Circling a strange contraption, the rat gnaws at its edges, pressing his paws against the clear Plexiglass walls. Inside the tube-shaped restrainer, trapped, is the rat he’s shared a cage with for two weeks.

The prisoner can barely do a 360-degree turn in his tight quarters and tiny squeaks betray his distress. Meanwhile, the free rat circles and circles, scraping his teeth against the restrainer, poking whiskers through its small openings.

For the past five days, it’s been the same routine for these cage mates: one free, one captive, both stressed. But today is different. After hours of trial and error of circling, biting, and digging into the restrainer, the free rat pushes its door with his head—and just the right amount of force. Suddenly, the plastic front falls away, as the researchers watching have designed it to do.

Both rats freeze, stunned. As the newly freed rat scurries out, the liberator follows in quick pursuit, jumping on him and licking him. It’s an unusual burst of energy that suggests he’s done what he meant to do: release his cage mate.

“It looks like celebration,” says Chicago neuroscientist Peggy Mason, who has observed the same interaction with dozens of rat pairs. For the past three years, Mason, psychology postdoctoral fellow Inbal Ben-Ami Bartal, AM’09, PhD’12, and empathy researcher Jean Decety, also a neuroscientist, have been putting the rodents in these sticky situations—and finding them more than willing to help each other out. 

The scientific term is “prosocial behavior,” which encompasses anything done for another’s benefit. We see it every day in the human world: a teenager helping his grandmother across the street, volunteers serving meals at a soup kitchen, human-rights advocates speaking out against torture. For us, it’s often motivated by empathy, that emotional tug of someone else’s distress.

Rats may not be so different. “The trapped buddy is sending out signals of distress that the other rat is picking up. He’s catching the distress and feeling pretty distressed himself,” explains Bartal, lead author on a 2011 Science paper detailing the researchers’ findings. “When that rat aids in terminating that distress, he gets a …” She trails off, searching for the right word.

Mason comes to her rescue.

“A big ‘Yahoo!’” she chimes in. “It’s ‘Yahoo for me!’”

Bartal nods. Helping, in and of itself, seems highly rewarding for the rats. Once the free rats learn how to open the restrainer door—on average this happens on day six of the 12-day experiment—they consistently repeat the behavior. As a control, researchers also tested free rats in a pen with empty restrainers and restrainers containing a toy rat. Neither prompted them to open the door, suggesting their earlier actions had been specifically motivated by the presence of the trapped cage mate.

But how far, the researchers wondered, would the rats really go for each other?

A second set of experiments upped the ante. This time, the free rat had three choices: liberate the cage mate, open an identical restrainer containing five milk-chocolate chips, or both. Normally, Bartal explains, a rat left alone with chocolate will gobble up the entire stash.

But that’s not what happened. The free animals not only released their cage mates just as frequently as they opened the chocolate-filled restrainer, but many left behind chips for the other rat to share. Even in instances where free rats pried open the chocolate restrainer before releasing their cage mate—and could have very easily hogged the food for themselves—they didn’t. Some even plucked the chocolate chips out of the restrainer and dropped them near the newly freed rat. 

“This just blew us away,” Bartal says. “It was very obvious they were purposefully leaving the chocolates.” Although apes and other primates also exhibit this sort of sharing behavior, she notes, “there is no such thing in the rat world.” Until now.

When it comes to sharing the chocolate, “we actually still can’t explain that,” says Mason, who has spent more than two decades studying rats to investigate pain processing and other concepts.

The researchers are now running a series of studies to better understand the rodents’ motivations. What they can explain in the meantime are some of the biological underpinnings that lead rats to free each other in the first place. Rats, explains Bartal, “actually share a lot of the neuronal structures that permit them to be attuned to the emotional state of another.” Like human empathy, the rat analogue takes place mainly in the brain’s subcortical region. “This behavior,” says Bartal, “is not a very highly complex cognitive function.”

The process starts when the free rat sees another in distress, then mimics some of that affective state. This mirroring, or emotional contagion, then produces in the animal a drive to do something. But first, the free rat has to get his own fear under control, what’s known in empathy research as downregulation.

“The rat not only has to feel motivated but has to feel bold enough to act,” says Mason. That includes venturing out into the middle of the arena to reach the imprisoned cage mate. “A rat, given its druthers,” says Mason, “will be plastered to the side” of the pen, where it feels safer. But time and time again, the animals overcome their own fear, moving forward to help out another.

Such selflessness makes evolutionary sense for any mammal, rats included. “You don’t get to live and reproduce if you can’t navigate the social world,” says Mason. By demonstrating rats’ sense of empathy, their findings suggest that helping out those in distress is instinctual and when we fail to do so, we are essentially going against a “biological mandate.” In short, “we are built to play well with others.”

Updated 09.23.2014