Why do we yawn? by Luciano Nicolini (n°149)
The idea at the basis of this article came to me fifteen years ago. I was on holiday at a seaside resort in the south of Italy and, as I had at my disposal a terrace overlooking the sea, I decided to invite for dinner a couple friends of mine who lived in a city approximately fifty kilometres away.
We dined and chatted amply. At one point I noticed that my extremely polite guests who, unlike me, had risen early to go to work, very silently and covering their mouths began to yawn. After about half an hour, the couple got into their car to return home.
In the days that followed I found out that, during the journey back, my lady guest had dozed off and the husband, who was driving, had had a fit of drowsiness: by pure chance he awoke just in time and was able to keep the vehicle on the road. If he had been a little less polite – I thought – he would have yawned loudly and awaken his wife and she, being aware of the serious danger, would have helped him to not fall asleep …
Given my training as an anthropologist, it comes spontaneous to me to generalise a hypothesis: perhaps it is precisely for this reason that, when we are tired, we yawn. Perhaps our ancestors, who probably lived in bands, had sentinels who, when they were about to fall asleep, would yawn loudly, thus warning those who were sleeping beside them of their state of loss of concentration. If that were the case, it would also explain the unique phenomenon of the contagiousness of yawning, which would make more effective, multiplying it, the sign of alarm!
Once my holiday was over, I attempted to seriously look into the matter. I immediately realised that the bibliography concerning the origin and function of yawning was endless. Many authors argued that its function was to provide oxygen to the brain (Trautmann 1901, who followed in the footsteps of what Hippocrates asserted in the 4th century B.C.), whiles others sustained that it awoke it (Baenninger 1997), and others more that the purpose was to cool it (a theory that has been recently revived and developed by Gallup and Gallup 2008). A handful, among which Charles Darwin (1872), took into consideration its communicative function. Moreover, in most works, yawning was considered behaviour common to all mammals, if not all vertebrates, including species for which an explanation in terms of alarm signal seemed indefensible.
I have recently had the opportunity to read an enlightening article by Adrian G. Guggisberg, Johannes Mathis, Armin Schnider and Christian W. Hess (2010) which appeared in “Neuroscience and Biobehavioral Reviews” which, at the end of an extremely close examination of experimental evidence regarding yawning, reached decidedly interesting conclusions.
The first important conclusion reached by Guggisberg et al. (2010), after having defined yawning as an involuntary sequence of mouth opening, deep inspiration, brief apnea and slow expiration, is that the homology of yawning among different species is debatable. And if the common phyletic origin of this behaviour is debatable, the analogy of function is even more disputable. This makes it possible to discuss its evolutionary meaning for our species without necessarily having to extend the hypothesised explanation to all species for which yawning was discussed (not always apropos).
Secondly, they prove that the respiratory hypothesis, according to which the function of yawning would be to provide a greater amount of oxygen to the brain, is not backed by experimental data. On the contrary, research carried out by Provine, Tate, and Geldmacher (1987) have shown that healthy subjects exposed to gas mixtures containing high levels of carbon dioxide do not yawn more frequently, and that the same occurs in such subjects during prolonged physical activity.
Guggisberg et al. (2010) affirm that yawning occurs mainly during periods of sleepiness, as provided for by the theory according to which it favours the awakening of the one who is yawning. However, despite the fact that many studies have analysed variations of EEG in people after yawning to test the hypothesis that the effect is awakening, the results were negative. In other words: consistently with what I hypothesised, one yawns when he is lacking concentration or when seized by drowsiness, but it does not seem to be the yawn itself that keeps the person yawning awake.
Even for the hypothesis of a thermoregulating effect of yawning, according to Guggisberg et al (2010), at present there is still insufficient experimental evidence. Its upholders basically hypothesise that yawning causes, by means of ventilation, cooling of the brain, something that has yet to be proven. On the contrary, it seems that yawning interrupts normal nasal respiration, which appears to be an even more efficient form of ventilation than the one proposed by them.
An even more interesting phenomenon is that in children contagious yawning cannot be induced before the age of five (Anderson and Meno 2003), i.e. in a period of life in which it is difficult to think that an individual can be entrusted with the role of group sentinel.
After having substantially excluded any physiological function of yawning, Guggisberg et al. (2010) can only hypothesise a communicative function.
According to this theory, yawning is “a non-verbal form of communication that synchronizes the behaviour of a group (Barbizet 1958, Provine 1986, Weller 1988, Deputte 1994)”. And this would explain, among other things, its contagious effect. Furthermore, the susceptibility of being infected by yawning in healthy individuals, would be in relation with empathic abilities (Platek et al. 2003), that is to say with the ability to participate intensely and immediately in the emotional situation of another person. It would instead be reduced in patients that have problems regarding social interaction, such as autism (Senju et al. 2007) and schizophrenia (Lehmann 1979; Haker and Rössler 2009).
These, and other less important observations regarding non-human primates, have lead Guggisberg, Mathis, Schnider and Hess to affirm that the communicative hypothesis is the one with the best experimental evidence among all the hypotheses formulated regarding the function of yawning and that, in their opinion, it is the only model able to explain the social effects of yawning, such as contagiousness and the different physiological states and social contexts that can cause it. Even if, cautiously, they do not venture a more precise definition of what is communicated to other members of the group through yawning.
In conclusion: the origin and evolutionary meaning of human yawning are probably destined to remain unknown. Behaviours, unlike organisms, do not leave behind fossils. However, it appears probable that the main function of yawning in protohuman hominins was that of communication.
The hypothesis that, through yawning, they communicated to other members of the group their state of drowsiness, or lack of concentration, seems consistent with what has been observed to this point and, if it were valid, the ability to yawn could have constituted an important evolutionary advantage in situations in which one was often a prey.
The fact that this explanation of the phenomenon has not been up to now proposed in a clear and explicit manner by the authors who have studied this subject can probably be traced back not only to understandable caution but to the limited attention dedicated to the sonority of yawning, avoided in contemporary man due to cultural conditioning, but probably associated to it in our ancestors.
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