According to “this page from the Economist”:http://www.economist.com/printedition/displayStory.cfm?Story_ID=2424049, the prairie vole has a chemical addiction to vasopressin and oxytocin, which causes them to recognize and be rewarded (in a chemical-in-brain sense) for having close relations with a particular recognized female.
bq. Helen Fisher, a researcher at Rutgers University, and the author of a new book on love, suggests it comes in three flavours: lust, romantic love and long-term attachment. There is some overlap but, in essence, these are separate phenomena, with their own emotional and motivational systems, and accompanying chemicals. These systems have evolved to enable, respectively, mating, pair-bonding and parenting.
bq. Because they are independent, these three systems can work simultaneously–with dangerous results. As Dr Fisher explains, “you can feel deep attachment for a long-term spouse, while you feel romantic love for someone else, while you feel the sex drive in situations unrelated to either partner.” This independence means it is possible to love more than one person at a time, a situation that leads to jealousy, adultery and divorce–though also to the possibilities of promiscuity and polygamy, with the likelihood of extra children, and thus a bigger stake in the genetic future, that those behaviours bring. As Dr Fisher observes, “We were not built to be happy but to reproduce.”
bq. And although both men and women express romantic love with the same intensity, and are attracted to partners who are dependable, kind, healthy, smart and educated, there are some notable differences in their choices. Men are more attracted to youth and beauty, while women are more attracted to money, education and position.