How Did Humans Change a Wolf to a Dog?人類是如何將狼變成狗的？

Most pet owners probably know what it’s like to cave to those “puppy dog” eyes—no matter the age of their canine1. When your dog looks at you with that curled brow and doleful stare， it’s difficult not to give it a loving scratch or meaty treat2. And why not： You and your furry friend have been conditioned3 by thousands of years of evolution for this moment， according to a growing body4 of research by biological anthropologists like Anne Burrows.

大部分養寵物的人可能都了解向“狗狗眼”屈服的感覺——無論狗狗年齡有多大。當狗狗皺著眉，憂郁地望向你，你很難不去給它一個愛的撓撓，或者請它吃肉肉大餐。何樂而不為呢？你和毛孩子歷經數千年的進化，才成全了這一時刻。越來越多的生物人類學家——像安妮·伯羅斯等——的研究為此提供了佐證。

“Dogs are our closest companions，” she says. “They’re not closely related to us [as a species]， but they live with us， they work with us， they take care of our children and our homes. So investigating different aspects of the dog-human bond， I thought， would help me understand human evolution and human origins.”

“狗是我們最親密的伙伴。”她說，“（從物種上來說，）它們并非人類的近親，卻與我們共同生活，一起工作，看護我們的孩子和家園。所以我認為，研究人犬關系的各個方面，能幫助我了解人類的進化和起源。”

From disarming5 looks to alarming barks， Burrows and her team at Duquesne University in Pittsburgh investigate the ways dogs have evolved to express themselves in order to earn the title of “man’s best friend.” The research group is taking a detailed anatomical6 approach to understand how dogs and their wild relatives， wolves， evolved to have different traits， such as facial expressions and vocalizations7.

從讓人卸下防備的表情到令人不安的吠叫，來自賓夕法尼亞州杜肯大學的伯羅斯及其團隊探究了狗為贏得“人類最好的朋友”這一美名，而進化出的自我表達方式。研究團隊采用了一種細致的解剖學方法，來研究狗和它那野生的近親狼是如何進化出不同特性的，比如各種面部表情和聲音。

The ancient relationship between people and canine companions can give anthropologists a window into human evolution， says Burrows. The timing is still contested8， but some 15，000 to 35，000 years ago， early Homo sapiens in parts of Europe and Siberia began to change their relationship with local wolf populations9. One theory to how that started is that a few bolder wolves began to cooperatively hunt with people for larger land game10， allowing increased success for both parties. Another is that nomads left behind remains from butchered mammals that wolves would then dine on， causing the canines to become more domestic11 （but scholars have widely debated this narrative）. “It could be something completely different， but the main hypotheses are that it somehow involved food，” says Burrows. Looking at the evolution of dogs in the context of these millennia-old interactions can show how human ancestors lived and survived in the past.

伯羅斯說，歷史悠久的人犬關系是一扇窗戶，人類學家能借此一窺人類進化。雖然具體時間尚有爭議，但大約在1.5萬至3.5萬年前，生活在歐洲和西伯利亞部分地區的早期智人，就已經開始改善與當地狼群的關系。至于是如何開始的，一個理論認為，一些膽子較大的狼開始與人類合作，狩獵體型更大的陸地動物，從而提高雙方的成功率。而另一個理論認為，狼進食了游牧民宰殺哺乳動物后留下的殘骸，因此變得更為馴化（不過，此觀點引起了學者的廣泛爭論）。“情況可能完全不同，但主流的假說都認為，這多少都與食物有關。”伯羅斯說。在數千年的人犬互動中觀察狗的進化，我們能看見人類祖先在過去是如何生活和生存的。

Burrows decided to focus on how dogs communicate with humans through their faces—a unique trait that’s rare between unrelated species， she says. This was inspired by her previous work studying facial muscles in primates. Chimps have displayed the ability to understand the facial expressions of other members of their species， similar to how humans depend on faces for context clues. In 2019， Burrows decided to hunt for similar signals between humans and dogs， and compare them to wolves. “Whether we know it or not， dogs and humans are constantly looking at one another’s face， and trying to understand what the other one is feeling， and what the other one intends，” Burrows says. “So， facial expression is our proxy12 for understanding the relationship between dogs and humans.”

伯羅斯決定重點研究狗是如何使用面部與人類溝通的。她認為，這種獨特的溝通方式在兩個沒有親緣關系的物種之間實屬罕見。該研究方向是受她從前對靈長類動物面部肌肉研究的啟發。黑猩猩表現出能夠理解其他同類面部表情的能力，與人類察言觀色相似。2019年，伯羅斯決定尋找人犬之間的類似表情信號，并將其與狼做對比。“不論我們是否意識到，狗和人都在不斷觀察彼此的臉，盡力了解對方的感受和意圖，”伯羅斯說道，“所以，我們能通過面部表情這個指標，來理解人犬關系。”

Previous studies have demonstrated that dogs can read and respond to human facial expressions—and even synchronize their emotions to match. “Dogs are watching us very closely—some of this is based on our gaze and body language， but also on the sounds we make and the scents we give off，” Monique Udell， an animal behaviorist and associate professor of animal sciences at Oregon State University in Corvallis， told National Geographic in 2021. Another study in July 2021 in the journal Current Biology found that dog puppies make more eye contact with humans than wolves， even when the wolf pups were reared by humans nearly from birth.

過去的研究已經證明，狗能夠讀懂人類表情并做出反應，甚至能調整自身情緒與之呼應。莫妮克·尤德爾是一名動物行為學家，在科瓦利斯的俄勒岡州立大學任動物科學副教授，2021年時她告訴《國家地理》雜志：“狗對我們的觀察很細致，其中，有些觀察是基于我們的眼神和身體語言，有些則是根據我們的聲音和散發的氣味。”2021年7月，另一篇發表在《當代生物學》雜志上的研究表明，相較于幼狼，幼犬與人類的眼神接觸次數更多，即便和那些幾乎生來便被人類撫育的幼狼相比，也同樣如此。

The evolutionary differences could be explained by fine-grained13 face muscles in both wolves and dogs. Burrows， along with collaborators Juliane Kaminski and Bridget Waller， found that a large range of dog breeds had defined musculature around the eye that lifts the brow up. The wolves they studied did not have the same attribute. This suggests that as humans domesticated wolves， they selected individuals with friendlier behaviors and traits， like a persuasive brow raise or smaller teeth and snouts. Since publishing these findings， Burrows has continued to collect data on other muscles that control facial expressions， known as mimetic muscles. “We know dogs’ facial expressions， but we don’t really know how their muscles work in the actual contractions，” she explains.

狼和狗的面部肌肉細密紋理可以解釋二者的進化差異。伯羅斯與其合作者朱利安娜·卡明斯基和布里奇特·沃勒發現，許多犬種的眼周都分布著清晰的肌肉組織，讓狗能夠抬起眉毛。而他們研究的狼卻沒有該特征。這說明人類在馴化狼的過程中，選擇了在行為和特性上更為友好的個體，例如，能夠令人信服地揚眉，擁有更短的牙齒和鼻子。這些研究成果發表后，伯羅斯繼續收集其他控制面部表情的肌肉（即表情肌）的數據。“我們能讀懂狗的面部表情，但它們面部肌肉的實際收縮機理，我們還沒弄明白。”她解釋道。

Humans mostly have fast-twitch muscles14 in their faces， but do have more slow-twitch muscles15 than chimpanzees， likely in part to form sounds for speech， Burrows says. Currently， her group is applying this reasoning16 and methodology to dog and wolf muscles by looking at the amount of fast- and slow-twitch fibers that control the duration and speed of contractions. Fast-twitch fibers allow for more spontaneity， but also tire more easily （think of smiling for a long period of time）; slow-twitch fibers take longer to start contracting， but are better for endurance （think of sustained walking or running）.

人類臉上的肌肉以快縮肌為主，但相較于黑猩猩，人類臉上的慢縮肌仍舊要多一些，部分原因可能是人類需要發聲講話，伯羅斯說。她的團隊正將此推斷和方法應用于狗和狼的肌肉研究中，即關注快縮型肌纖維和慢縮型肌纖維的數量。這兩種肌肉纖維控制肌肉收縮的時長和速度，快縮型肌纖維的自發性更強，但也容易疲憊（試想一下長時間微笑），而慢縮型肌纖維的啟動時間較長，耐力卻更好（如長時間走路或長跑）。

For their latest research， Burrows and her graduate students sampled cross sections of facial muscles in humans， dogs， and wolves and determined the amount of each fiber type. Burrows notes that the sample size of the preliminary data is small， with six wolf specimens and 10 specimens of different dog breeds. From this initial data， the team expected that the muscle profiles of dogs and humans would look similar， while the wolves would be distinct. However， they found that humans and wolves were actually more alike with more slow-twitch fibers overall， while dogs had more fast-twitch fibers.

在最新的研究中，伯羅斯及其研究生對人、狗、狼的面部肌肉橫截面進行了采樣，并測定了每種肌肉纖維的數量。伯羅斯指出，初步數據的樣本量很小，只有6只狼和10只品種不同的狗。根據這個樣本數據，研究團隊曾預測人和狗的肌肉剖面會相似，狼則不同。然而他們發現，實際上人和狼才更相似，二者面部以慢縮型肌纖維為主，而狗臉上卻是快縮型肌纖維居多。

“At first we were horrified，” says Burrows. “But as we thought about what muscle fibers of the face do， it kind of began to make a little bit more sense. Humans use speech， and that means we have to slow down our lips so that we can clearly articulate speech sounds. Wolves use howling and it’s a protracted vocalization—they kind of make a funnel out of their lips.” Meanwhile17， a dog’s bark is a much shorter vocalization， so it doesn’t require them to hold their lips in one position for an extended amount of time.

“最初我們都被嚇到了，”伯羅斯說，“但考慮到面部肌肉纖維的作用，這就開始有點道理了。人要說話，意味著我們要放慢唇動頻率才能清晰地發音。狼嚎叫，要拖長聲音，就得把嘴撅成漏斗形。”然而，狗的吠叫要短促得多，因此狗不需要長時間保持一定幅度的張嘴。

The findings have led Burrows to suspect that humans may have favored wolves that had shorter staccato18 vocalizations during the process of dog domestication. Anthropologists have suggested that as humans domesticated dogs， they sought animals that could guard or warn them of any sudden threats. This alarm call—or bark—could have been important in the process of dog domestication. Now， “dogs just bark for a living，” Burrows says.

根據這些發現，伯羅斯猜測，在馴化過程中，人類或許傾向于選擇叫聲更為短促刺耳的狼。人類學家認為，人類馴化狗時，尋找的是能保護自己，或對突發威脅預警的動物。這種警報聲，也就是吠叫，在狗的馴化過程中可能很重要。而如今，“狗吠叫只是為了討生活。”伯羅斯說。

And while both animals do exhibit a range of vocalizations， they have a tendency to stick to their specific styles， Burrows says. Wolves only bark occasionally when they want to alert a nearby pack. And with the exception of certain breeds like huskies and hounds， dogs are much less partial to howls. “We seem to have kind of created this weird creature， this dog that uses vocalizations very differently than the way wolves use them，” Burrows explains.

伯羅斯表示，雖然狼和狗都展現出一系列不同的發聲方式，但他們卻傾向于保持特定的發聲風格。狼很少吠叫，只在想要警示附近狼群時才這么做，至于狗，除了哈士奇和獵犬等特定犬種，都不愛嚎叫。“某種程度上，似乎是我們造就了狗這種奇異的生物，它們使用嗓音的方式和狼截然不同。”伯羅斯解釋道。

The team is planning on completing another year’s worth19 of data collecting before publishing their next study. Personally， Burrows would like to investigate how the facial muscles of ancient dog breeds， like huskies， malamutes， and chow chows， compare to wolves， as well as younger dog breeds. The older breeds might be able to help anthropologists really break down the relationships that turned wolves into dogs.

該團隊打算在下一項研究成果發表前，先完成另一個年度的數據收集工作。就個人而言，伯羅斯想研究哈士奇、愛斯基摩犬、松獅犬等古老犬種的面部肌肉，將其和狼以及其他更為近代的犬種進行對比。古老的犬種或許能夠幫助人類學家真正破解使狼變成狗的人犬關系之謎。

“Our evolutionary history of becoming human is intimately tied to the process of dog domestication，” Burrows says. “When we look at a dog today， we see what was important to Upper Paleolithic20 people， 30，000-plus years ago. Dogs just get us in a way that no other animal does.”

“我們變成人類的進化史，與狗的馴化史密切相關。”伯羅斯說，“今天我們看著狗，能看到三萬多年前對舊石器時代晚期人類來說意義重大的東西。狗狗拿下我們的方式，是其他動物無法比擬的。”

（譯者為“《英語世界》杯”翻譯大賽獲獎者）

1 canine 〈正式〉犬，狗。 2 treat款待。 3 condition使適應，使有準備。 4 body 大量。

5 disarming使人消除戒心的。 6 anatomical解剖的；（人體或動物體）結構上的。 7 vocalization發出的聲音。 8 contest爭辯，就……提出異議。 9 population動物種群。 10 game獵物。 11 domestic馴養的。

12 proxy指標。

13 fine-grained紋理細密的。

14 fast-twitch muscle快縮肌。 15 slow-twitch muscle慢縮肌。 16 reasoning推理。

17 meanwhile與之相比，對比之下。 18 staccato（聲音）短促刺耳的。

19 worth持續的時間。 20 Upper Paleolithic舊石器時代晚期的，upper指地質或考古中“晚期的，較近期的”。