mise à jour du 8 avril 2002
1962 nov 17;732:152
 On yawning
Ashley Montagu
Princeton, NJ
Yawning: an evolutionary perspective Smith EO
YAWNING is a form of behavior which has defied all attempts to explain it. The explanation offered in the present communication may be no more successfull than others have been, but at least it will have the merit of being more comprehensive.
A yawn is a long, deep inspiration with the mouth wide-open, followed by a slow expiration. I dont know whether all mammals yawn, but certainly a large number of them do. All monkeys and apes do so. A trait so widely distributed among mammals and especially in the order to which man belongs, the primates, is likely to be functionally "basic or at least to serve some very real organic need". In man, the needs served are likely to be even more involved than they are in less sophisticated mammals.
Let us consider the conditions under which yawning occurs; by this means we may perhaps obtain some clue as to the meaning of yawning. In man, yawning occurs at all ages and in both sexes. The following are the conditions under which yawning occurs: sleepiness, weariness, on awakening from unrefreshed sleep, and boredom. All these conditions have in common a lowered state of critical consciousness. "Critical consciousness" here means the state of normal, active awareness of and relatedness to the environment. Any significant decline in critical consciousness may lead to yawning. Reduced critical consciousness is often associated with an increased C02 saturation of the blood. Yawning, by its massive inspiration of oxygen and exhalation of C02 serves to restore the depleted oxygen content of the blood. The wide opening of the mouth is associated with inspiration of air through the mouth as well as nose, and at the same time by movements of the mandible the branches of the internal and external carotids and the vessels emptying into the jugulars are compressed. Compression probably also occurs of the carotid bodies, all of which contributes to the acceleration of cerebral oxygenation and venous drainage. Virtually all cephalic structures are in consequence stimulated. The excursion of the mandibular condyle upon the prearticular surface of the sphenoid and its return into the mandibular fossa probably produces some encephalic reverberation. All of these changes will tend to contribute to the heightening of consciousness, and this, it is here suggested, constitutes the adaptive value and the principal function of yawning.
The extreme extension of the muscles of the jaw is akin to the stretching of the extremities when a feeling of lowered body tone is experienced. The stretching has a stimulating, revitalizing effect. It is of interest to note that body-stretching is often accompanied or followed by yawning. In order to yawn it is necessary to be conscious, but anything that tends to lower the normal level of consciousness is likely to induce yawning. The reduction in stimulation which occurs in consequence of sitting in a close, stuffy room underscores the importance of the effect of oxygen reduction as well as of the reduction in the critical aw areness of external. stimuli in producing yawming. It is, in this connection, interesting to observe that it is not as easy to yawn when one feels cold as when one feels warm. The stimulation of the cold tends to heighten consciousness; heat tends to reduce it.
That yawning can be induced by nothing more than a reduction in critical consciousness is shown by the fact that any monotonous situation may induce yawning. The monotony or repetitiveness of the situation may lead to a reduction in oxygenation of the blood as a direct consequence of the decrease in the depth of respiration. It is well known that yawning is contagions, but it would appear to be so principally under conditions of reduced critical consciousness. Thus, the social and biological function of contagious yawning would be to heighten the consciousness of one's fellows by inducing yawning in them in a resulting reciprocally interstimulating situation. Tearing from the lacrimal glands is often associated with yawning, but whether this is due to pressure on the lacrimal gland or is nervous in origin, or both, I do not know. The pressure upon the eyes during yawning can be quite considerable. Possibly one of the functions of lacrimation in yawning is to keep the eye well lubricated during the changes m pressure to which it is exposed in yawning. The flow of tears through the nasolacrimal ducts is often considerable enough to lead to noseblowing.
Yawning warns one of the reduction in critical consciousness and, as in sleepiness or wearmess, suggests that one ought to sleep or rest or, as in boredom, that one ought to do something about the boredom.