mystery of yawning
Le bâillement, du réflexe à la pathologie
Le bâillement : de l'éthologie à la médecine clinique
Le bâillement : phylogenèse, éthologie, nosogénie
 Le bâillement : un comportement universel
La parakinésie brachiale oscitante
Yawning: its cycle, its role
Warum gähnen wir ?
Fetal yawning assessed by 3D and 4D sonography
Le bâillement foetal
Le bâillement, du réflexe à la pathologie
Le bâillement : de l'éthologie à la médecine clinique
Le bâillement : phylogenèse, éthologie, nosogénie
 Le bâillement : un comportement universel
La parakinésie brachiale oscitante
Yawning: its cycle, its role
Warum gähnen wir ?
Fetal yawning assessed by 3D and 4D sonography
Le bâillement foetal

mise à jour du
24 janvier 2011
South African J of Sci
The tonsillar evacuation hypothesis
of yawning behaviour
Andrew A. McKenzie
Mammal Research Institute, University of Pretoria, South Africa


We do not know why we and other animals yawn. Yet because this behaviour is so commonplace, we are obliged to accept either that yawning has no physiologic function or that, because it involves gaping of the mouth, it has some obscure respiratory function. It is suggested that yawning is a powerful reflex to evacuate the palatine tonsillar fossae. As this strong reflex does not have any immediate urgency, it he become allocated to times of minimal inconvenience. This is reflected in the circadian pattern of yawning in our own species, and explains why we have come to associate yawning with drowsiness and boredom, even though this association is not a causal one. If yawning does indeed have this function, then by our social sanctions against we may be supressing its effectiveness, leading to endemic tonsillitis in our species.

L'auteur suggère que la fonction du bâillement serait d'évacuer, par compression, la sécrétion amygdalienne. Comme ce puissnt réflexe n'a pas d'urgence immédiate, il surviendrait aux moments de moindre interférences avec la vie quotidienne. Les conventions sociales en refrénant les opportunités seraient contraire aux bénéfices attendus pour la santé. De là à penser que bâiller pourrait réduire les indications à l'amygalectomie et les prescriptions antibiotiques, voilà un pas que l'auteur n'a pas peur de franchir. Hypothèse, hypothèse...

The familiar reflex known as yawning has been defined as "the involuntary act of opening the mouth widely, accompanied by deep inspiration, and frequently stretching of the arms, shoulders and chest to assist in the inspiratory act, followed by relaxation of the muscles involved, usually performed when sleepy or bored". This definition incorporates, without formal statement, the conventional wisdom, as yet unproven, that yawning has an inspiratory function. It is a remarkable fact that to this day the cause, physiological function and, most intriguing of all, the contagiousness of this everyday habit remain unexplained. This article explores an alternative function for yawning behaviour, by looking laterally, not straight down the throat but slightly to the side.
Yawning, as in the act of opening the mouth widely, has been recorded in all vertebrate classes with the exception of the Agnatha. Among mammals this stereotyped behaviour is most obvious in the carnivores and primates. Several authors have cautioned that resemblances between taxa may be superficial only, and that the role of yawning may not be the same in all groups of animals. This discussion will be limited to mammals, yet even within this group it is recognized that yawning serves two discrete purposes: the first is the as yet unsatisfactorily defined physiological/homeostatic function, and the second is a communication function. Provine and Hamernik have appropriately labelled the signalling function in humans as "a paralinguistic signal for boredom", emphasizing the strong relationship between yawning and fatigue or drowsiness.
The mystery surrounding the function of yawning has intrigued us for centuries: Hippocrates is recorded as having been fascinated by its role. Numerous hypotheses have been proposed to fill this rather worrisome gap in our knowledge about ourselves - none of which has adequately explained all the observed features of this complex behaviour. The hypothesis proposed by Russel, that yawning is an "automatic impulse" caused by "bad air in the lungs" intended to "awaken the respiratory organs into activity" and to effect a stimulation of the brain through increased activity of the circulation, seems to form the basis of present-day conventional wisdom on yawning.
However, Provine et al. have demonstrated that anoxia does not play a role in the regulation of yawning. This can be confirmed by the reader by holding his or her breath for as long as possible - this does not result in a yawn! We may therefore bury the old ideas about yawning and take a look at some of the alternative explanations: yawning is an integrated discharge in a bulbo-reticular motor structure occurring at a particular level of activity of the reticular formation corresponding to a decrease of wakefulness preceding sleep; yawning has no physiological role; yawning stretches the facial and neck muscles; yawning prevents a loss of lung compliance during normal breathing; yawning synchronizes sleep in group-living animals; yawning provides a pulse of thyroid hormones by squeezing the thyroid gland (unknown source in ref. 6); yawning is an attempt of the body to delay onset of sleep and reinforce wakefulness after sleep; yawning is a means of increasing attention when sleep is pressing in the face of a danger or social circumstance; yawning is an arousal defence reflex.
The following conclusions can be drawn from current knowledge on the subject.
Yawning is a reflex act whichmay be modified or induced by higher cerebral activity. It involves paroxysmal contraction of facial and neck musculature which, once initiated, cannot be fully supressed by concious effort (going) to completion with the inevitability of a sneeze (ref. 2, p. 119). The strength of the reflex is such that it occasionally results in dislocation of the jaw and may even fracture the styloid process at the base of the skull. Yawning is often, but not invariably, associated with stretching. It is regulated by a centre in the brain, possibly in the medulla. (This may account for the yawn's strong, involuntary nature, as the medulla is the site of regulation of several involuntary physiological reflexes.)
Yawning is mediated through an as yet undefined neurochemical sequence involving ACTH, dopamine D2 receptors and muscarinic cholinergic neurons (reviewed in ref. 26). Oxytocin has also been implicated in the cascade of events leading to yawning, while adrenergic activity antagonizes the yawning reflex. Yawning is associated with inactivity and proximity of sleep in animals, and with inactivity, boredom, fatigue and proximity of sleep in humans, although the causality of this relationship has not yet been defined.
Yawning is contagious within groups of social animals and in humans; in the case of humans this contagiousness is extended to include auto-suggestion evoked by thinking or reading about yawning. It is the contagiousness of yawning which eludes explanation by the various hypotheses on its physiological role. Because of its close association with drowsiness and boredom, yawning has become an unmistakeable signal of these states, possibly leading to the present-day suppression of yawning as a socially unacceptable human behaviour and to its occasional use purposefully to indicate boredom.
The tonsillar evacuation hypothesis
Based on three disparate yet equally incongruous observations, I formulated an hypothesis to explain yawning behaviour. The observations were:
1) The conventional respiratory explanation of yawning does not appear to be valid although it remains as established conventional wisdom.
2) A yoga exercise (called Simhasana, or the Lion Posture) is claimed to help prevent or even heal a sore throat. (This exercise consists of elevating the head and protruding the tongue.)
3) How do the tonsils of humans, other primates, and carnivores avoid beConurig a permanent site of infection? The dark, moist, sheltered tonsillar fossae, with their continual supply of masticated food and associated bacteria, seem an ideal site for the proliferation of microbes and eventual ulceration and abscessation.
These three unrelated observations gave rise to the following hypothesis:
The yawn is a complex, centrally mediated, reflex muscular contraction which ensures intermittent evacuation of the palatine tonsillar fossae, thereby ensuring that the tonsils are exposed to new antigens and preventing excessive accumulation of foreign material, micro-organisas and inflammatory products.
The function of the palatine tonsils is to identify antigens passing via the pharynx, and to initiate antibody production against potentially harmful micro-organisms which have entered the body via the mouth. Occlusion of the tonsillar fossae with foreign material and inflammatory products would eliminate this function. The tonsillar fossa is a blind diverticulum, open to the external environment, which is lined with lymphatic tissue and mucous membranes, and which is devoid of any apparent evacuating reflex to clear itself of obstruction. Such involuntary reflexes exist for the eyes (blink, lachrymation), the nose (sneeze), the mouth (swallow, gag) and the lungs (cough). As this diverticulum is located in the pharynx, where it is continually exposed to food particles and bacteria, it is reasonable to expect that some mechanism would exist to evacuate foreign material.
A question immediately raised by this alternative explanation of yawning is: Why then is this reflex associated with drowsiness, boredom or proximity to sleep? This can be explained in terms of the tonsillar evacuation hypothesis. A mechanism to ensure that the tonsillar fossae are frequently evacuated would have to be of an involuntary nature; if it remained a voluntary function, it would be improperly or insufficiently performed. If there was such a reflex, it could be one of two types. Centrally mediated reflexes controlled by centres in the medulla oblongata maintain essential homeostasis. Some of these reflexes may serve a protective function - for example, related to the vomition centre, the satiety centre, etc. Other, more directly released protective reflexes mediate a reaction to potentially harmful circumstances - for example, blinking, swallowing, gagging, sneezing and coughing. While reflex control of yawning is indeed required, it need not be of the immediate type as is encountered in the direct reflexes such as sneezing and coughing: even total occlusion of the tonsillar fossae is not a life-threatening event.
Complete evacuation of the fossae is likely to require a strong and co-ordinated contraction of a complex array of phaxyngeal and associated muscles. If this reflex was of the direct type it could be elicited at any time by partial occlusion of one of the fossae. If an animal or person was sleeping, chewing, drinking, vocalizing or mating at the time that a yawn was initiated, this strong and prolonged (3-11 seconds) reflex would be highly inconvenient. If the animal was swallowing food, fighting, subduing prey or escaping predation, a yawn could be a life-threatening event. The allocation of a paroxysmal reflex such as yawning to an appropriate time in order to avoid potential disaster would be an essential event in the evolution of animal behaviour. Yawning is not unique in this regard. Spontaneous blinking is another maintenance reflex which needs to occur sufficiently often to perform its function of keeping the eyes clean and moist, but which is allocated to a more appropriate time if other - potentially life-threatening - matters become more urgent. Indeed, there is evidence that blinking and yawning have a common neuropharmacological basis.
Most earlier explanations of yawning considered that its association with drowsiness and the proximity of sleep has some causal significance. The implication of the tonsillar-evacuation hypothesis of yawning is that its association with sleepiness is a reflection of its disassociation from activity, during which its strong, irresistible nature could be inappropriate. This is supported by the fact that yawning is generally of a low frequency during times of high cerebral activity and during times of physical activity? Furthermore, it has been shown that adrenergic activity, characteristic of fight-or-flight situations, antagonizes drug-induced yawning.
Suggestions that cerebral anoxia during inactivity is the cause of yawning (list in ref. 14) do not address the logical extension of this hypothesis - why do animals and humans not yawn when they are asleep, when such anoxia could potentially be greatest? The present hypothesis implies that there is no association between yawning and sleep. In fact, yawning could disrupt sleep and, from a lack of any records of yawning during sleep, it would appear that the reflex is supressed during the hours of sleep just as it is supressed during other times of inappropriate activity.
The previously inexplicable contagiousness of yawning conforms to the tonsillar evacuation hypothesis. Social animals take many of their behavioural cues from conspecifics within their group - a phenomenon which has been dubbed social facilitation ? The social milieu in which an animal finds itself at any one time is thus a cue to its own behaviour. As explained above, yawning in an animal is a reflection of a state of minimal cerebral, physical and adrenergic activity: a yawn by an animal is thus a signal of such an internal state. If another animal in the group witnesses the yawn this signal (or releaser) may invoke a yawn through social facilitation, only if the observer is in an appropriate physiological state; the induced yawn is a reflection of the state of relaxation in the observer, not of sottie magical contagiousness of the reflex itself. In the wild, times of relaxation for potential prey animals (including our own ancestors, who were heavily preyed upon by leopards and other African predators) may be unpredictable, irregular and scarce. If yawning is to occur at least a few times per day in order to fulfil a maintenance function, then it should, for reasons of survival, happen at times of minimal predation risk as signalled by relaxation in group members: being caught in the middle of a 5-8-second yawn could mean the difference between life and death. This may he the reason for the strength of the contagiousness of yawning in our and other species,
The tonsillar evacuation hypothesis may lead to an explanation of speciesspecific differences in yawning behaviour among mammals. It is well known that carnivores are the most prodiguous yawners, leading, through the association between yawning and sleep, to the common description of lions as lazy animals! As the food of carnivores contains particularly high levels of bacteria, and provides an ideal, protein-rich culture medium for bacterial multiplication, they may be obliged to yawn more frequently than other mammals in order to keep their tonsils clear.
Finally, it is suggested that the tonsillar-evacuation hypothesis of yawning may usefully explain the published observations that uninhibited yawning leads to a feeling of refreshment, and that yawning is frequently followed by swallowing
The hypothesis presented here suggests that yawning has important health implications for Homo sapiens: if yawning does indeed function to evacuate the tonsillar fossae, then by supressing yawning in public through our social sanctions against this behaviour we may have encouraged the development of endemic tonsillitis in our children, leading first to the high incidence of tonsillectomy, and later to the widespread use of antibiotics in its place. This hypothesis suggests that accumulated material is removed from the tonsillar fossae by the act of yawning (if it is not supressed), and the encouragement of this reflex could become an adjunct to the routine treatment of tonsillitis by clinicians. After all, few clinicians would consider antibiotic therapy for an abscess without assisting the healing process by promoting drainage of the accumulated material. The yoga exercise Simhasana - which stretches the pharyngeal muscles to the extreme may well be valuable physiotherapy for both the prevention and treatment of tonsillitis. For this reason, I hope this article will motivate research and clinical trials to test the hypothesis.
1. Gennaro A.R. (ed.) (1979). Blakinston's Gould Medical Dictionary, 4th edn. McGraw-Hill, New York.
2. Provine R.R. (1986). Yawning as a stereotyped action pattern and releasing stimulus. Ethology 72,109-122.
3. Baenninger R. (1987). Some comparative aspects of yawning in Relia splendens, Homo sapiens, Panlhera leo and Papio sphinx. J. comp. PsychoL 101, 349-354.
4. Melis M.R., Argiolas A. and Gessa G.L. (1987). Apomorphine-induced penile erection and yawning: site of action in the brain. Brain Res. 415, 98-104.
5. Izquierdo L (1988). Now you'll start yawning and you won't know why. Trends pharmac. Sel. 91,119.
6. Heusner A.P . (1946). Yawning and associated phenomena. Physiol. Rev. 26, 156-168.
7. Hediger H. (1955). Animals Asleep: Yawning. J.R. Geigy SA., Basic,
8. Barbizet J. (1958). Yawning. L Neurol. Neuro. Psychiat 21, 203-209.
9 .Rasa OA. (1971). The causal factors and function of 'yawning' in Microspathodon chrysurus (Pisces: Pomacentritidae). Behaviour 39 39-57.
10. Hadidian J. (1980). Yawning in an Old World monkey Macaca nigra (Primates: Cereopithecidae). Behaviour 75, 133-147.
11. Moore JE. (1942). Some psychological aspect of yawning. J. gen. Psychol. 27,289-294.
12. Provine R.R., Hamernik HB. and Curchack B.C. (1987). Yawning: relation to sleeping and stretching in humans. Ethology 76, 152-160.
13. Russel H. (1891). Yawning. In The Delsarse Series, No 1. US Book Company,New York.
14. Provine R.R., Tate B.C. and Geidmacher L. (1987). Yawning: no effect of 3-59k CG5, 100% 02, and exercise. Behav. Neurol. Riot 48 382-393.
15. Laskiewicz A. (1953). Yawning with regards to the respiratory organs and the ear. Acta oto laryngol 43,267-270.
16. Gilbert D. (1988). Is yawning a brainstem phenomenon? Lancet 8585, 596.
17. Weller MP, (1988). Yawning (letter). Lancet, 8591, 950.
18. Ferrari W, Gessa, GL, Vargiu L (1963). Behavioral effects induced by intra-cistern ally injected ACTH and MSH. Ann. N.Y. Acad. Sci. 104,330-343.
19. Argiolas A et al. (1990). Role of calcium in the espression of ACTH-induced stretching, yawning and penile erection. Brain Res. Bull. 24, 853-856.
20. Askenasy JJ. (1989). Is yawning an arousal defense reflex? J. Psychol. 123, 609-621.
21. Tesfaye Y, Lal S. (1990). Hazard of yawning. Can. med. Ass. J 142, 533.
22. Tesfaye Y, Lal S. (1990). Hazard of yawning. Can. med. Ass. L 142,15.
23. McCorkell S. (1985). Fractures of the styloid process and ttylohyoid ligament an uncommon injury. J. Trauma 25, 1010-1012.
24. Cronin TG. (1988). Yawning: an early manifestation of vasovagal reflex. AIR 150, 209.
25. Wimalaratana HS, Capildeo R. (1988). Is yawning a brainstem phenomenon? Lancet 8580, 300.
26. Dourish CT, Cooper SJ (1990). Neural basis of drug-induced yawning. In Neurobiology of Stereotyped Behaviour, eds. S.L Cooper and C.T. Dourish, pp. 91-116. Clarendon Press, Oxford.
27. Argiolas A et al. (1986). Oxytocin: an extremely potent inducer of penile erection and yawning in male rats. Europ J. Pharmacol Chemical 130,265-272.
28. Yamada K et al. (1989). Involvement of central adrenorecepcors in the regutation of yawning responses. NaunynSchmiedebergs Arch. Pharmacol. 340,26-30.
29. Dellini-Stula A, Hunn V (1990). Effects of single and repeated treatment with antidepressants on apomorphine-induced yawning in the rat: the implication of n-1 adrenergic mechanisms in the D-2 receptor function. Psychopharmacologia 101, 62-66.
30. Provine R.R. and Hamemik H.B. (1986). Yawning: effects of stimulus interest. Bull. Psychon. Soc. 24, 437-438.
31. -Lehmann H (1979). Yawning an a horneosutic reflex and its psychological significance. Bull. Menninger Clin. 43,123-136.
32. Hewin B. (1990). The Complete Yoga Book Rider, London.
33. Scadding G.K. (1990). Immunology of the tonsil: a review. L R. Soc. Med. 83,104-107.
34. Blin O, Masson G, Azulay JP, Fondarai J, Serratrice G (1990). Apomorphine-inducoTi blinking and yawning in healthy volunteers. Br. L clin. Pharmac. 30, 769-773.
35. Dumpert Y. (1921). Zur Kenntssis des Wesens und der physiologischen Bedeutung des Gahnens. J. Psychol. Neurol. 27, 82-95.
36. Allport F.H. (1924). Social Psychology. Houghton Mifflin, Boston.
37. Brain C.K. (1981). The Hunters or the Hunted? University of Chicago Press, Chicago.
38. Rosenfeld R.M. and Green R.P. (1990). Tonsillectomy and adenoidectomy: changing trends. Ann. Obi, Rhinol. Lar. 99,187-191.