你或许会注意到，聚会的一群人中只要有一个人打了哈欠，其他的人也会接二连三地开始打哈欠，甚至你家的宠物狗狗也会被你的哈欠传染。人为什么会打哈欠，打哈欠又为什么会传染呢？
You know the feeling. It’s impossible to resist. You just need to yawn.
你知道这种感觉。那种冲动很难抗拒，你就是想打个哈欠。
A yawn consists of an extended gaping of the mouth followed by a more rapid closure.
打哈欠时，人们张大嘴巴，之后以更快的速度闭上嘴。
In mammals and birds, a long intake of breath and shorter exhale follows the gaping of the mouth, but in other species such as fish, amphibians and snakes there is no intake of breath.
哺乳动物和鸟类张嘴打哈欠会吸入一大口气，之后短暂呼气。但其他动物打哈欠时并不吸气，比如鱼类、两栖动物和蛇。
But why does it occur?
但哈欠是怎么来的呢？
In the past, people have had many hypotheses. As far back as 400 BC, Hippocrates thought yawning removed bad air from the lungs before a fever.
在过去，人们有很多假想。在公元前400年，古希腊名医希波克拉底认为，发烧之前，哈欠会把废气从肺部排出。
In the 17th and 18th century, doctors believed yawning increased oxygen in the blood, blood pressure, heart rate and blood flow itself.
在17和18世纪，医生们认为哈欠可以增加血液中的氧气、使血压和心率上升、促进血流。
More recently, consensus moved toward the idea that yawning cools down the brain, so when ambient conditions and temperature of the brain itself increase, yawning episodes increase.
近年来，更多人认为哈欠会让大脑冷静，所以当外界条件趋于紧张，大脑温度增加时，打哈欠的频率也会增加。
Despite all these theories, the truth is that scientists do not know the true biological function of a yawn.
虽说有这么多的理论，但真相是，科学家并不知道哈欠真实的生物功能。
What we do know is that yawning occurs in just about every species. It happens when an animal is tired. It can be used as a threat display in some species.
我们只知道，几乎每种动物都会打哈欠。动物疲劳的时候会打哈欠。一些动物用哈欠作为威胁。
Yawning can occur during times of social conflict and stress, something researchers call a displacement behavior.
哈欠还可以在遇到社会冲突和压力时产生。一些研究人员称之为替代行为。
And that wide-open mouth can be contagious, especially in social species such as humans, chimpanzees, bonobos, macaques and wolves.
而且，打哈欠会传染，特别是在社会型动物中，比如人类、黑猩猩、倭黑猩猩、猕猴和狼。
Watching someone yawn - heck, even reading about yawns - can lead you to yawn yourself. Why?
看到别人打哈欠，甚至读到打哈欠的情节，自己也会打哈欠，这又是为什么呢？
Research on humans tell us that people who are more empathetic tend to be more susceptible to contagious yawning. When you see someone else yawn, the networks in your brain responsible for empathy and social skills are activated.
关于人类的研究告诉我们，越有同理心的人越容易被哈欠传染。当你看到别人打哈欠，你的大脑中负责共鸣和社交技能的神经网络就会被激活。
Is yawning contagious for dogs, too? In 2011, UK biologists tested for contagious yawning between people and man’s best friend.
打哈欠可以传染给狗吗？2011年，英国生物学家对人和狗之间打哈欠是否传染进行了试验。
Although five of the 19 dogs they studied did yawn in response to an unfamiliar person’s yawn, the researchers couldn’t prove the yawns were contagious.
尽管在参与测试的19只狗中，有5只的确受到陌生人打哈欠的行为感染也打了哈欠，但研究人员仍不能证实这种传染性。
In 2013, cognitive and behavioral scientists at the University of Tokyo once again tested contagious yawning in canines while controlling for stress.
2013年，东京大学认知和行为学家在控制压力的条件下再次测试了狗是否会被打哈欠传染。
This time the researchers found that dogs were more likely to yawn in response to a familiar person. They concluded that dogs can “catch” a yawn from humans and that yawning is a social rather than a stress-based behavior.
这次研究人员发现，狗在见到熟悉的人打哈欠时更容易被传染。他们得出结论称，狗会被人打哈欠的行为传染，打哈欠是一种社会行为，而非基于压力的行为。
In 2017, University of Nebraska psychologists looked at contagious yawning in shelter dogs.
2017年，内布拉斯加大学的心理学家观察了收容所的狗打哈欠传染的现象。
They found that some dogs that yawned when exposed to human yawning had elevated cortisol levels - a proxy for stress.
他们发现，被人打哈欠传染的狗皮质醇水平升高，皮质醇是一种压力荷尔蒙。
Levels of the cortisol stress hormone did not rise in dogs that didn’t yawn in response to a human yawn.
看到人打哈欠时毫无反应的狗狗体内皮质醇压力荷尔蒙水平并未升高。
This finding suggests some dogs find human yawning stressful and others do not. More research is needed to evaluate this aspect of the human-dog relationship.
这一发现表明，一些狗狗认为人打哈欠给它们带来了压力，而另一些则没有。我们还需要更多的研究来评估人与狗之间在打哈欠时产生的关联。
So the jury’s still out on the true why of yawning. But when it comes to inter-species yawning, you can collect your own anecdotal data.
所以，关于打哈欠的原因目前还没有定论。但是若论到打哈欠互相传染，你可以从自己的生活中搜集一些证据。
Try an experiment at home: Yawn and see if your pet yawns back.
可以在家里做个小试验：打个哈欠，看看你的宠物会不会也回给你一个哈欠。