Why do we yawn? Scientists claim it evolved as a social cue to warn others we are less alert that usual – and might need protection from predators
- Researcher has reviewed previously published studies to find out why we yawn
- The results suggest yawning primarily serves as a cue gained through evolution
- Thousands of years ago a yawn would have warned others that we are less alert
- Contagious yawning may have spread this message among social animal groups
Many theories have been proposed to explain why animals yawn, such as replenish oxygen supplies, cool the brain and even stretch the lungs.
Now, a new study claims yawning evolved as a social cue to warn others that we are less alert, and so they need to be extra vigilant in the lookout for predators.
Meanwhile, the phenomenon known as ‘contagious yawning’ – reflexively yawning after seeing or hearing another individual yawn – is thought to have further spread this cue among groups of social animals.
Seeing as this behaviour evolved on the plains of Africa thousands of years ago and no longer applies to modern-day humans, it’s possible the yawn could die out.
Previous analysis has already suggested a positive correlation between yawn duration and brain size meaning the bigger the brain, the bigger the yawn.
Yawning is a shared trait across multiple species, but until recently little has been known about the actual function of a yawn, according to a researcher at the State University of New York Polytechnic Institute
WHAT IS A YAWN?
A yawn is a reflex involving the simultaneous inhalation of air and stretching of the eardrums, following by the exhalation of the air.
It is most commonly associated with being tired and happens before and after sleep, although tedious activities can trigger a yawn, as can a yawn.
Most vertebrates have been known to yawn, and a contagious yawn has been demonstrated in humans, chimps, dogs, cats, birds and reptiles, even between different species.
There have been multiple explanations for why we yawn and why they seem to be contagious.
A recent study suggests that longer yawns are linked to larger and more active brains that get hotter.
This led the team to suggest that we yawn to cool our brains down.
Rather than tiredness or inattentiveness we are cooling our minds and preparing for the next activity.
Another study suggests that contagious yawning evolved as a mechanism to keep animals, particularly prey animals, alert.
The study was conducted by Professor Andrew C. Gallup, a behavioural sciences researcher at the State University of New York Polytechnic Institute, and published in the journal Animal Behaviour.
‘Yawning is a neurophysiological adaptation that is omnipresent across vertebrates, and the detection of this action pattern in others appears to be biologically important among social species,’ he says in his paper.
‘[It] serves as a cue that enhances individual vigilance and promotes motor synchrony through contagion.’
Yawning is a shared trait across multiple species, but until recently little has been known about the actual function of a yawn.
To learn more, Professor Gallup conducted a review of previously-published scientific studies to assess the causes and consequences of yawning in animal groups.
He explored the ‘psychological and social significance’ of yawning in mammals and birds.
According to the research, recent studies have presented the idea of yawning being a type of social cue, warning observers that the yawning individual is less alert.
Yawns, he says in his paper, can be either spontaneous or contagious – the former seemingly coming from nowhere, while the latter resulting in seeing someone else yawn.
By definition, every contagious yawn can be traced back to an original spontaneous yawn, and for this reason, contagious yawning must have evolved more recently in time.
‘Evidence suggests yawning evolved originally as a spontaneous event, and therefore physiological in nature,’ Professor Gallup said.
Elephants can ‘catch a yawn’ from humans they know well, according to a 2020 study.
Researcher Zoe Rossman from the University of New Mexico worked with a herd of captive elephants at the Knysna elephant park in South Africa.
Handlers who yawned in front of African elephants found they were more likely to yawn if they saw a human do so than any other type of human mouth movement.
While spontaneous yawning is common across all vertebrate classes, contagious yawning is less common and has been observed only in a few species of animal.
‘Contagious yawning emerged thereafter, and has only been documented in social species.’
Contagious yawning has only been documented in social species – those that are genetically inclined to group together – and and does not develop until after infancy.
According to Professor Gallup, numerous hypotheses have been proposed to explain the physiological significance of yawning, but most lack empirical support or have been proven wrong.
For example, there’s a common but incorrect belief that yawning functions to equilibrate blood oxygen levels.
But experiments on human subjects have demonstrated that yawn frequency is not altered by breathing enhanced or decreased levels of oxygen or carbon dioxide.
‘It has therefore been concluded that yawning and breathing are controlled by different mechanisms, and it is now widely accepted in the scientific literature that respiration is not a necessary component of yawning,’ Professor Gallup says.
Last year, researchers from Utrecht University said yawning helps to cool the brain and does not function to oxygenate our blood.
They collected over 1,250 yawns from more than 100 species of mammals and birds by visiting zoos with cameras.
Other studies on humans, non-human primates, rats and birds have all showed that yawn frequency can be reliably manipulated by changes in ambient temperature, offering support to this argument.
Professor Gallup does not think previous yawning theories are necessarily wrong, and acknowledges such physiological functions of yawning.
But overall, current evidence suggests that yawning primarily serves ‘as a cue rather than as a signal’, according to the author.
‘Future studies could further examine whether spontaneous yawns evolved specifically to communicate internal states and/or alter the behaviour of observers in some species,’ he says.
Yawning is contagious for lions, too! Animals in South Africa found to mimic behaviors to help group cohesion
Some animals, including humans, will start yawning after someone else nearby yawns, and a new study suggests this ‘contagious yawning’ helps fuel group cohesion.
Lions can trigger each others’ yawns, just like humans. But while its believed ‘contagious yawning’ in humans is a sign of empathy, experts say for lions its a way to sync up behaviors and be a more cohesive pride
Researchers observing lions in South Africa found the animals do not just mimic each other’s yawns, they would copy subsequent behaviors.
If one lion yawned, then got up and moved somewhere else, another cat was almost sure to do the same.
Scientists believe such synchronized behavior enables the pride to work as a team, finding food and spotting threats to the group.
Animals yawn for different reasons, according to researchers from the University of Pisa—sometimes it’s a transition state from awake to asleep, other times it’s a reaction to ‘high social tension.’
But a number of social animals — from wolves to birds to monkeys — have been observed engaging in ‘contagious yawning,’ in which one member of a group triggers another’s yawn.
‘Owing to their high social cohesion and synchronized group activity, wild lions are a good model to investigate both spontaneous yawning from the physiological domain and, possibly, contagious yawning, from the social communicative domain,’ the scientists wrote in a report published in the journal Animal Behavior.
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