You know the feeling. It’s impossible to resist. You just need to yawn.
你知道這種感覺。那種衝動很難抗拒，你就是想打個哈欠。
A yawn consists of an extended gaping of the mouth followed by a more rapid closure.
打哈欠時，人們張大嘴巴，之後以更快的速度閉上嘴。
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.
哺乳動物和鳥類張嘴打哈欠會吸入一大口氣，之後短暫呼氣。但其他動物打哈欠時並不吸氣，比如魚類、兩棲動物和蛇。
But why does it occur?
但哈欠是怎麼來的呢？
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.
在過去，人們有很多假想。在公元前400年，古希臘名醫希波克拉底認為，發燒之前，哈欠會把廢氣從肺部排出。
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世紀，醫生們認為哈欠可以增加血液中的氧氣、使血壓和心率上升、促進血流。
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.
近年來，更多人認為哈欠會讓大腦冷靜，所以當外界條件趨於緊張，大腦溫度增加時，打哈欠的頻率也會增加。
Despite all these theories, the truth is that scientists do not know the true biological function of a yawn.
雖說有這麼多的理論，但真相是，科學家並不知道哈欠真實的生物功能。
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.
我們只知道，幾乎每種動物都會打哈欠。動物疲勞的時候會打哈欠。一些動物用哈欠作為威脅。
Yawning can occur during times of social conflict and stress, something researchers call a displacement behavior.
哈欠還可以在遇到社會衝突和壓力時產生。一些研究人員稱之為替代行為。
And that wide-open mouth can be contagious, especially in social species such as humans, chimpanzees, bonobos, macaques and wolves.
而且，打哈欠會傳染，特別是在社會型動物中，比如人類、黑猩猩、倭黑猩猩、獼猴和狼。
Watching someone yawn - heck, even reading about yawns - can lead you to yawn yourself. Why?
看到別人打哈欠，甚至讀到打哈欠的情節，自己也會打哈欠，這又是為什麼呢？
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.
關於人類的研究告訴我們，越有同理心的人越容易被哈欠傳染。當你看到別人打哈欠，你的大腦中負責共鳴和社交技能的神經網絡就會被激活。
Is yawning contagious for dogs, too? In 2011, UK biologists tested for contagious yawning between people and man’s best friend.
打哈欠可以傳染給狗嗎？2011年，英國生物學家對人和狗之間打哈欠是否傳染進行了試驗。
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.
儘管在參與測試的19隻狗中，有5隻的確受到陌生人打哈欠的行為感染也打了哈欠，但研究人員仍不能證實這種傳染性。
In 2013, cognitive and behavioral scientists at the University of Tokyo once again tested contagious yawning in canines while controlling for stress.
2013年，東京大學認知和行為學家在控制壓力的條件下再次測試了狗是否會被打哈欠傳染。
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.
這次研究人員發現，狗在見到熟悉的人打哈欠時更容易被傳染。他們得出結論稱，狗會被人打哈欠的行為傳染，打哈欠是一種社會行為，而非基於壓力的行為。
In 2014, University of Nebraska psychologists looked at contagious yawning in shelter dogs.
2014年，內布拉斯加大學的心理學家觀察了收容所的狗打哈欠傳染的現象。
They found that some dogs that yawned when exposed to human yawning had elevated cortisol levels - a proxy for stress.
他們發現，被人打哈欠傳染的狗皮質醇水平升高，皮質醇是一種壓力荷爾蒙。
Levels of the cortisol stress hormone did not rise in dogs that didn’t yawn in response to a human yawn.
看到人打哈欠時毫無反應的狗狗體內皮質醇壓力荷爾蒙水平並未升高。
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.
這一發現表明，一些狗狗認為人打哈欠給它們帶來了壓力，而另一些則沒有。我們還需要更多的研究來評估人與狗之間在打哈欠時產生的關聯。
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，關於打哈欠的原因目前還沒有定論。但是若論到打哈欠互相傳染，你可以從自己的生活中搜集一些證據。
Try an experiment at home: Yawn and see if your pet yawns back.
可以在家裡做個小試驗：打個哈欠，看看你的寵物會不會也回給你一個哈欠。