THURSDAY, Dec. 8 (HealthDay News) -- Rats may have gotten a bum rap.

Far from being self-centered scroungers, a new study found that the rodents showed what looks like real empathy -- repeatedly freeing trapped companions, even when they're given the opportunity to eat chocolate instead.

This first evidence of empathy-motivated behavior in rodents suggests that this type of pro-social helping behavior developed earlier in animal evolution than was previously thought, the University of Chicago researchers added.

Anecdotal observations of empathetic behavior have been noted in non-human primates and some other wild animal species, but it had not been seen in laboratory rodents.

In the study, companion rats that normally shared a cage were put in a separate space where one was under restraint and the other wasn't. The restrained rat was kept in a tube with a door that could only be opened by a push from outside.

The researchers say the free-ranging rat appeared agitated when the other rat was kept in the tube, a state the scientists called "emotional contagion" -- feeling the distress of others.

A few days into the experiment, the free-roaming rat learned how to open the tube door, freeing the other rat. Once this was learned, the free-ranging rat made this his/her first action upon being placed in the new area.

"There was no other reason to take this action, except to terminate the distress of the trapped rats," said the study's lead author, graduate student Inbal Ben-Ami Bartal. "In the rat model world, seeing the same behavior repeated over and over basically means that this action is rewarding to the rat."

Even the lure of a tasty treat didn't seem to sway the rat from helping a trapped companion: When the researchers placed chocolate chips inside another tube, the free-ranging rat nearly always opened the tube containing the other rat first, before going for the chocolate.

"That was very compelling," said co-author Peggy Mason, a professor of neurobiology. "It said to us that essentially helping their cage mate is on a par with chocolate. He can hog the entire chocolate stash if he wanted to, and he does not. We were shocked."

"This is the first evidence of helping behavior triggered by empathy in rats," Jean Decety, a professor of psychology and psychiatry, added in a University of Chicago Medical Center news release. "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. We put together in one series of experiments evidence of helping behavior based on empathy in rodents, and that's really the first time it's been seen."

The study was published Dec. 8 in the journal Science.

More information

The Stanford Encyclopedia of Philosophy has more about empathy.