At the very least, the new experiment reported in Science is going to make people think differently about what it means to be a "rat." Eventually, though, it may tell us interesting things about what it means to be a human being.

In a simple experiment, researchers at the University of Chicago sought to find out whether a rat would release a fellow rat from an unpleasantly restrictive cage if it could. The answer: Yes.

The free rat, often hearing distress calls from its compatriot, learned to open the cage and did so with greater efficiency over time. It would release the other animal even if there wasn't the payoff of a reunion with it.

The successful release of the caged rat led to what strongly resembled a triumphal celebration between the two.

Astonishingly, if given access to a small hoard of chocolate chips, the free rat would usually save at least one treat for the captive, which is a lot to expect of a rat.

The researchers came to the unavoidable conclusion that what they were seeing was empathy, and apparently selfless behavior driven by that mental state.

"There is nothing in it for them except for whatever feeling they get from helping another individual," said Peggy Mason, the neurobiologist who conducted the experiment along with graduate student Inbal Ben-Ami Bartal and fellow researcher Jean Decety.

The idea that animals have emotional lives and are capable of detecting emotions in others has been gaining ground for decades.

Empathic behavior has been observed in apes and monkeys, and testified to by numberless pet owners, especially dog owners.

Recently, scientists demonstrated "emotional contagion" in mice, a situation in which one animal's stress worsens another's. But empathy that leads to helping activity — what psychologists term "pro-social behavior" — hasn't been formally shown in nonprimates until now.

Communicating distress

The University of Chicago team first paired rats of the same gender for three weeks. Then they placed one of the pair in a small, Plexiglas restraint cage, locked by a door that could only be opened from the outside. The cage was placed in a larger enclosure where the rat's partner roamed free.

By means the researchers aren't sure of, the caged rat seemed to communicate its distress to the freed rat, and the freed rat sprang into action.

"The free rat jumps on the restraining cage immediately, pushing it, biting at it, touching its nose and whiskers through the openings in the restraining cage with those of the trapped rat," Mason said. "Clearly it wants to help out the trapped rat."

After about six days, the free rat would accidentally open the door and from then on, it quickly learned how to deliberately open it, and then excitedly interact with its now-free partner as they raced around the enclosure.

"I can't say that they are celebrating," said Mason. "But sure looks like a celebration."

Saving chocolate

Because rats love chocolate, in some experiments the scientists placed two restraint cages in an enclosure with a rat that already knew how to open the cage door. One cage contained a rat, the other five chocolate chips.

"We wanted to ask how much the free rat valued being able to liberate the caged rat," Mason said. "They like their chocolate chips, but the free rat would open both cages in no particular order.

"The free (rat) could have done all manner of things to monopolize the chocolate chips, but on average it always left one and a half chocolate chips for the liberated rat. That's impressive — a hard thing for primates to do — showing it puts equal value on chocolate and freeing its partner."

Eventually rats that did not know each other were used, and the free rat still worked hard to liberate the stranger from the cage.

Females also showed more consistent empathy than males, Mason said. All six females freed their trapped partner; 17 of the 24 males did so. This confirms other studies that show females demonstrating more pro-social behavior than males, she said.

There were days the male rats took the day off from helping their trapped partner, but the females never did, she added.

Rigorous study

If the experiment reported Thursday holds up under scrutiny, it will give neuroscientists a method to study empathy and altruism in a rigorous way.

"The study is truly groundbreaking," said Frans de Waal, a primatologist at Emory University who has written extensively about empathy. What is particularly interesting, he said, is there appears to be no clear cost/benefit trade-off going on. "We are entering a distinctly psychological realm of emotions and reactions to the emotions of others, which is where most human altruism finds its motivation."

Jeffrey Mogil, the McGill University neuroscientist who showed "emotional contagion" in mice in 2006, said that "what is amazing about this is that it shows empathy in such a robust way. This is not something that rats would otherwise be doing."

A major question that needs to be answered next is whether the free rat liberates the captive one to relieve its own stress or the stress of the other animal.

"It's more likely to be the former," Mogil said. "But even if it is the former, I'm not sure that's so different from humans."

Mason said the tireless behavior the rats showed in helping out another rat in distress is something humans should pay attention to, showing that empathy is somewhat hard-wired into our mammalian brain.

"I would suggest that helping is what we are biologically programmed to do. You have to suppress that biological tendency to not help. If we owned up to our biological inheritance a little bit more than we do, we would be better off."

Material from The Chicago Tribune and The Associated Press is included in this report.