关于爱情的科学传说非常无辜地始于田鼠。草原田鼠作为一种社会性生物，它是仅存3%的“一夫一妻制”哺乳动物中的一类。对“恋爱”中的草原田鼠来说，交配是一项耗时24小时的宏伟工程。一旦“结婚”，小俩口便终生相伴，直到天荒地老。“夫妻俩”宁愿在一起共度时光，为彼此梳理毛发，同筑爱巢也不去搞“婚外恋”。平时，“丈夫”是“妻子”好斗的“护花使者”，而幼崽一旦出生，它们又即刻成为挚爱并专一的父母。然而，另一种被称作山区田鼠的，作为草原田鼠近亲，除了一夜情之外，它们对稳定的伴侣关系毫无兴趣。令人不解的是这些行为中的巨大差异却仅仅源于一小撮基因。如就基因而言，这两个物种超过99％的部分绝无二致。

Why do voles fall in love? / 为什么田鼠会坠入爱河？

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.

像往常一样，最让人着魔的是田鼠爱情故事的进展细节。当草原田鼠性交时，其体内会释放两种称作催产素和抗利尿激素的荷尔蒙。如果这些荷尔蒙的释放被阻断，草原田鼠的性生活便成了短暂的艳遇，它们就会像生性放荡的山区堂兄那样去尽享受风流韵事。 相反，如果给草原田鼠注射以上荷尔蒙，虽然阻止它们性交，它们依然会钟情于已选择的伴侣。换句话说, 不过就一剂注射，研究者们便能让草原田鼠落入情网，不管草原田鼠的感觉如何，反正它们会产生与爱相类似的神经反应。

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?

研究者找到一条与正在发生情形相关的线索，这一线索与如何使上述结果作用于人类有关。线索的结论是：当把这一魔术般的汁液注入山区田鼠体内，其反应与草原田鼠如出一辙。这就证实了，在忠诚的草原田鼠大脑内，与奖赏与强化相关联的区域中，具有一种催产素和抗利尿激素的荷尔蒙受体, 然而山区田鼠却没有。 问题是: 人类——据称是3%实行一夫一妻制的哺乳动物中的另一物种，是否也具有和草原田鼠相似的大脑结构？

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. ()