The details of what is going on—the vole story, as it were—is a fascinating one. When prairie voles have sex, two hormones called oxytocin and vasopressin are released. If the release of these hormones is blocked, prairie-voles' sex becomes a fleeting affair, like that normally enjoyed by their rakish montane cousins. Conversely, if prairie voles are given an injection of the hormones, but prevented from having sex, they will still form a preference for their chosen partner. In other words, researchers can make prairie voles fall in love—or whatever the vole equivalent of this is—with an injection.

像往常一樣，最讓人著魔的是田鼠愛(ài)情故事的進(jìn)展細(xì)節(jié)。當(dāng)草原田鼠性交時(shí)，其體內(nèi)會(huì)釋放兩種稱作催產(chǎn)素和抗利尿激素的荷爾蒙。如果這些荷爾蒙的釋放被阻斷，草原田鼠的性生活便成了短暫的艷遇，它們就會(huì)像生性放蕩的山區(qū)堂兄那樣去盡享受風(fēng)流韻事。 相反，如果給草原田鼠注射以上荷爾蒙，雖然阻止它們性交，它們依然會(huì)鐘情于已選擇的伴侶。換句話說(shuō), 不過(guò)就一劑注射，研究者們便能讓草原田鼠落入情網(wǎng)，不管草原田鼠的感覺(jué)如何，反正它們會(huì)產(chǎn)生與愛(ài)相類似的神經(jīng)反應(yīng)。

A clue to what is happening—and how these results might bear on the human condition—was found when this magic juice was given to the montane vole: it made no difference. It turns out that the faithful prairie vole has receptors for oxytocin and vasopressin in brain regions associated with reward and reinforcement, whereas the montane vole does not. The question is, do humans (another species in the 3% of allegedly monogamous mammals) have brains similar to prairie voles?

研究者找到一條與正在發(fā)生情形相關(guān)的線索，這一線索與如何使上述結(jié)果作用于人類有關(guān)。線索的結(jié)論是：當(dāng)把這一魔術(shù)般的汁液注入山區(qū)田鼠體內(nèi)，其反應(yīng)與草原田鼠如出一轍。這就證實(shí)了，在忠誠(chéng)的草原田鼠大腦內(nèi)，與獎(jiǎng)賞與強(qiáng)化相關(guān)聯(lián)的區(qū)域中，具有一種催產(chǎn)素和抗利尿激素的荷爾蒙受體, 然而山區(qū)田鼠卻沒(méi)有。 問(wèn)題是: 人類——據(jù)稱是3%實(shí)行一夫一妻制的哺乳動(dòng)物中的另一物種，是否也具有和草原田鼠相似的大腦結(jié)構(gòu)？

To answer that question you need to dig a little deeper. As Larry Young, a researcher into social attachment at Emory University, in Atlanta, Georgia, explains, the brain has a reward system designed to make voles (and people and other animals) do what they ought to. Without it, they might forget to eat, drink and have sex—with disastrous results. That animals continue to do these things is because they make them feel good. And they feel good because of the release of a chemical called dopamine into the brain. Sure enough, when a female prairie vole mates, there is a 50% increase in the level of dopamine in the reward centre of her brain.

為對(duì)上述疑惑刨根問(wèn)底，就需要“挖”得更深一些。一位來(lái)自佐治亞州，亞特蘭大Emory大學(xué)，研究社會(huì)附屬關(guān)系的學(xué)者Larry Yong，他對(duì)此的解釋是，田鼠(以及人類和其他動(dòng)物)的大腦內(nèi)具有一套獎(jiǎng)賞系統(tǒng)用以鼓勵(lì)它(他)們?nèi)プ錾�物�?yīng)該完成的行為。如若不然，動(dòng)物將忘記進(jìn)食，飲水和性行為，從而招致災(zāi)難性的后果。動(dòng)物們不斷重復(fù)這些行為是因?yàn)槟鞘顾鼈兏械娇鞓?lè)。這種快感是一種稱作多巴胺的化學(xué)物質(zhì)在大腦中的釋放使然。當(dāng)雌性草原田鼠交配時(shí)，在大腦獎(jiǎng)賞系統(tǒng)中樞，多巴胺水平會(huì)有50%的上升，而這已完全足夠讓這些“女士”們產(chǎn)生上面提到的那種快感。

Similarly, when a male rat has sex it feels good to him because of the dopamine. He learns that sex is enjoyable, and seeks out more of it based on how it happened the first time. But, in contrast to the prairie vole, at no time do rats learn to associate sex with a particular female. Rats are not monogamous.

與草原田鼠近似，雄性家鼠性交時(shí)，因多巴胺分泌同樣會(huì)感到快樂(lè)。當(dāng)它體驗(yàn)到性讓它感到愉悅，便會(huì)參照初次經(jīng)歷去搜尋更多機(jī)會(huì)。但與草原田鼠相反，家鼠決不會(huì)學(xué)習(xí)把性與某個(gè)特定的雌性個(gè)體關(guān)聯(lián)起來(lái)。畢竟家鼠不是“一夫一妻制”的哺乳動(dòng)物。

This is where the vasopressin and oxytocin come in. They are involved in parts of the brain that help to pick out the salient features used to identify individuals. If the gene for oxytocin is knocked out of a mouse before birth, that mouse will become a social amnesiac and have no memory of the other mice it meets. The same is true if the vasopressin gene is knocked out.

抗利尿激素和催產(chǎn)素就是從這里進(jìn)入這個(gè)科學(xué)傳說(shuō)的。它們參與了大腦一些部分的工作，以幫助選出用于辨別個(gè)體的顯著特征。 如果在老鼠出生前，DNA中的抗利尿激素基因即被剔除, 那只老鼠將會(huì)成為一個(gè)社會(huì)性失憶個(gè)體，它也不會(huì)對(duì)遇到的其他老鼠留下任何印象。如果催產(chǎn)素基因缺損，以上命題同樣成立。

The salient feature in this case is odour. Rats, mice and voles recognise each other by smell. Christie Fowler and her colleagues at Florida State University have found that exposure to the opposite sex generates new nerve cells in the brains of prairie voles—in particular in areas important to olfactory memory. Could it be that prairie voles form an olfactory “image” of their partners—the rodent equivalent of remembering a personality—and this becomes linked with pleasure?

這里的顯著特征是氣味。耗子、老鼠和野鼠靠嗅覺(jué)區(qū)分彼此。Christie Fowler和她的同事在佛羅里達(dá)州大學(xué)的研究中發(fā)現(xiàn)，把草原田鼠暴露給異性可使其大腦產(chǎn)生新的神經(jīng)細(xì)胞——在對(duì)嗅覺(jué)記憶至關(guān)重要的一些區(qū)域尤其如此。人類是靠個(gè)性特征來(lái)記住某一個(gè)人的。那么草原田鼠會(huì)構(gòu)造一個(gè)嗅覺(jué)“形像”去記住伴侶嗎？這與性愉悅是否存在關(guān)聯(lián)呢？

Dr Young and his colleagues suggest this idea in an article published last month in the Journal of Comparative Neurology. They argue that prairie voles become addicted to each other through a process of sexual imprinting mediated by odour. Furthermore, they suggest that the reward mechanism involved in this addiction has probably evolved in a similar way in other monogamous animals, humans included, to regulate pair-bonding in them as well.

Young博士和他的同事在上個(gè)月《比較神經(jīng)學(xué)期刊》的文章中提到了這種想法。他們認(rèn)為草原田鼠通過(guò)一個(gè)由氣味做媒介的性烙印過(guò)程使配偶彼此沉溺于對(duì)方。此外，他們還提到，與這彼此沉溺上癮相關(guān)的獎(jiǎng)賞機(jī)制在其他的一夫一妻制動(dòng)物（包括人類）生理系統(tǒng)中，或許也進(jìn)化成了相似的方式，以規(guī)范他們的配偶聯(lián)結(jié)關(guān)系。