Yawning is a reflex consisting of the simultaneous intake of air and reflexive stretching of the eardrums, followed by an exhalation of breath. There are two leading theories of the purpose of yawning, both in humans and other animals. Because yawning is common to most vertebrates, biologists assume that it plays an important role in survival. Two competing theories, dubbed “A” and “B,” seek to explain how.
Supporters of theory A argue that the primary purpose of yawning is to keep the brain cool. The human brain is quite sensitive to even small temperature increases: our reaction times increase and our recall is diminished when the temperature of the brain differs a few tenths of a degree from the perfect temperature of 98.6° F. The proponents of theory A point out that, in terms of escaping from a predator, these tiny temperature changes in the brain could easily be the difference between life and death.
However, critics of theory A argue that yawning is not more common in warmer climates, and that the body has much more sophisticated methods of maintaining the optimal temperature in the brain—the circulatory system, for one example, and sweating, for another. They advocate theory B, which claims that yawning plays a primarily social role based on the fact that yawning is “contagious.”
Because yawning is so demonstrative and affects the body so little, say supporters of theory B, the reflex is most likely a social mode of communication that happens to have some slight physiological effects. The contagiousness of yawning has been shown to be stronger among group members who feel closer to each other, implying that it has a major social component. Based on this information, theory B claims that the primary purpose of yawning is to communicate an increased need for alertness throughout a group. This alertness, according to theory B, is only slightly encouraged by the yawning itself; the real benefit of the contagious yawn is that the yawning animal is reminded to stay alert to the other members of the group and to the surroundings.