Why Do We Yawn?

tags: yawning, thermoregulation, budgerigars, Melopsittacus undulatus, animal behavior

Yawning. Everybody does it. In fact, I am yawning now as I write this piece. Yawning is interpreted to have a variety of meanings, ranging from tiredness to boredom. Perhaps more interesting is the fact that yawning is contagious among humans, at least: watching someone else yawn, seeing a photograph or reading about -- and even the mere thought of -- yawning is enough to induce this behavior in observers. (Tell me: have you yawned yet?) Even though yawning is very common, the physiological and evolutionary reasons for yawning behavior are poorly understood.

Yawning is a familiar behavior characterized by a large gaping of the mouth, accompanied by a deep inhalation of air, followed a shorter expiration. This behavior is under involuntary control, so it cannot be consciously controlled or suppressed. Further, yawning is a stereotyped behavior expressed by all classes of animals, and is correlated by a variety of neurochemical changes in the brain. Previous research suggests that yawning is a biological mechanism in humans and non-human apes, such as chimpanzees, to keep the brain from overheating.

"Brains are like computers," reports Andrew Gallup, a researcher in the Department of Biology at Binghamton University who led the study. "They operate most efficiently when cool, and physical adaptations have evolved to allow maximum cooling of the brain."

This is known as the "radiator hypothesis" or the "brain-cooling hypothesis," which can be tested.

"Based on the brain-cooling hypothesis, we suggest that there should be a thermal window in which yawning should occur," Gallup proposed. "For instance, yawning should not occur when ambient temperatures exceed body temperature, as taking a deep inhalation of warm air would be counterproductive. In addition, yawning when it is extremely cold may be maladaptive, as this may send unusually cold air to the brain, which may produce a thermal shock."

To test this hypothesis, Gallup and his colleagues, Michael Miller and Anne Clark, studied yawning in wild budgerigars, Melopsittacus undulatus. They chose budgerigars because they have relatively large brains for their body size, they live wild in arid regions of Australia that are subject to frequent and dramatic temperature changes, and -- most important for their experimental design, where they tested the birds in small groups -- budgerigars do not experience contagious or sympathetic yawning, as humans and some other animals do.

To do this work, the researchers divided the small parrots into groups of four, placed them into a wire cage and filmed them while they were subjected to changes in the ambient temperature from 22 to 38 degrees Celsius. Later, these films were scored by observers on the basis of cooling behaviors (yawning and gular fluttering -- the rapid and continuous opening and closing of the beak, which acts to flap membranes in the throat and thereby increase evaporative cooling). These observers were unaware of the ambient temperature that the experimental birds were subjected to.

Their data revealed that the budgerigars were more than twice as likely to express cooling behaviors such as yawning as the ambient temperature rose. Yawning behavior peaked at 30 degrees Celsius, then decreased, to be supplanted by gular fluttering (figure 1). Apparently, yawning is the initial thermal regulatory response but as the ambient temperature further increases and heat dissipation becomes more difficult, gular flutterring is triggered.

These data indicate that yawning is, indeed, a biological thermoregulatory behavior, triggered by increased ambient temperatures.

But why do we care about yawning? There is a growing body of medical and physiological research that links increased ambient temperatures with the unpleasant symptoms of certain medical conditions. Further, it has also been noted that yawning often precedes an epileptic seizure or a migraine headache among those who suffer from them, and increased yawning is also associated with multiple sclerosis. Furthermore, individuals with these health problems tend yawn excessively anyway. Additionally, people who are taking paxil (paroxetine HCl) or Celexa (citalopram) often experience excessive yawning during the first three or so months of treatment. Thus, excessive yawning in humans may be indicative of health problems or can be associated with medications that should be investigated further.

But there are positive aspects to yawning too, because it serves to reinstate an optimal brain temperature that facilitates mental alertness and arousal, and its infectiousness may have evolved to facilitate group vigilance.

"Yawning more accurately reflects a mechanism that maintains attention, and therefore should be looked at as a compliment!" concluded Gallup.

So tell me: have you yawned yet?

Sources:

A Gallup, M Miller, A Clark (2009). Yawning and thermoregulation in budgerigars, Melopsittacus undulatus. Animal Behaviour, 77 (1), 109-113 DOI: 10.1016/j.anbehav.2008.09.014.

Andrew C. Gallup, Gordon G. Gallup Jr. (2007). Yawning as a Brain Cooling Mechanism: Nasal Breathing and Forehead Cooling Diminish the Incidence of Contagious Yawning. Evolutionary Psychology, 5 (1), 92-101 [free PDF].