We can be triggered to yawn when we see another person (or animal) yawning. Even seeing the word ‘yawn’ may have triggered some readers to do so. And just try stopping in mid-yawn. You can’t. Once you begin to yawn, this instinctive, hard-wired fixed action pattern (FAP) must run its course, from beginning to end.

Although fixed action patterns are most common in animals with simpler cognitive capabilities, humans also demonstrate fixed action patterns. Yawning is a great example. Yawns last around 6 seconds and are difficult to stop once started.

So it seems like a pretty simple question. Why do people (and animals for the matter) yawn? The answer is not as simple as you many think.

Why Do We Yawn?
Oxygen Levels
It's assumed that yawning occurs because we are tired or bored, or is triggered when we see someone else yawn. Scientists do agree that a yawn is an involuntary respiratory reflex, opening of the mouth followed by the deep inhalation and slow exhalation, which regulates the carbon dioxide and oxygen levels in the blood.

Fatigue and Boredom
A yawn may be triggered by fatigue and boredom because when we experience either, our breathing becomes shallow, and the shallow breaths take in less oxygen to support our cells.

During the act of yawning, our alertness is heightened. The sudden intake of oxygen increases heart rate, rids our body of toxic carbon dioxide buildup, forces oxygen through blood vessels in the brain, and restores normal breathing.

Oxygen Hypothesis Doesn’t Explain It All
There are several problems with the explanation given above:the lungs do not necessarily sense oxygen levels
 

  • fetuses yawn in the womb, although they do not take oxygen into their lungs until birth
  • different regions of the brain control yawning and breathing
  • breathing more oxygen does not decrease yawning and breathing more carbon dioxide does not increase yawning

The oxygen hypothesis also doesn’t address why yawns are so contagious or why individuals with high concentrations of oxygen in their blood streams yawn. What else do we know about yawning that could shed some light on this mysterious behavior?

General Sign of Changing Alertness
Since we often yawn when awakening or during other times when our alertness is changing, perhaps yawning is a much more general sign of changing conditions within the body.

A variation on this theory is that yawning stretches the lungs and lung tissue. Stretching and yawning may be a way to flex muscles and joints, increase heart rate, and feel more awake.

Contagiousness of Yawning
There is evidence suggesting that yawning is a means of communication, a ways of conveying changing environmental or internal body conditions to others. This would somewhat explain the contagious nature as a form of communication within groups of animals, perhaps a way of synchronizing behavior.

If this is true, yawning in humans is most likely ‘left over’ mechanism that is no longer useful, having, over time lost its evolutionary significance.

The Brains Paraventricular Nucleus
Low oxygen levels in the paraventricular nucleus (PVN) of the brain’s hypothalamus can induce yawning, and research suggests that the PVN of the hypothalamus is, among other things, the "yawning center" of the brain.

The PVN contains several chemical messengers that can induce yawns, including dopamine, glycine, oxytocin and adrenocorticotropic hormone (ACTH). ACTH increases at night and just before awakening, inducing yawning and stretching behavior in humans.

A Means of Cooling the Brain
A hypothesis recently proposed by Andrew and Gordon Gallup in 2007 states that yawning may be a wayto keep the brain cool. Endotherms are animals that regulate their temperature and are not dependent on the environmental temperature for warming and cooling their bodies and the brain of mammals works best when it is cool.

These researchers showed people videos of other people yawning. When the subjects viewed the videos while holding heat packs up to their foreheads they yawned often. But when they held cold packs up to their foreheads or breathed through their noses (another means of brain cooling), they did not yawn at all.

So there is obviously much more to be learned about the common, but still mysterious behavior of yawning.

Additional Animal Behavior Resources
The Animal Behaviorist
The Animal Behavior Society (ABS)
Ethology Wikipedia


Sources
Gordon G. Gallup. 2007. Good Morning America - The Science of Yawning.

Gallup AC & Gallup GG Jr (2007). "Yawning as a brain cooling mechanism: Nasal breathing and forehead cooling diminish the incidence of contagious yawning." Evolutionary Psychology 5 (1).

Richard Horner: news.utoronto.ca/bios/askus49.htm

Photo Credit
Yawn taken by Stuart Butterfield and posted to Flickr.