The researchers observed that the free rat acted more agitated when its cagemate was restrained, compared to its activity when the rat was placed in a cage with an empty restrainer. This response offered evidence of an “emotional contagion,” a frequently observed phenomenon in humans and animals in which a subject shares in the fear, distress or even pain suffered by another subject.

While emotional contagion is the simplest form of empathy, the rats’ subsequent actions clearly comprised active helping behavior, a far more complex expression of empathy. After several daily restraint sessions, the free rat learned how to open the restrainer door and free its cagemate. Though slow to act at first, once the rat discovered the ability to free its companion, it would take action almost immediately upon placement in the test arena.

“We are not training these rats in any way,” Bartal said. “These rats are learning because they are motivated by something internal. We’re not showing them how to open the door, they don’t get any previous exposure on opening the door, and it’s hard to open the door. But they keep trying and trying, and it eventually works.”

To control for motivations other than empathy that would lead the rat to free its companion, the researchers conducted further experiments. When a stuffed toy rat was placed in the restrainer, the free rat did not open the door. When opening the restrainer door released his companion into a separate compartment, the free rat continued to nudge open the door, ruling out the reward of social interaction as motivation. The experiments left behavior motivated by empathy as the simplest explanation for the rats’ behavior.

“There was no other reason to take this action, except to terminate the distress of the trapped rats,” Bartal said. “In the rat model world, seeing the same behavior repeated over and over basically means that this action is rewarding to the rat.”

As a test of the power of this reward, another experiment was designed to give the free rats a choice: free their companion or feast on chocolate. Two restrainers were placed in the cage with the rat, one containing the cagemate, another containing a pile of chocolate chips. Though the free rat had the option of eating all the chocolate before freeing its companion, the rat was equally likely to open the restrainer containing the cagemate before opening the chocolate container.

“That was very compelling,” said Mason, Professor in Neurobiology. “It said to us that essentially helping their cagemate is on a par with chocolate. He can hog the entire chocolate stash if he wanted to, and he does not. We were shocked.”

Now that this model of empathic behavior has been established, the researchers are carrying out additional experiments. Because not every rat learned to open the door and free its companion, studies can compare these individuals to look for the biological source of these behavioral differences. Early results suggested that females were more likely to become door openers than males, perhaps reflecting the important role of empathy in motherhood and providing another avenue for study…