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Yawning circumstances
O. Walusinski & B. Deputte
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Yawning is a stereotyped behaviour akin to a reflex. It is mysterious as to its function, and medical people have tried ever since antiquity to assign it a physiological role. All theories about breathing and circulation proposed from the days of Hippocrates until the middle of the XXth century have been debunked by contemporary physiological explorations. Neurophysiology is now starting to explain its cerebral mechanisms. Psychopharmacology has integrated it to preclinical tests on Man and animals in order to better classify mechanisms of action in the various ancient and developing psychotropic families.
Yawning circumstances
Among mammals, there are three types of morphologically identical yawns occurring in three distinct situations: situations relative to circadian rest-activity rhythms, situations relative to feeding, situations relative to sexuality or social interactions.
Situations relative to circadian rest-activity rhythms. In human, the most pronounced occurrences at at awakening in the morning, in association with the stretching of muscles (pandiculation), and as sleep is about to occur, without stretching, as well as in any condition of lessened vigilance. Repetitive and monotonous activities trigger repeated yawns as have shown studies of individuals at their work stations. Activities involving a weekly change of shift, called 3 times 8, help trigger episodes of somnolence preceded by salvos of yawning, lack of sleep and perturbations of circadian rhythms (cortisol). When driving a vehicle, particularly on long stretches of highways, repeated yawns are an alarm signal warning the driver about the risk of falling asleep. R. Provine proposed to students that they watch both stimulating and monotonous videos. Not surprisingly, he found a significant correlation between the frequency of yawns and the viewing of monotonous videos. Environmental conditions such as being confined to a small room or subjected to excessive heat proved to be factors that augment the frequency of yawns caused by boredom. M. Greco et R. Baenninger have established that the frequency of yawns is high under four daily life situations: reading, travelling in public transport, driving and waiting. In children, there is also a relationship between yawning and schooling rhythms and lifestyles. When children move from kindergarten, which is not really stressful, to the first year of grade school, where they learn to read and count, the proportion of yawners increases markedly.
G. Schino et F. Aureli have shown, through the study of subway users in Rome at rush hour, that there is no difference in the frequency of yawns between men and women (38). Yawning evolves over the course of human ontogeny: from high frequency during the first year of life to a gradual reduction as age advances, in parallel with the decrease in the the total time spend asleep during the nycthemeron. Just as there are light and deep sleepers, there are light and deep yawners. Among non-human primates, unspecific yawning is common to all species regardless of age and sex, and constant as to its parameters in a given male before canines push out and after they have reached full size. Yawning happens before sleep but mainly after sleep (or various phases of daytime rest). This type of yawning which occurs during changes of life rhythms and activities is common throughout the animal kingdom. For instance, the ostrich yawns and stretches as soon as it has hatched, as well as throughout its life following a prolonged period of immobility, when it wakes up, etc.
Situations relative to feeding. While observing lions at the Philadelphia zoo, R. Baenninger noted an increased frequency of yawning before feeding times, as well as in function of the outside temperature (maximum of 20 to 24°C). In the wild, carnivores yawn when they wake up and before they go hunting. Carrion-eating animals, such as hyenas, yawn repeatedly while circling a dead animal before tearing it to pieces. Similar observations of anticipatory yawning at meal time have been noted among mandrill monkeys in captivity.
Among birds, yawning immediately after eclosion can be interpreted as a feed-me signal addressed to the parents. The approach of any other member of the species elicits a fear recoil, which shows that the yawn is specifically related to the food-providing adults.
B. Holmgren has experimented on the effect the feeding rhythm of Sprague-Dawley rats has on the schedule of yawns, but under constant lighting conditions to eliminate the light-darkness rhythm. After three weeks of getting them used to one meal a day always at the same time, he found increases in the number of yawns in the hour preceding feeding time, as well as in the moving-about activity. After three days of food deprivation, this behaviour faded away, as well as all spontaneous yawns or those pharmacologically induced with apomorphine. It would appear that, beyond a day-night rhythm, there is a rhythm associated with ingestion which is expressed by an increase in the frequency of anticipatory yawns. This condition is in correlation with a rise in the concentration of circulating corticosteroids.
Of note is the fact that hypocretine or orexine is a neuromediator involved in the stimulation of vigilance and appetite. It so happens that the injection of hypocretine in the paraventricular nucleus of the hypothalamus also triggers yawns.
Though long claimed non-existent, yawning is also found in herbivores, but with a reduced frequency. The energy contribution of grasses is low, as opposed to that of meat. Herbivores spend a great amount of time feeding, do not sleep as much as carnivores and yawn infrequently. There is a correlation between the number of yawns, the duration of sleep, particularly of paradoxical sleep, and the calorific value of food intake. It seems logical to propose the following: the more an animal is under pressure from predators, the case of herbivores for instance, the less it sleeps and the less it yawns. The more it ingests a high amount of calories in a short span of time, the case of predators for instance, the more it can afford to sleep for long stretches and the more it yawns. The exception to this rule is found in non-human primates, the frugivorous, whose paradoxical sleep and number of yawns is comparable to that of carnivores.
In human, fasting is also a trigger of yawns (hypoglycemia). Culinary excesses with overeating and alcohol consumption quickly lead to somnolence accompanied by yawns.
Situations relative to sexuality or social interactions. Yawning also occurs outside the periods of sleep or rest, after various social interactions linked notably to sexuality or conflicts. Data is not available for all animal species. Examples used here refer to a fish and to macaque monkeys.
Microspathodon chrysurus, a fish of the Pomacentridae family, exhibits natural aggressivity. Yawning in fish, as observed in aquariums, is associated with an interruption of swimming motions, the stiffening of the body as a whole and of the fins, along with a wide opening of the mouth. This interruption of swimming motions translates into a tendency to sink vertically. In examining the behaviour of this fish, it is possible to identify yawns related to internal stimuli. As mentioned earlier, yawns develop before feeding time and during reduced motor activity; turbulent water forces regular swimming motion, which inhibits yawning. However, Microspathodon chrysurus also yawns in response to external stimuli. Two types of experiments involving calm waters have been conducted. Dangling a lure mimicking a conspecific or dangling a grey moving ball triggers an increase in the degree of excitation. Chronologically, there are 2 to 4 yawns in the 30 seconds following the dangling, then the body takes on a dark-green hue, swimming picks up speed with frequent direction changes at 90°, signalling a state of excitation that will last about 10 minutes. An immobile ball does not trigger the same reaction as the image of a conspecific. This fish lure may be the target of agressive attacks and will sustain this state of excitation as long as it is dangled, whereas the grey ball in motion will gradually lose its triggering effect. These observations confirm that yawning is the first manifestation that the level of excitation in the fish has increased, either because the water is being agitated or because a conspecific has been seen. Adding a surrenal- stimulating hormone (ACTH) to the aquarium water, which triggers the release of the stress hormone cortisol, starts the same chronology of events: yawning, darkening of the skin, agitated swimming.
Males yawn more frequently than females, be it among rodents (rats, cobayes) or non-human primates. This behavioural characteristic is widely used in neurophysiological experiments.
Among non-human primates, the frequency grows as secondary sexual traits develop (testicular drop, growth of canines), and reaches its maximum in adult males. Yawning is partially governed by androgens; the castration of adult macaques causes a marqued reduction of yawns, while injections of exogenous dihydrotestosterone re-establishes their frequency. The injection of a non-steroid anti-androgen (hydroxyflutamide) blocks the effects of testosterone injections, thus reproducing the effects of castration.
Such yawns, which are basically restricted to adult males and dominant males in the group can be qualified as "emotivity yawns" (BL Deputte), which indicates that they are triggered by a "psychic tension". What communication value can yawning have in a social group? In such a context, anything that can be witnessed by a partner can have meaning. Over the course of evolution, actual communication signals have arisen to act as codes shared by all members of a given species and have led to the modification of the fellow creature's behaviour to whom the signal is adressed or who has noticed it. The yawn can be perceived by a fellow creature without being specifically adressed to same. Such signal always appears at the end of an interaction during which a more or less significant quantity of signals, stricto sensu, have been exchanged. It has not been possible to detect any modification of the fellow creature's behaviour at the sight of a yawn. Therefore, it can only be seen as having a secondary communication value. Since the emotiviy yawn is associated with a given individual, it can serve as a reinforcement of the particular rank this individual has in the group. Such a function cannot be assigned to an unspecific yawn which, though morphologically identical, is not associated with any individual in particular.
The testosterone-dependent yawn uncovers and exposes the male's long canines. Non-human primates being essentially vegetarians, a utilitarian perspective would explain this behaviour as a ritualised exhibition of "weapons of dissuasion". But, this hypothesis is not supported by observation, certainly not among the white-cheeked mangabeys and the long-tailed macaques. In the case of the emotivity yawn, more particular to males therefore more susceptible to expose powerful canines, it occurs at the end of interactions involving exchanges of numerous communicative signals allowing for the specific pattern of this interaction. To the contrary of other signals, the yawn, and of course the canines, have very little chance of being noticed. An electromyographic analysis has shown that during the yawn the exposure of the canines is purely passive and dependent on the maximal opening of the mouth. This study has underlined an essential difference between a threat, a specific signal to stay away, and a yawn. Three characteristics support this difference:
a- Whereas the duration of a threat and its intensity are totally dependent of the partner's behaviour, the sequence of events during a yawn is unchanging and entirely independent of any behavioural trait of the partner.
b- Among all primates, a threat comprises at least an intense visual fixation aimed by the emitting party at the threatened one. While this visual fixation is accompanied solely by a retractation of the scalp among white-cheeked mangabeys, it involves the opening of the mouth among long-tailed macaques. This visual fixation is held as long as the partner has not backed off or ended the interaction. Yawning does not necessarily imply, at its inception, that the yawner is looking at a fellow being; once triggered, it involves the lifting of the head, usually accompanied by the closing of the eyes.
c- Finally, when a threat is a specific signal devoid of ambiguity, the canines remain hidden, whereas they are largely exposed during a yawn that is not in any way directed at a given partner. Under no circumstances can yawning be seen as a type of threat.
U. Halder and R. Schenkel have described the repeated yawns of bovids which, after having calved, lick the amniotic liquid and the embryonic sac. Is this an olfactive component of yawning resting of the mother's specific recognition of her calf later? (or a yet to be investigated Ocytocine effect?).
Abstract in English ..Why do people yawn ?
Yawning : an evolutionary perspective
The neural basis of contagious yawning
Contagious yawning: the role of self-awareness and mental state attribution
Empathy and contagion of yawning
 Video of a typical Yawn