Empathy lets us feel another person's pain and drives us to help ease it. But is empathy a uniquely human trait? For decades researchers have debated whether nonhuman animals possess this attribute. Now a new study shows that rats will free a trapped cagemate in distress. The results mean that these rodents can be used to help determine the genetic and physiological underpinnings of empathy in people.
A few years ago, neuroscientist Jeffrey Mogil of McGill University in Montreal, Canada, reported in Science that mice possess a simple kind of empathy  called emotional contagion. They sense what another mouse is feeling and feel it themselves. For example, when one mouse receives a painful chemical injection into its paw, the mouse and its cagemate lick their paw to ease the pain.
That's a necessary first step toward empathy, but it's not sufficient, says neuroscientist Jean Decety of the University of Chicago in Illinois, a co-author of the new study. To truly empathize, one needs to understand on some level what the other individual is experiencing, as when a mother senses what's upsetting her child. Only then can she help, Decety says.
To find out whether rats can feel true empathy and act on it, Decety and his University of Chicago colleagues, neuroscientist Peggy Mason and graduate student Inbal Bartal, placed pairs of unrelated rats in plastic cages for 2 weeks so they became familiar with each other. They then put one of the rodents into a small Plexiglas container inside the cage. Using a commercial bat detector, the team showed that many of the trapped rats emitted high-pitched squeaks, indicating that they were distressed. The small container was outfitted with a door that was rigged to fall to the side when the free rat bumped or nudged it.
After running rat pairs through a week of daily testing sessions, the researchers found that three-quarters of the rats with trapped cagemates had learned how to open the door (see video), whereas only one rat in six without a trapped cagemate learned to do this. This difference showed that rats with trapped cagemates were motivated to learn how to free them.
But what motivated the rats in the first place? To find out, the Chicago team kept up daily tests on the rodents that had learned how to open the container. Each free rat kept liberating its trapped cagemate for a month, which ruled out simple curiosity as a motivation. What's more, the free rat would liberate its cagemate even when the trapped animal exited into a separate cage, which showed that the free rat wasn't simply seeking the reward of schmoozing with its friend. The rats also freed trapped cagemates even when they had the option of bumping open an identical container and obtaining five chocolate chips for themselves, which showed that their motivation to help was on par with their desire for a tasty treat. In fact, half of the time they even shared chips by leaving one or two for the trapped rat, the team reports online today in Science.
The results are the first to demonstrate that rodents take action in response to another's distress , Decety says. Similar behavior has been observed in monkeys and chimpanzees. But unlike those animals, rats can be readily used in laboratory studies to investigate which brain structures underlie empathy and helping behavior and whether empathy is acquired through nature or nurture.
"The study ... is truly groundbreaking," ethologist Frans de Waal of Emory University in Atlanta, writes in an e-mail. It "shows for the first time that rodents are not just affected by the emotions of others, but that empathy motivates altruism."
Evolutionary anthropologist Joan Silk of the University of California, Los Angeles, is not so sure, however. "I think experiments like this are extremely important," she says. But she adds that if the free rats are acting to relieve their own distress at their cagemate's suffering—a possibility the authors admit—then that's very different from acting altruistically to relieve the distress of others. Nevertheless, she allows, "it's a step in the right direction."