Lab rats have feelings, too. Given a choice between munching on a tasty chocolate treat or helping a fellow rat escape from a restraint, test rodents often preferred to liberate a pal in need, indicating that their empathy for others was reward enough. The observation by University of Chicago neuroscientists, published on Thursday in the journal Science, suggests that even these primitive creatures are wired to show benevolence for their own kind. "This is the first evidence of helping behavior triggered by empathy in rats," said researcher Jean Decety, professor of psychology and psychiatry at the University of Chicago. "There are a lot of ideas in the literature showing that empathy is not unique to humans, and it has been well demonstrated in apes, but in rodents it was not very clear." Researchers started by housing 30 rats together in pairs, each duo sharing the same cage for two weeks. Then, they moved them to a new cage where one rat was held in a restraining device while the other could roam free. The free rat could see and hear his (or her -- six of the rats were female) trapped buddy, and appeared more agitated while the entrapment was going on. The door to the trapping enclosure was not easy to open, but most rats figured it out within three to seven days. Once they knew how, they went straight to the door to open it every time they were put in the cage. To test the rats' true bond to their cagemates, researchers also ran the experiment with toys in the restraint to see if the rats would freethe fake stuffed rats like they did their comrades. They did not. "We are not training these rats in any way," said first author Inbal Ben-Ami Bartal. "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." Even when researchers rearranged the experiment so that the trapped rat would be set free into another enclosure, away from his hero friend, the rats still opened the door, indicating they were not motivated by companionship. "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." In one final test to truly measure the resolve of the rats, scientists presented them with a pile of chocolate chips in the cage. The rats were not hungry, and in prior experiments showed they liked chocolate because they would eat it instead of rat chow given the chance. Still, free rats tended to act benevolently. Even if they munched on a few chips first, they would then free their pal and allow him to eat the remaining chips. "It said to us that essentially helping their cagemate is on a par with chocolate. He can hog the entire chocolate stash if he want(s) to, and he does not. We were shocked," said co-author Peggy Mason, a professor of neurobiology. Rats shared their chips in 52 percent of all the trials. In control experiments when the rats were alone with no one to help and a pile of chocolate, they ate virtually all the chips. Researchers switched the rats' roles so that those who were once trapped later were the ones free and faced with a companion who was constrained. In those cases, all six female rats became door openers and 17 out of 24 male rats did, "which is consistent with suggestions that females are more empathic than males," said the study. Since most, but not all the rats became door-openers for their pals, the next step could be to look "for the biological source of these behavioral differences," the study said. Mason said the study offered an important lesson for humans. "When we act without empathy we are acting against our biological inheritance," she said. "If humans would listen and act on their biological inheritance more often, we'd be better off."