In the new experiment, the pairs of rats were put in the experimental condition for an hour a day for 12 days. (They had previously spent two weeks together in a cage and knew each other.) The rat opened the door to the trapped rat’s cage by chance the first time, usually freezing in fright when it fell over noisily. In an average of seven days, however, it had learned to open the door intentionally and was no longer spooked when the door fell over.
In 13 percent of the sessions, the trapped animal gave an alarm call, but vocalized distress was clearly not necessary to put the free rat to work. When the cage was empty or occupied by a rat doll, the free rat sometimes opened it, but over the course of days lost interest in doing so.
After liberation, the rats nuzzled and explored the experimental arena. But when the setup was changed so that the captive exited into a different area, the free rat still opened the door for the captive one.
When a cage with five chocolate chips was added to the arena, the free rat opened it, too. That animal consumed all the treats if the other cage was empty. But if it contained a captive rat, the free rat shared the chocolate about half the time, letting its compatriot have 11
2 pieces on average.
“To actually share food — this is a big deal to a rat,” Mason said. “I didn’t think they would do that.”
Mason sees two processes at work. The first is one animal’s ability to identify and share another animal’s stress. But equally important is the ability to control the “acquired” stress and keep from becoming overwhelmed. But that was something not every rat could do. All six female rats in the experiment learned to open the captive’s cage, but seven of 26 males never did.
“I don’t think it’s because they didn’t have empathy. I don’t think they had the ability to down-regulate their own stress and act on the empathy,” she said.
Mason thinks that empathy and altruism evolved with females caring for helpless offspring. Natural selection favored those maternal traits, which then became generalized to both sexes. They helped forge social bonds that aided the survival of individuals and groups. She suspects the behavior is “sub-cortical” — closer to a reflex than a thought, and driven by ancient parts of the brain. De Waal, who in 2009 wrote a book called “The Age of Empathy” whose cover featured a chimpanzee shaking hands with a man, agrees up to a point.
“It is an intelligent response, but the motivation is, as in humans, an empathic process that is fairly automatic,” he said.
Jaak Panksepp, a neuroscientist at Washington State University who wrote an accompanying commentary in the journal, said that many people still doubt that animals have emotional lives that can be studied.
“Some skeptics are bound to say that this interpretation is a bit far-fetched,” he said in an interview. “What this provides is reasonably good evidence for empathy, and a model system to study the underlying processes further.”