After pairing nine male voles with females and letting them bond for five days (an eternity for these short-lived rodents), researchers wrested the fledgling couples apart. Compared to voles who had not been separated—and to a control group of males who had been bonded to other males in "friendship," then separated—the voles who had been taken away from their "romantic" partners showed signs of suffering. The level of stress hormones in their blood rose sharply, they grew passive and sluggish, and their performance on "forced swim and tail suspension tests"—which are just about as dauntingly unfun for the voles as they sound—decreased quite a bit from pre-separation levels. In short, they were depressed: bonding with an opposite-sex partner had measurably changed the neurochemistry of the voles' brains. The researchers found that they could reverse this effect by administering a drug that blocked a chemical called corticotropin-releasing factor (CRF), which regulates the body's stress responses. This suggests that CRF plays a part in generating the array of emotional and physical sensations that we call grief.