawning (Regehr et al., 1992). Furthermore, EEG power after yawning was not significantly different from EEG power after postural adjustments without yawning (Laing and Ogilvie, 1988). In our analyses of EEG power spectra from patients undergoing MWTs, we observed that the increase in delta power over the vertex that was found before yawning (as compared to delta activity before postural adjustments without yawning, see 3.2.1 above) persisted to the same amount also after yawning (Fig. 1, right panel). Thus, yawning did not reverse the increased sleep pressure and drowsiness that seemed to have triggered it. Besides delta power, alpha oscillations (~ 7.5 - 12.5 Hz) also reflect the individual vigilance level. They become faster and smaller in amplitude when the arousal level increases. Figure 2A gives an example of the EEG power spectrum 30 min after oral ingestion of 250 mg caffeine (Barry et al., 2005). In contrast, drowsiness is associated with a slowing of alpha oscillations, and with a shift of alpha oscillations from mainly occipital towards central brain regions (Tanaka et al., 1997; De Gennaro et al., 2001b; De Gennaro et al., 2001a). Figure 2B shows that alpha power after yawning showed a pattern that is typical for sleepiness: alpha rhythms decelerated, and shifted towards central brain regions after yawning, as compared to the data segments before yawning. Conversely, we did observe EEG markers of increased arousal levels after simple postural adjustments, as shown in Figure 2C: alpha rhythms became faster and smaller after body movements. Hence, if yawning had an arousing effect &endash; even if it were as small as the effect of simple postural adjustments &endash; we would have detected it with our EEG analyses. Instead, we observed signs of progressive drowsiness after yawning.
3.2.3 Conclusions The experimental data suggests that yawning indeed occurs during progressive drowsiness, which is compatible with the notion that it is triggered by states of low vigilance. However, no specific arousing effect of yawning on the brain or the autonomic nervous system could be observed. Experimental evidence therefore suggests a rejection of the arousal hypothesis. The absence of an arousing effect of yawning does obviously not exclude that it might have some other form of activating function on brain metabolism or neuropharmacology, but these effects should not be named arousal.
3.3 The Sleepiness Hypothesis Rather than attributing an arousing effect to yawning, some authors have suggested that it might lower the arousal level (Deputte, 1994). Studies assessing the arousal level after yawing have indeed found signs of decreasing wakefulness (see section 3.2.2 above), which would be compatible with this notion. However, the observations could simply represent the drowsiness underlying yawning that continues to progress also after yawning. Thus, there is no established causal link between yawning and subsequent drowsiness. Moreover, if yawning had a soporific effect apart from being induced by drowsiness, it would be a self-reinforcing mechanism and would need to be controlled by other processes in order to ensure stability of the sleep-wake balance.
3.4 The Thermoregulation Hypothesis Recently, another physiological function of yawning has been proposed: the regulation of brain temperature. It is postulated that yawning might cool down the brain when its temperature increases. The advocates of this model give a detailed description of their arguments in (Gallup and Gallup, Jr., 2008). Here, we provide a brief critique of the corresponding experimental evidence.
3.4.1 Does Brain Hyperthermia Trigger Yawning? .......
The same concern also applies to a second study of the same group performed in birds which were exposed to different ambient temperature conditions. A rapidly increasing room temperature was associated with more frequent yawns than relatively stable cold or warm temperatures (Gallup et al., 2009), which may again be due to uncontrolled factors such as differences in drowsiness or related to rapidly changing vs. stable temperatures. The proponents of the thermoregulation hypothesis also advance anecdotal data of yawning frequency in patients with different brain diseases, but in the absence of direct comparisons and controls, the evidence remains inconclusive.
3.4.2 Yawning Does Probably Not Cool Down the Brain The greatest challenge for the proponents of the thermoregulation hypothesis lies in demonstrating how yawning would be able to cool down the brain. It is suggested that the inflow of cool air during yawning ventilates heat off the brain. However, the proposition faces similar problems as the respiratory hypotheses discussed above. Yawning actually interrupts normal nasal breathing which seems to be a more efficient way of ventilation.
3.4.3 Conclusions There is currently insufficient evidence for a thermoregulatory effect of yawning. The thermoregulation hypothesis seems to be counterintuitive and has important explanatory gaps which seem to be difficult to close.
3.5 The Ear Pressure Hypothesis Yawning has the much appreciated capacity to equalize air pressure in the middle ear with outside air pressure. It can thus relieve discomfort in the ear and hearing problems due to rapid altitude changes in air planes or elevators. This is achieved through contraction and relaxation of tensor tympani and stapedius muscles during yawning, which results in an opening of the Eustachian tubes and the aeration of the tympanal cavities (Laskiewicz, 1953; Winther et al., 2005). This observation has led to the postulation that yawning might be a "defence reflex" of the ear, which is triggered by rapid altitude changes or other conditions leading to air trapping in the middle ear (Laskiewicz, 1953). However, there is to our knowledge no systematic investigation that would confirm increased yawning rates under rapidly changing ear pressure conditions. Also, yawning is not the only mechanism to open the Eustachian tube; swallowing, chewing, and the Valsalva manoeuvre have the same effect (Laskiewicz, 1953; Winther et al., 2005). The middle ear pressure release of yawning does therefore not represent by itself an indispensable evolutionary advantage. Equalization of ear pressure seems to be a useful effect that yawns have in common with other contractions of oropharyngeal muscles rather than the primary purpose of yawning.
3.6 The State Change Hypothesis Rather than suggesting a single physiological function of yawning, Provine attempted to combine the multiple behavioural state changes associated with yawning (wakefulness to sleep, sleep to wakefulness, alertness to boredom, etc) within a single framework. He proposed that "yawning is a vigorous, widespread act that may stir up our physiology and facilitate these transitions" (Provine, 1986; Provine, 2005). This approach has the advantage that it might integrate findings from different research fields. However, the proposition does not go beyond a mere description of the behavioural changes associated with yawning and does not give insights into how or why the proposed state changes might be achieved. Given the current scarcity of experimental evidence for any physiological function of yawning, the combination of several physiological states within a single concept also lacks empirical support.
3.7 Other Physiological Hypotheses Several other variants of a regulatory function of yawning on body physiology have been proposed (Smith, 1999). To name only a few: yawning prevents lung atelectasis (Cahill, 1978); yawning renews surfactant films in lungs (Forrester, 1988); yawning ensures intermittent evacuation of the palatine tonsillar fossae (McKenzie, 1994). None of these propositions has been experimentally tested. 4 The Social/Communication Hypothesis In many cultures, yawning is interpreted as a sign of boredom and sleepiness and is therefore considered to be rude (Schiller, 2002). Thus, yawning seems to communicate a message that is almost universally understood. Moreover, yawning frequently occurs in social contexts. A communicative function of yawning has therefore long been suspected. The hypothesis states that yawning is a non-verbal form of communication that synchronizes the behaviour of a group (Barbizet, 1958; Provine, 1986; Weller, 1988; Deputte, 1994).
4.1 Yawning Has Physiological and Social Triggers Yawning can be triggered by several different physiological body states as well as social contexts. Drowsiness (see above) and boredom (Provine and Hamernik, 1986) are well documented precursors of yawning. Observations in animals further suggest that yawns may be facilitated by hunger or mild psychological stress (Deputte, 1994). The communication hypothesis accounts for all these inductors by stating that they generate yawning to transmit the corresponding information to other members of a social group. The number of possible yawning triggers must of course not be unlimited; otherwise the transmitted message would be too ambiguous. Indeed, all triggers of yawning mentioned above have in common that they are mildly to moderately unpleasant while not presenting an immediate threat.
4.2 Social Effects of Yawning The social hypothesis predicts that yawning has some impact on the behavioural organization of a social group. Communication should result in better synchronization of group behaviour. Such effects have indeed been observed in Ostriches (Sauer and Sauer, 1967), but studies that test the prediction in a controlled fashion are lacking.
4.3 Contagious Yawning Yawning has a well-known contagious effect in humans (Baenninger, 1987; Provine et al., 1987b; Provine, 1989a; Provine, 1989b; Platek et al., 2003) and this effect is now frequently used to induce yawning for research purposes. Recent studies have accumulated evidence that this contagiousness depends on an intact social competence of the yawning individual. The susceptibility to contagious yawning correlates with empathic skills in healthy humans (Platek et al., 2003) and is reduced in patients with disorders affecting the ability of social interaction, such as autism (Senju et al., 2007) and schizophrenia (Lehmann, 1979; Haker and Rossler, 2009). In patients with schizophrenia, the occurrence of yawns has been interpreted as a positive sign indicating that the patient is in an accessible mood (Lehmann, 1979).
Watching or hearing other persons yawn activates a complex network of brain regions related to motor imitation, empathy, and social behaviour. Figure 3 illustrates the brain regions that have been reported to activate in different functional magnetic resonance imaging (fMRI) studies when human subjects observe yawns of others. The so-called mirror neuron system is important for action understanding and imitation (Rizzolatti and Craighero, 2004) and mirror neurons in the right posterior inferior frontal gyrus also seem to be recruited for contagious yawning (Arnott et al., 2009). The mirror neuron activity is however not specific to yawning but occurs to the same amount also during observation of other movements (Nahab et al., 2009; Arnott et al., 2009). Activations that are more specific to contagious yawns have been observed in the posterior cingulate (Platek et al., 2005), the bilateral superior temporal sulcus (Schurmann et al., 2005), or the ventromedial prefrontal cortex (Nahab et al., 2009). The fMRI activations in these areas were significantly greater when the study subjects watched other persons yawn than when they watched control face movements of others. Although different studies have reported divergent areas to be implicated in contagious yawning, all of them seem to be part of a distributed neural network related to empathy and social behaviour (Saxe et al., 2004; Carrington and Bailey, 2009). In children, no contagious yawning can be induced before the age of five (Anderson and Meno, 2003), suggesting that the contagiousness of yawning depends on mechanisms that have to develop during childhood in parallel with the empathic capacity to understand mental states of others (Saxe et al., 2004).
In animals, contagious yawning has been consistently observed in chimpanzees (Anderson et al., 2004; Campbell et al., 2009; Vick and Paukner, 2010), whereas it seems to be absent in lions (Baenninger, 1987). In old-world monkeys (Baenninger, 1987; Paukner and Anderson, 2006; Palagi et al., 2009) and dogs (Joly-Mascheroni et al., 2008; Harr et al., 2009), different studies showed divergent results, but contagious yawning occurs at least in some individuals. The findings from animal studies therefore also support the notion that contagious yawning mostly occurs in individuals and species with advanced empathic and social skills. In monkeys, the contagiousness of yawning correlates with the level of grooming contact between individuals (Palagi et al., 2009), i.e., it is higher in animals that are socially and emotionally close to each other. In summary, research on contagious yawning has revealed that yawns are part of the action repertoire of empathic and communicative processes in adult humans and some mammals, which provides strong evidence for a social role of yawns in these species.
4.4 Other Social Modulators of Yawning Social contexts were found to have an important impact on the yawning rate. In animals, the hierarchical position within a social group influences the frequency of yawning: group leaders initiate more yawns than subordinates (Hadidian, 1980). This difference in the yawning rate may correspond to the greater importance of communications from leaders than from other individuals for the synchronized behaviour of the group (Sauer and Sauer, 1967), and may thus also be explained within the framework of the communication hypothesis. There are however also yawns that are independent of social modulation. Yawning also occurs when individuals are alone and in non-social animals. This might be used as an argument against the communication hypothesis and for the need to postulate an additional physiological effect of yawning. However, the existence of yawns during aloneness does not contradict the communication hypothesis in general; it merely shows that the generators of yawning lack a negative feedback mechanism checking for the presence of other individuals. Hence, the message of yawning seems to be triggered by certain body states and "sent out", no matter whether there are other individuals that might actually receive it. In humans, the presence of other humans may even have a suppressive effect on the yawning rate. If human subjects feel socially observed, they completely stop yawning even if the usual conditions of yawning are met (Baenninger and Greco, 1991; Provine, 2005). This suppression may result from arousing effects inherent to social observation. Alternatively, the negative connotation of yawning in human society may push the individuals to hide or inhibit yawns when they are felt to be inappropriate.
4.5 Conclusions The communication hypothesis has the best experimental evidence among all propositions on the function of yawning. It is the only model that can account for social effects of yawning such as contagiousness and for the different physiological states and social contexts that can trigger it.
Missing elements of this model include controlled studies observing a regulating effect of yawning on synchronized group behaviour and data on the neuropharmacological mechanisms underlying the social inductors and effects of yawning. It is also far from clear whether the findings of contagious yawns derived mostly from studies in humans and primates can be generalized to other forms of yawns and to yawns in other species. The social aspects of spontaneous (non-contagious) yawns, particularly in species and individuals who are not susceptible to contagious yawning, have received little research interest so far.
5 Discussion
In 1986, Robert R. Provine, the pioneer in yawning research, wrote that "yawning may have the dubious distinction of being the least understood, common human behaviour." (Provine, 1986). Today, more than two decades later, this may well still be the case. In particular, the centuries-old question of why we yawn still awaits a corroborated answer. None of the numerous propositions on the function of yawning has currently sufficient experimental support or links to neuropharmacological mechanisms. Nevertheless, the preceding sections (which are summarized in Table 1) may have demonstrated that the emphasis of models on yawning has changed. Whereas traditional hypotheses were mostly characterized by the quest for a physiological function of yawning in individuals, these propositions now face severe explanatory problems or lack empirical evidence. In contrast, the idea that yawning might rather serve a social function in groups of individuals receives increasing support from studies in different fields. It emerges that yawning might communicate unpleasant but not immediately threatening states to other members of a group in order to enhance behavioural synchronization.
This social hypothesis of yawning is also the only model that can account by itself for all elements associated with yawns. For instance, contagious effects or social contexts of yawning cannot be explained when assuming a purely physiological function. Physiological hypotheses therefore have to postulate social effects in addition to a physiological effect of yawning, whereas the physiological triggers of yawning form an integral part of social models. Hence, the social hypothesis has not only the best experimental support but is also the most parsimonious model.
From an evolutionary perspective, the communicative value of yawning may yield sufficient advantage to explain its persistence and frequent usage in many vertebrate species. The capacity to exchange information about the physical and mental state of each individual seems indeed to be crucial for the survival of a group. There is therefore no need to postulate additional physiological functions of yawning to explain its selection during evolution. One may argue that the difficulties with physiological models results from an oversimplification of a complex phenomenon.
There might be different types of yawning that assume different functions which are unrecognized if all yawns are inappropriately pooled. However, the data from observational studies does not support this notion. Although numerous yawn morphologies and contexts have been described (Provine, 1986; Deputte, 1994; Baenninger, 1987; Palagi et al., 2009; Vick and Paukner, 2010), the different studies did not converge on a consistent classification into well-delimited types. Furthermore, most studies found no functional or contextual differences among the different yawning morphologies (Provine, 1986; Deputte, 1994; Baenninger, 1987; Palagi et al., 2009). Vick and Paukner (2010) interpreted differences in the scratching rate after "full yawns" vs. "modified yawns" with additional voluntary face movements of chimpanzees as evidence for a selective arousal effect of modified yawns only, but we have seen above that indirect behavioural markers of arousals are problematic. The current limited data therefore seems to suggest that yawning is a single mechanism associated with a continuum of behavioural manifestations rather than a discrete set of functional entities.
On the species level, the generators and functions of yawning may have evolved differently in different species, and yawns may even be a residual of earlier life forms with no remaining function at all in some species. However, in the absence of evidence for systematic differences in the mechanisms and functions of yawning between species or yawn morphologies, this call for more complexity does not withstand the simplicity and elegance of the social model of yawning. In conclusion, current data suggests that we might have to get used to the idea that yawns have a primarily social rather than physiological function.
6 Future Research Directions Several lessons can be learned from research of the last 3 decades. Experience with the respiratory and arousal hypotheses demonstrates that one must be careful when interpreting indirect or anecdotal evidence. Although both hypotheses had some arguments and indirect evidence on their side, direct measurements showed negative results. In order to differentiate between specific features of yawning and nonspecific coexisting elements, it is important to include control groups or conditions during experiments. The lack of controlled experimental studies on yawning illustrates the need for research programs in all related fields. Some of the specific questions that could be addressed are listed in Table 2.
All current models on the function of yawning are derived from observations of the phenomenology and contexts of yawning, which may result in a negligence of aspects that are not behaviourally evident. An exploration of the neural and metabolic mechanisms may give new hints on the functions of yawning that were hitherto unsuspected or on the mechanisms of existing concepts. Future research should therefore systematically assess behavioural, physiological, and social features of yawning and combine observational with interventional techniques. This requires interdisciplinary strategies that would overcome limitations of traditional techniques. For example, a combination of interventional approaches [e.g., administration of yawn-inducing or -inhibiting drugs (Argiolas and Melis, 1998), experimental lesions of brain structures involved in yawn-generation such as the PVN (Argiolas et al., 1987), manipulation of environmental conditions] with systematic behavioural observations during wakefulness may increase the value of both animal models and observational approaches.
A multimodal approach of this kind also seems to be necessary to resolve long-standing controversies on whether different types of yawning exist and on whether yawns in different species are homologous. Future studies addressing these issues should systematically compare not only behavioural but also social, functional, and physiological parameters when trying to classify yawns within and across species. Besides this explorative approach, there is also a need for hypothesis-driven research based on the current models of yawning. Numerous open questions related to the hypotheses discussed above remain unanswered; Table 2 lists only a few. Page 26 of 37
7 Acknowledgments The authors would like to thank Olivier Walusinski for his invaluable online archive of articles on yawning ( which greatly facilitated the literature research for this article.

mise à jour du
26 décembre 2010
Neurosci Biobehav Res
Why do we yawn?
The importance of evidence
or specific yawn-induced effects
Adrian G. Guggisberg, Johannes Mathis, Armin Schnider,
Christian W. Hess
Gallup (Gallup, 2010) believes that our recent review on the function of yawning (Guggisberg et al., 2010) is unbalanced and that it ignores evidence for his thermoregulation hypothesis. Here we address these criticisms and show them to be untenable. While we never claimed that the social hypothesis of yawning has "definite experimental support", we emphasize the importance of experimental evidence for specific effects of yawns when considering why we yawn. The only specific effect of yawning that could be demonstrated so far is its contagiousness in humans, some nonhuman primates, and possibly dogs, whereas all studies investigating physiological consequences of yawns were unable to observe specific yawn-induced effects in the individual of any species. The argument that from an evolutionary perspective, yawns must have a "primitive" physiological function arises from imprecise reasoning.

Yawning is a multifaceted, ubiquitous, and frequent behavior that has been largely neglected by research. The existing scientific literature on yawning is characterized by a relative abundance of theoretical considerations and hypotheses which contrasts with a scarcity of experimental data. In an attempt to encourage future research and to provide an overview of the current state of empirical investigations, we recently published a review that purposely focused on experimental studies (Guggisberg et al., 2010).
When going through the literature, it was important to carefully distinguish between triggers and effects of yawns in order to correctly interpret the causal chain of this complex behavior. With this approach, it became evident that yawning has several rather well documented triggers, which can be physiological, psychological, or social. However, when it comes to the effects of yawning, there is a striking absence of evidence for any specific physiological effect of yawning with several studies reporting negative results (Baenninger and Greco, 1991; Greco and Baenninger, 1991; Guggisberg et al., 2007; Laing and Ogilvie, 1988; Provine et al., 1987; Regehr et al., 1992). This is in contrast to the relatively abundant evidence from research in different fields showing that yawns have a specific contagious effect in humans, some non-human primates, and possibly dogs (Anderson et al., 2004; Baenninger, 1987; Campbell et al., 2009; Haker and Rossler, 2009; Harr et al., 2009; Joly-Mascheroni et al., 2008; Palagi et al., 2009; Paukner and Anderson, 2006; Provine, 1989; Provine, 1989), which is associated with activations in neural networks responsible for empathy and social skills (Arnott et al., 2009; Nahab et al., 2009; Platek et al., 2003; Platek et al., 2005; Schurmann et al., 2005).
Our review concluded that no model of the function of yawning has currently sufficient experimental support, and that there is more evidence for social effects than for physiological effects of yawns. This constellation forces us to consider the possibility that yawning might actually have a mainly social function.
Our article provoked criticisms by Andrew Gallup who prefers to advocate a thermoregulatory function of yawning (Gallup, 2010). Here we show that these criticisms are ill-founded and based on precisely those mistakes that we had tried to overcome with our article.
The commentary of Gallup fails to distinguish between triggers and effects of yawning. By merely looking at the pooled evidence for the different models of yawning, he supposes that physiological and social models of yawning both have similar evidence and dismisses our conclusions as unbalanced. However, any hypothesis on the function of yawning must be supported not only by evidence for a corresponding specific trigger, but also for a specific effect. In the example of the arousal hypothesis, there are at least 6 studies suggesting that yawns can be triggered by sleepiness (Giganti et al., 2007; Greco et al., 1993; Guggisberg et al., 2007; Provine et al., 1987; Zilli et al., 2007; Zilli et al., 2008), but at least 5 studies that were unable to observe a specific arousing effect of yawning with at least 3 different techniques that are considered the gold standard for measuring vigilance (Baenninger and Greco, 1991; Greco and Baenninger, 1991; Guggisberg et al., 2007; Laing and Ogilvie, 1988; Regehr et al., 1992). In this situation, we must conclude that the main prediction of the arousal hypothesis is not supported by experimental evidence. Gallup does not agree with our argument that the non-specific variation in heart rate accompanying yawns is insufficient to conclude on an activating function of yawns. However, we keep insisting that an ubiquitous behavior such as yawning should yield more evolutionary advantage than to induce variation in heart rate which already occurs hundreds of times each hour with each movement and respiration. Unlike claimed by Gallup, this lack of specificity does not apply to the contagious effect of yawns, because only yawn-related behavior can provoke other yawns.
Gallup also accuses us of having ignored evidence for his thermoregulation hypothesis. However, the experimental studies published by his group at the time we published our review reported only changes in brain temperature as triggers of yawns and provided no evidence for a brain cooling effect. After the appearance of our review, Gallup's group published a new study measuring cortical temperature with implanted thermocoupled probes in rats which showed an increase of -O.2 t in brain temperature starting about 1 minute before yawns and stretches, which then starts to decrease again about 20-40 seconds after the onset of yawns and stretches (Shoup-Knox et al., 2010). This is the first study that provides direct evidence for an association between brain temperature and yawning. However, the question remains whether this association is causal, i.e., whether the decrease in brain temperature is produced by the yawns as such. The observation that the temperature decrease after yawning was similar as after stretching suggests that the respiratory component of yawns (inflow of cool air) does not play a significant role. What remains is an increase in blood flow associated with yawns and stretches. However, the increase in cerebral blood flow which is supposed to cool down the brain according to the thermoregulation hypothesis occurs within a few seconds after the yawns.
Conversely, the brain continued to warm up with the same speed as before until -20-40 seconds after the yawns and stretches of the observed rats. Even if we allow for a certain delay in venous blood drain and thermal convection, this time difference appears to be too large. The thermometer seems to have been placed close to the dura and therefore should have rapidly captured a blood flow induced temperature change. Hence, as (Shoup-Knox et al., 2010) discuss in their article, the observed variation in brain temperature associated with yawns is probably mediated by concomitant thermoregulatory brain processes and not caused by the yawns themselves as would have been predicted by the thermoregulation hypothesis (Gallup and Gallup, 2008). Moreover, (Elo, 2010) has recently shown for humans that even small yawn-induced decreases of temperature (Gallup and Gallup, 2010) are physically impossible as long as there is no massive sweating associated with the yawning. Despite the criticisms of Gallup, we also keep insisting that the design of his earlier studies exploring temperature as a trigger of yawns did not include recordings of physiological parameters such as brain temperature and vigilance. It is therefore impossible to exclude confounding effects in these studies.
We agree with Gallup that yawning needs to be studied from a phylogenic perspective in different species. However, we prefer to avoid premature and problematic conclusions such as "any social or communicative value of yawning among humans and non- human primates is likely a derived feature, while the underlying primitive feature or function is physiological". Let's start from the experimental finding of contagious yawns in species and individuals with empathic skills. We agree that empathic contagion has developed late in evolution, and that it must have been derived from a basic communicative value of yawns. Indeed, we cannot be empathic to states of others if this state is not somehow communicated to us. Hence, the existence of contagious yawning strongly supports a communicative value of yawns at least in these species. Gallup goes a step further and proposes that basic communication must in turn be derived from "primitive" physiological features. Now, there is no doubt that the components of yawns such as opening and closing of the mouth and respiration have physiological functions. In this sense, yawns are indeed derived from more primitive physiological features. However, this does not mean that the combination of these components, i.e., the yawns as such, also must have a physiological function. Basic communication is a phylogentically old phenomenon; even primitive cells and organisms maintain communications with the exterior. Yawning as a non-verbal form of communication could therefore have evolved in vertebrates independent of a physiological function. Gallup's proposition that contagious yawns may help to spread an arousing effect of yawns to all individuals of a social group would therefore need to be supported by empirical evidence, which is, as we have discussed above, negative. Hence, all we know so far is that yawning has a communicative function in humans, some non-human primates, and possibly dogs. Even though this communicative function has so far only been convincingly demonstrated in species with empathic skills, it does not necessarily depend on empathy and could therefore exist also in non-empathic animals and contexts. Some observational studies suggest that this may be the case (Deputte, 1994; Sauer and Sauer, 1967), but we repeat that controlled studies of social effects in non-empathic species are lacking. Yawning could additionally have physiological functions in the individual, but the claim that this is likely the case arises from imprecise reasoning and the corresponding experimental studies are essentially negative. Until systematic comparative data across species becomes available, we should also avoid premature statements such as "it is likely that instead of serving one purpose, yawning is multifunctional across a number of species".
We are further criticized for having given only a "vague and imprecise" description of a possible social function of yawning and for not having integrated "theories on signal evolution" in this framework. We believe that we need more controlled empirical studies on social effects of yawns in different species before we start to construct precise theories. Precise hypotheses that turn out to be wrong are of no use for the advancement of yawning research.
Gallup distorts our article's content. Unlike claimed we do discuss the lack of experimental data for a social effect of yawns beyond its contagiousness, and we conclude that "none of the numerous propositions on the function of yawning has currently sufficient experimental support or links to neuropharmacological mechanisms." Nevertheless, given that there is no evidence for any specific physiological effect of yawns in any species, the evidence for social effects is still better. Unlike claimed, we do include all empirical studies in our discussion of the arousal hypothesis, including the ones mentioned in the article of Baenninger (1997). Unlike claimed we do consider experimental data from all species and discuss the possibility that yawning might be multifunctional across species.
In conclusion, we cannot agree with the criticisms expressed by Gallup (Gallup, 2010). Discussions on the function of yawning should be based on experimental evidence and not on problematic assumptions.
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Andrew C. Gallup. Yawning and the thermoregulatory hypothesis