The English language is not especially kind to rats. We say we "smell a rat" when something doesn't feel right, refer to stressful competition as the "rat race," and scorn traitors who "rat on" friends. But rats don't deserve their bad rap. According to a new study in the December 9 issue of Science, rats are surprisingly selfless, consistently breaking friends out of cages—even if freeing their buddies means having to share coveted chocolate. It seems that empathy and self-sacrifice have a greater evolutionary legacy than anyone expected.



In 2007 neuroscientist Peggy Mason of the University of Chicago wrote about the neurobiology of empathy for Scientific American. Inbal Ben-Ami Bartal, a new PhD student in integrative neuroscience who worked across the street from Mason in a different lab, saw the article and proposed a collaboration. "Scientific American really brought us together," Mason says.



In the new study, Mason, Bartal and University of Chicago colleague Jean Decety placed pairs of rats in Plexiglass pens. One rat was trapped in a cage in the middle of the pen, whereas the other rat was free to run around. Most free rats circled their imprisoned peer, gnawing at the cage and sticking their paws, noses and whiskers through any openings. After a week of trial and error, 23 of the 30 rats in the experiment learned to open the cage and free their peers by head-butting the cage door or leaning their full weight against the door until it tipped over. (The door could only be opened from the outside.) At first the rats were startled by the noise of the toppling door. Eventually, however, they stopped showing surprise, which suggests that they fully intended to push the door aside. Further, the rodents showed no interest in opening empty cages or in those containing toy rats, indicating that a break out was their genuine goal.



In this first set of experiments, most rats seemed quite willing to help their peers, but Mason wanted to give them a tougher test. She placed rats in a Plexiglass pen with two cages: in one was another rat, in the other was a pile of five milk chocolate chips—a favorite snack of these particular rodents. The unrestricted rats could easily have eaten the chocolate themselves before freeing their peers or been so distracted by the sweets that they would neglect their imprisoned friends. Instead, most of the rats opened both cages and shared in the chocolate chip feast.



"In our lab we called it the 'chocolate versus pal' experiment," Mason says. "The rat could have put his butt in the opening of the cage containing chocolate to block the other guy, but he didn't. They were sharing food with their pals. In rat land, that is big—I was shocked." Mason says that free rats typically took the chocolate out of the cage before eating it and that sometimes the free rats placed the chocolate chips in front of or very near their recently sprung peers, "as if delivering it."



Mason's new study is one of the most recent in a series of experiments changing how scientists think about empathy and altruism in the animal kingdom. At first, most people agreed that true altruism was a uniquely human characteristic requiring an awareness of one's actions as selfless. Now it seems that many animals have evolved instincts to help others, even at a cost to themselves, and that we inherited these same instincts. "The bottom line is that helping an individual in distress is part of our biology," Mason says. "It's not something that develops or doesn't develop because of culture."



In earlier work, McGill University psychologist Jeffrey Mogil and his colleagues showed that mice recognize their peers' pain—what researchers call "emotional contagion"—and spend more time with suffering cage mates. His team also developed a scale to measure pain expressed on the faces of mice.



Mogil was impressed with Mason's study, but had some questions about the findings. "This is surprising because it's not clear what the motivation for the prosocial behavior is, although the prosocial behavior is clearly there," says Mogil. Both Mogil and Mason point out that because trapped rats squeak out alarm calls now and then, which stress out any fellows that hear them, the rats opening the doors might be trying to silence their peers. Mason thinks the alarm calls aren't frequent enough to motivate the rats, but Mogil is not so sure.



In future research, Mason wants to investigate why some male rats never learned to break open their friends' cages. All six female rats in the first set of experiments figured out how to liberate their peers, but only 17 out of 24 male rats obliged. Mason's best guess is that some rats are paralyzed by alarm calls. Recognizing distress in another is not enough; the rats need to suppress their own panic before they can help. Mason cites research suggesting that females are generally more empathic than males as a possible explanation, but the findings are controversial.



Mogil says he plans to follow up the research as well. For starters, he is interested in whether another common lab animal—the mouse—can learn to spring its peers, too. And he wants to perform the experiments with rodents that are strangers to one another, rather than ones that have been raised together.



In the last couple decades research on empathy and helping behaviors in animals has become more prevalent. "At first people were scared away from this research because they didn't want to be derided as anthropomorphic," Mogil says. "More and more evidence is coming along that all mammals can do this sort of thing. I think fear over the word anthropomorphism is starting to subside."



If anything, recent science shows us that we are not as guilty of endowing animals with uniquely human qualities as we are of failing to understand just how many qualities animals and people share.