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Why Do We Yawn?
From a whole-roasted turkey to carb-loaded turkey stuffing (my favorite pick of the bunch, by the way) and mashed potatoes, a traditional Thanksgiving meal is filled with gluttony, and is a surefire way to put yourself into a food-induced coma after [several rounds of] eating. As if to let other members of the party know of your drowsy state, you let out a large yawn, perhaps throw in a stretch of your arms and twist your back a bit, like this historical figure below:

18th Century French Painter Joseph Ducreaux’s self portrait of himself pandiculating.
A very flattering choice of self-portrait position, I must say....
Both you and the painter above are undergoing a bodily process called pandiculation, an involuntary, reflexive movement involving the stretching of your soft tissues, that is a conserved behavior performed amongst all vertebrate, mammalian species. Yawning is considered to be a specific subtype of pandiculation that involves the stretching of facial, upper back, and the respiratory system tissues, and typically happens concomitantly with the pandiculation of the rest of your body.

So, why do we yawn? While there have been many theories behind the significance of this phenomenon, the answer is: it’s still a mystery.
I’m out of gas. Or… am I?
A widespread theory (one that you may have heard of before) as to why people yawn is that it’s an involuntary mechanism to intake a large volume of oxygen after accumulating carbon dioxide in your body, especially in the brain. While there hasn’t been conclusive evidence to refute this theory, experimental results, as well as common observations, suggest that other factors are involved. For instance, documented experiments where subjects were exposed to rooms containing higher carbon dioxide/low oxygen environments didn’t alter yawning patterns, or their frequency1. Instead, as you might predict, subjects simply started breathing rapidly to compensate for the lack of oxygen in the environment, which is the natural, involuntary bodily response in an attempt for the body to quickly exchange carbon dioxide for oxygen. By the same token, undergoing heavy exercise, or being at high altitudes, which causes carbon dioxide buildup and has low oxygen content respectively, doesn’t noticeably cause more yawning. Given some of these observations, the cause of yawns and its biological function cannot simply be linked solely to carbon dioxide and oxygen levels in your body and atmosphere.
Wake Up! A Wolf is Going to Eat Us!
Alternative theories suggest that yawning evolutionarily developed amongst vertebrates as a method of communication, both to keep oneself and other members in a pack to stay alert. As you probably notice after yawning, you feel slightly more awake and alert (though, biological mechanisms for how yawning allows you to feel more awake are also unknown) and some believe that yawning signals others to also follow suit and put themselves in a similar, alert state. In wild animals, this could be used as a way to stay prepared as a pack to ready themselves against potential predator ambushes, and allow them to escape. Thus giving rise to the notion of Contagious Yawning (CY), a phenomenon where one animal’s yawn causes others in proximity to also yawn at a higher frequency.
Contagious Yawning (CY) has been observed in other non-human, mammalian species, including dogs.
Though the existence of CY in both humans and other mammals is a topic of contention in the research field (and even in an episode on Mythbusters2), you may have also anecdotally experienced it yourself. Perhaps it was in your office amongst your sleepy colleagues on Monday mornings, or during that 8am research symposium talk you had to attend, delivered by everyone’s favorite, monotone, barely audible professor.
Wanna Know How I Really Feel?
In addition to communicating danger, yawning has been proposed to be a method to convey emotion to one another. Consistent with this theory, functional MRI (fMRI) studies of subjects yawning resulted in a significant increase of neuronal firing within the ventromedial prefrontal cortex3, a portion of the brain associated with processing emotion and empathy. Additionally, the accumulation of neurotransmitters associated with both consciousness and emotion, such as serotonin, nitric oxide, and dopamine, leads to increased yawning. Recent studies also suggest that the average duration of a yawn also correlates with brain weight and, in particular, the number of cortical neurons5. These observations suggest, especially in mammalian species capable of processing emotion, yawning may have developed as a way to share feelings. Furthermore, CY may be a way to collectively empathize with one another regarding their serotonin-filled, drowsy state. As you may have also experienced (though, hopefully not), nothing really conveys emotion like a collection of big-wig professors belting out huge yawns during your research seminar talks, right?

Hopefully you’re not seeing a lot of this amongst your audience while you give your presentation… Image credit: Mr. Bean, Tiger Aspect Productions.
Excessive yawning, defined as more than once per minute4, has also been linked to several diseases, including brain tumors and strokes that affect the brain stem, Multiple Sclerosis (MS), Epilepsy, and heart disease. Now, if you’ve just read the sentence above and thought…wait, I think I yawned several times over the course of the last minute, do I have a medical condition I don’t know about? Don’t fret; you’re probably okay. Simple drowsiness and fatigue from a lack of sleep can easily induce yawns at a rate considered to be excessive, so there’s probably no need to plan out a visit to the doctor’s office just yet. Furthermore, I must note that excessive yawning isn’t a defining symptom associated with the above listed diseases, and the link between increased yawning and these conditions is not entirely clear. For instance, increased cortisol levels and elevated brain temperatures have been postulated to increase yawning in MS patients6, though there hasn’t been solid scientific evidence that link these symptoms to yawning. So go and get yourself a good night sleep or two, and try counting your yawns again.
While people have always pondered why animals yawn for generations, there is still no clear explanation for the precise function of this intriguing bodily phenomenon. Regardless of how much we know about its biological function, a full Thanksgiving meal is sure to get you yawning and eventually falling asleep!

Do you do research on yawning? What’s your theory on why animals yawn? How many times have you yawned reading this blog post? Let us know at tech@biolegend.com!

A belated happy Thanksgiving from BioLegend! Perhaps eating a lot of food these past few days, like pumpkin pie ultimately gave you Stretch-Yawning Syndrome?
References:
  1. Yawning: no effect of 3-5% CO2, 100% O2, and exercise
  2. MythBusters - Contagious Yawning
  3. Contagious Yawning and the Frontal Lobe: An fMRI Study
  4. Excessive Yawning and What You Can Do About It
  5. Yawn duration predicts brain weight and cortical neuron number in mammals
  6. Born to Yawn? Understanding Yawning as a Warning of the Rise in Cortisol Levels: Randomized Trial
  7. Pandiculation: nature's way of maintaining the functional integrity of the myofascial system?
Contributed by Kenta Yamamoto, PhD.
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