A rottweiler (King Kong) yawning in Mauritius.
|Biological system||Nervous system|
|Health||Unaffected or Beneficial|
|Method||Complete extension of jaw, inhalation, eyes close, stretching of the eardrums, exhalation|
Yawning is commonly associated with tiredness, stress, overwork, lack of stimulation and boredom, though recent studies show it may be linked to the cooling of the brain. In humans, yawning is often triggered by others yawning (e.g., seeing a person yawning, talking to someone on the phone who is yawning) and is a typical example of positive feedback. This "infectious" yawning has also been observed in chimpanzees and dogs.
Yawn comes from Old English 'Ginian' and 'Gionian' meaning to "Open the mouth wide, gape," which in turn comes from the Proto-Germanic base gin-.
Proposed causes 
There are a number of theories that attempt to explain why animals and people yawn. It is likely that there are a number of triggers for the behavior. However, there are a few theories that attempt to explain the primary evolutionary reason for the yawn. None of them has been empirically substantiated.
One study states that yawning occurs when one's blood contains increased amounts of carbon dioxide and therefore becomes in need of the influx of oxygen (or expulsion of carbon dioxide) that a yawn can provide. Yawning may in fact reduce oxygen intake compared to normal respiration. But giving people additional oxygen didn't decrease yawning, and decreasing the amount of carbon dioxide in a subject's environment also didn't prevent yawning.
Another speculated reason for yawning is the desire to stretch one's muscles. The tongue and throat are some of the most overused muscles in the body, used for speech, breathing and swallowing. The need to stretch these muscles and refresh them arises just like in other muscles of the body, to ensure that they do not cramp. Yawns are often accompanied by the urge to stretch. Prey animals must be ready to physically exert themselves at any given moment. There have been studies that suggest yawning, especially psychological "contagious" yawning, may have developed as a way of keeping a group of animals alert. If an animal is drowsy or bored, it may not be as alert as it should be to spring into action. Therefore, the "contagious" yawn could be an instinctual reaction to a signal from one member of the group reminding the others to stay alert.
The desire to yawn when one is nervous or afraid is linked to the 'flight or fight' response and is related to the amount of adrenalin in the muscles and the readiness to use it. If one is in this aroused adrenal state and the adrenalin isn't used, yawning results as a way of stretching and using the muscles and expending this excess of adrenalin. This is a cycle, so the yawning does not diminish until the 'danger' has passed or the 'fight' has occurred. One way or another the adrenalin must be expended.
Nervousness has also been suggested as a possible reason. Nervousness often indicates the perception of an impending need for action. Anecdotal evidence suggests that yawning helps increase the state of alertness of a person. Paratroopers have been noted to yawn in the moments before they exit the aircraft.
Another notion states that yawning is the body's way of controlling brain temperature. In 2007, researchers including a professor of psychology from the University of Albany proposed that yawning may be a means to keep the brain cool. Mammalian brains operate best within a narrow temperature range. In two experiments, they demonstrated that both subjects with cold packs attached to their foreheads and subjects asked to breathe strictly nasally exhibited reduced contagious yawning when watching videos of people yawning. A similar recent hypothesis is that yawning is used for regulation of body temperature. Similarly, a study by Jared Guttmann at Worcester Polytechnic Institute found that when a subject wearing earplugs yawned, a breeze is heard caused by the flux of the air moving between the subject's ear and the environment. Researcher Guttmann determined that a yawn causes one of three possible situations to occur: the brain cools down due to an influx or outflux of oxygen, the pressure in the brain is reduced by an outflux of oxygen, or the pressure of the brain is increased by an influx of air caused by increased cranial space.
Another hypothesis is that yawns are caused by the same chemicals (neurotransmitters) in the brain that affect emotions, mood, appetite, and other phenomena. These chemicals include serotonin, dopamine, glutamic acid, and nitric oxide. As more (or less) of these compounds are activated in the brain, the frequency of yawning increases. Conversely, a greater presence in the brain of opioid neurotransmitters such as endorphins reduces the frequency of yawning. Individuals in opioid withdrawal exhibit a greatly increased frequency of yawning. Patients taking the selective serotonin reuptake inhibitors Paxil (paroxetine HCl) or Celexa (citalopram) have been observed yawning more often. Excessive yawning is more common during the first three months of taking the SSRIs. Anecdotal reports by users of psilocybin mushrooms often describe a marked stimulation of yawning while intoxicated, often associated with excess lacrimation (tearing) and nasal mucosal stimulation, especially while "peaking" (undergoing the most intense portion of the psilocybin experience). While opioids have been demonstrated to reduce this yawning and lacrimation provoked by psilocybin, it is not clear that the same pathways that induce yawning as a symptom of opioid abstinence in habituated users are the mode of action in yawning in mushroom users. While even opioid-dependent users of psilocybin on stable opioid therapy often report yawning and excess lacrimation while undergoing this entheogenic mushroom experience, there are no reports on mushrooms in the literature regarding habituated users[clarification needed] experiencing other typical opioid withdrawal symptoms such as cramping, physical pain, anxiety, gooseflesh, etc.
Recent research carried out by Garrett Norris, a lecturer in psychology at the University of Leeds, involving monitoring the behavior of students kept waiting in a reception area indicates a connection (supported by neuro-imaging research) between empathic ability and yawning. "We believe that contagious yawning indicates empathy. It indicates an appreciation of other peoples' behavioral and physiological state." says Norris.
|This section requires expansion. (February 2013)|
It is also possible that yawning is a low-impact territorial reflex. Usually being associated with boredom or lack of interest, yawning is often displayed when the subject is faced with a worrying or dangerous situation. Therefore, to yawn in the presence of a rival for territory would portray the subject as unthreatened by the rival in order to deter it from entering the subject's territory. This behavior is seen in many primates as well as some feline species. This type of response is related to the ethological phenomenon of displacement activity.
Another proposed theory is that yawning has evolved to give sleeping animals a better chance of hearing approaching predators. Because yawning stretches the eardrums, the listener can often hear quieter sounds after a yawn, and since animals rely on their hearing while sleeping, this may have provided an evolutionary advantage.
To look at the issue in terms of a possible evolutionary advantage, yawning might be a herd instinct. For example, theories suggest that the yawn serves to synchronize mood in gregarious animals, similar to the howling of the wolf pack. It signals tiredness to other members of the group in order to synchronize sleeping patterns and periods.
The yawn reflex has long been observed to be contagious. In 1508, Erasmus wrote, "One man's yawning makes another yawn.," and the French proverbialized the idea to "Un bon bâilleur en fait bâiller sept." ("One good gaper makes seven others gape"). Often, if one person yawns, this may cause another person to "empathetically" yawn. Observing another person's yawning face (especially his/her eyes), even reading, or thinking about yawning, or looking at a yawning picture can cause a person to yawn. The proximate cause for contagious yawning may lie with mirror neurons in the frontal cortex of certain vertebrates, which, upon being exposed to a stimulus from conspecific (same species) and occasionally interspecific organisms, activates the same regions in the brain. Mirror neurons have been proposed as a driving force for imitation which lies at the root of much human learning such as language acquisition. Yawning may be an offshoot of the same imitative impulse.
A 2007 study found that young children with autism spectrum disorders do not increase their yawning frequency after seeing videos of other people yawning, in contrast to typically developing children. In fact, the autistic children actually yawned less during the videos of yawning than during the control videos. This supports the claim that contagious yawning is related to empathic capacity.
The relationship between yawn contagion and empathy is strongly supported by a 2011 behavioral study, conducted by Ivan Norscia and Elisabetta Palagi (University of Pisa, Italy). The study revealed that - among other variables such as nationality, gender, and sensory modality - only social bonding predicted the occurrence, frequency, and latency of yawn contagion. As with other measures of empathy, the rate of contagion was found to be greatest in response to kin, then friends, then acquaintances, and lastly strangers. Related individuals (r≥0.25) showed the greatest contagion, in terms of both occurrence of yawning and frequency of yawns. Strangers and acquaintances showed a longer delay in the yawn response (latency) compared to friends and kin. Hence, yawn contagion appears to be primarily driven by the emotional closeness between individuals.
Yawning has been observed among various primates. In these cases the yawn is a threat gesture, a way of maintaining order in the primates' social structure. Specific studies were conducted on chimpanzees and stumptail macaques. A group of these animals was shown a video of other conspecifics yawning; both species yawned as well. This helps to partly confirm a yawn's "contagiousness".
The Discovery Channel's show Mythbusters also tested this concept. In their small-scale, informal study they concluded that yawning is contagious.
Gordon Gallup, who hypothesizes that yawning may be a means of keeping the brain cool, also hypothesizes that "contagious" yawning may be a survival instinct inherited from our evolutionary past. "During human evolutionary history, when we were subject to predation and attacks by other groups, if everybody yawns in response to seeing someone yawn the whole group becomes much more vigilant and much better at being able to detect danger."
A study by the University of London has suggested that the "contagiousness" of yawns by a human will pass to dogs. The study observed that 21 of 29 dogs yawned when a stranger yawned in front of them, but did not yawn when the stranger only opened his mouth. A recent study from Lund University showed that dogs, like humans, develop a susceptibility to contagious yawning gradually, and that while dogs above 7 months 'catch' yawns from humans, younger dogs are immune to contagion. The study also indicated that nearly half of the dogs responded to the human's yawn by becoming relaxed and sleepy, suggesting that the dogs copied not just the yawn, but also the physical state that yawns typically reflect.
Animal yawning 
In animals, yawning can serve as a warning signal. For example, Charles Darwin, in his book The Expression of the Emotions in Man and Animals, mentioned that baboons yawn to threaten their enemies, possibly by displaying large canine teeth. Similarly, Siamese fighting fish yawn only when they see a conspecific (same species) or their own mirror-image, and their yawn often accompanies aggressive attack. Guinea pigs also yawn in a display of dominance or anger, displaying their impressive incisor teeth. This is often accompanied by teeth chattering, purring and scent marking. Adelie Penguins employ yawning as part of their courtship ritual. Penguin couples face off and the males engage in what is described as an "ecstatic display," opening their beaks and pointing their faces skyward. This trait has also been seen among Emperor Penguins. Researchers have been attempting to discover why these two different species share this trait, despite not sharing a habitat. Snakes yawn, both to realign their jaws after a meal and for respiratory reasons, as their trachea can be seen to expand when they do this. Dogs often yawn after seeing people yawn and when they feel uncertain. Fish can also yawn, and they increase this behavior due to lack of oxygen or excessive heat.
Some cultures lend yawning spiritual significance. An open mouth has been associated with letting good immaterial things (such as the soul) escape or letting bad ones (evil spirits) enter, and yawning may have been thought to increase these risks. Covering the mouth when yawning may have been a way to prevent such transmission. Exorcists believe that yawning can indicate that a demon or possessive spirit is leaving its human host during the course of an exorcism. Superstitions regarding the act of yawning may have arisen from concerns over public health. Polydore Vergil (c. 1470–1555), in his De Rerum Inventoribus, writes that it was customary to make the Sign of the Cross over one's mouth, since "alike deadly plague was sometime in yawning, wherefore men used to fence themselves with the sign of the cross...which custom we retain at this day."
Yawning is often perceived as implying boredom, and yawning conspicuously in another's presence has historically been a faux pas. In 1663 Francis Hawkins advised, "In yawning howl not, and thou shouldst abstain as much as thou can to yawn, especially when thou speakest". George Washington said, "If You Cough, Sneeze, Sigh, or Yawn, do it not Loud but Privately; and Speak not in your Yawning, but put Your handkerchief or Hand before your face and turn aside." These customary beliefs persist in the modern age. One of Mason Cooley's aphorisms is "A yawn is more disconcerting than a contradiction." A loud yawn may even lead to penalties for contempt of court.
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- "Fish do yawn! An occasional yawn is normal! If all the fish in an aquarium are yawning every few minutes or so, call us. This may indicate a dissolved oxygen or temperature problem." http://www.aquariumpros.com/articles/fishfacts.shtml[dead link] "Fish facts" on aquariumpros.com[unreliable source?]
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Further reading 
- Provine, Robert R. Curious Behavior: Yawning, Laughing, Hiccupping, and Beyond (Harvard University Press; 2012) 246 pages; examines the evolutionary context for humans
|Wikimedia Commons has media related to: Yawning|
- Why Do We Yawn? The unexpected truth behind the science of yawning, LiveLeak (originally aired on Good Morning America), 30 July 2007
- A Real Yawner: Causes, Concerns and Communications of the Yawn, Mary Bridget Reilly, University of Cincinnati, 23 October 2003