Why do we yawn?

Babies: A common cause of yawning worldwide.
Image credit: Björn Rixman (Flickr/Creative Commons)

With a newborn baby at home, you probably won't be surprised to hear I yawn a lot these days. But why do I do it? Although the answer seems obvious: "I'm tired", the question "why do we yawn" is very much unsolved.

Boredom and tiredness are the two most stereotypical reasons for yawning, but what's the connection between these two conditions? Neither explains why we yawn because we saw somebody else doing so, and there's even a good chance that simply reading this article will make you yawn - and hopefully not because you're bored! Why do we yawn? Is there a physiological reason? Or a psychological one? How about evolutionary? Why can't we control whether or not we do it?

It has been suggested, and even taught, that yawning is a response to a need for more oxygen in the brain, but this has been shown to be wrong¹. But what about temperature? It could be that we yawn because the brain is getting too hot, and that yawning helps cool it down again. The cooling effect is thought to come from both the air flowing through the skull as a result of the deep breath, and by increasing the blood flow to the brain by stretching the jaw. This research so far seems promising, but this still wouldn't explain why it's contagious.

Dogs will yawn if they hear their owners doing so.
Photo credit: Jessica Spengler (Flickr/Creative commons)

In January this year, a team of researchers from across Europe won the Ig Nobel prize in physiology, for their study which found that although tortoises yawn (along with all other animals) they won't yawn just because their mate did² (unlike dogs, who can "catch" yawns from humans³, and will yawn just because they heard you doing so⁴). If you haven't heard of the Ig Nobel prizes before, they're awarded to research which "first makes you laugh, then makes you think", and are well worth checking out.

So why do we have different trigger mechanisms for yawning? Could it be that it somehow benefited us as a species - a so-called "evolutionary" reason? Is it empathy? Some researchers imply that the stronger the bonds are between the subjects, the more contagious the yawn is⁵.

In other words, why we yawn remains very much a Thing We Don't Know.

References
why don't all these papers have links?
¹ Provine R. R., Tate B. C., Geldmacher L. L. (1987). Yawning: no effect of 3-5% CO2, 100% CO2, and exercise. Behav. Neural Bio. 48, 382–393.  
² "No Evidence Of Contagious Yawning in the Red-Footed Tortoise Geochelone carbonaria," Anna Wilkinson, Natalie Sebanz, Isabella Mandl, Ludwig Huber, Current Zoology, vol. 57, no. 4, 2011. pp. 477-84.
³ Joly-Mascheroni, R., Senju, A. & Shepherd, A. J. 2008 Dogs catch human yawns. Biol. Lett. 4, 446–448.
⁴ Silva K, Bessa J, de Sousa L  Auditory contagious yawning in domestic dogs (Canis familiaris): First evidence for social modulation. Anim. Cogn.2012
⁵ Norscia I, Palagi E (2011) Yawn Contagion and Empathy in Homo sapiens. PLoS ONE 6(12): e28472. doi:10.1371/journal.pone.0028472