Why do we yawn?
— Photo: V.V. Krishnan
Good models: Parakeets do not practise contagious yawning as humans and some other animals do. They thus made good animal models for this study.
There are several things in daily life, considered trivial, yet for which we have no easy explanation. Yawning is one such. Why do we yawn? What makes me start yawning when I see you do so? why is it contagious? Why do we yawn shortly before hitting the bed and soon after we wake up? Why do we stretch often as we yawn?
As a wag once said, a yawn is like economics — everyone has his pet theory and explanation for it. The most talked about explanation is that we yawn when we feel the need for more oxygen. As it turns out, this theory is not valid.
Dr. Robert Provine of the University of Maryland, a long-time researcher on yawning, put experimental subjects in a room and changed the composition of the air in it, first to contain 3-5 per cent carbon dioxide and measured the frequency of yawning; then he did the same upon changing it to 100 per cent oxygen. The frequency of yawning remained pretty much the same.
Do we yawn more when we are tired? Apparently not so! The frequency of yawning does not change measurably before, during or after exercise.
Does yawning depend on age? It turns out that even a newborn, nay, the growing baby inside the mother’s womb too yawns.
Foetal yawning in utero at 20 weeks gestation has been recorded by ultrasound monitoring. This behaviour appears just as important as breathing movements, eye movements and swallowing for postnatal functions. It is an involuntary act, not inhibited nor elicited by individual command.
A yawn is not a simple act. It lasts about 6-7 seconds on an average — a deep inhalation of air and a sharp exhalation. The heart rate shoots up by about 30 per cent during a yawn, and a whole lot of neurochemical changes occur in the brain.
Arousal and activity
Yawning is thus associated with arousal and activity. This might seem to explain why people with neurological disorders (epilepsy, multiple sclerosis) experience bouts of excessive yawning. A yawn might even act as a ‘psychological decompressor.’
Clearly then, yawning is not a simple expression of boredom, apathy or want of oxygen. It has a deeper adaptive or biological significance, particularly when we note that yawning is common in many animals, particularly vertebrates. It should thus have an evolutionary aspect to it. Charles Darwin, who observed and pondered about every behavioural act of animals, noted that baboons yawn to threaten enemies, and as they gape their mouths they show off their large canine teeth.
He also thought that guinea pigs yawn to show anger. And that dogs and cats are kings of the act; watch the way cats stretch themselves to about twice their normal size as they yawn!
Brain temperature
There is now a growing body of evidence that yawning is a way to regulate the temperature of the brain. As a theory, it has been proposed for over fifty years but experimental testing of the idea has been more recent.
The latest results come from the laboratory of Dr. Andrew Gallup of Binghampton University, New York State (Animal Behavior 2009; 77, 109). They chose to work with the birds, parakeets (also called budgerigars).
Why parakeets? Because they have relatively large brains, live wild in areas of Australia where the ambient temperature fluctuates often. More importantly, they do not practise contagious yawning as humans and some other animals do. They thus make good animal models for this study.
A total of twenty birds were taken, placed in thermal boxes in which the temperature could be adjusted between 20 and 40 degrees Celsius.
Experiments were done when the birds were kept at 22 degrees, then 34-38 degrees, and also when it was rapidly increased between 22 and 34 degrees. The frequency of yawning at each condition was monitored through video.
The researchers found that at 22 degrees, the birds yawned once per set interval of time, and at 34-38 they yawned twice as much. But when they were placed in the condition when the temperature rapidly rose from 22 to 34 degrees, they yawned four times as often as in the other two situations.
The authors conclude that their results support the idea that yawning helps cooling the brain. “Brains are like computers. They operate most efficiently when cool, and physical adaptations have evolved to allow maximum cooling of the brain.”
Cooling effect
In effect, yawning operates like a radiator in an air-cooled engine. Gaping of the mouth and deep inhalation during a yawn cools the circulating blood, dilates the vessels and dissipates heat through emissary veins. A yawn is thus a thermoregulator.
While this research supports the idea of the yawn being a brain-cooling mechanism, it does not answer why it is contagious. Why when you yawn, I too (indeed even your pet dog) do so?
The researcher Andrew Gallup’s father, Gordon suggests that mirror neurons might hold a clue. This is the explanation suggested for why monkeys ape and mimic us.
D. BALASUBRAMANIAN
dbala@lvpei.org
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