The presence of a rat trapped in a restrainer elicits focused activity from his cagemate, leading eventually to door-opening and consequent liberation of the trapped rat.

Credit: Science/AAAS

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LONDON: Rats will choose to liberate a trapped companion even when offered a reward of chocolate as an alternative, report researchers.

A new study, published in Science today, demonstrates that rats are capable of 'pro-social' or voluntary helping behaviour motivated purely by the distress of their fellow rats. This type of behaviour has not previously been observed in rodents.

"Our results indicate that pro-social behaviour is much older evolutionarily than previously thought, extending beyond primates to rodents," said study author Inbal Ben-Ami Bartal from the University of Chicago in the U.S.

"It is widely agreed today that the roots of empathy lie in the mother-offspring bond. Mothers need to be responsive to the needs of the offspring to ensure its survival, and this ability is also important in social groups, where the group's survival is promoted by helping between individuals. As rats are very social animals it's not surprising that they demonstrate this ability," she added.

Social rats feel pain

Mice and rats are known to show 'emotional contagion' towards their fellows, in that they can detect and mimic each others emotional state, but actively helping other rodents requires an extra empathic step.

"We wanted to see if rats would go beyond emotional contagion to actively help another rat in distress," said co-author Peggy Mason, also from the University of Chicago.

"This is really asking quite a bit of a rat as active helping requires that the helper rat do more than freeze in fear and distress. The helper rat has to down-regulate the fear experienced through emotional contagion. In other words, the helper has to suppress the natural response of frozen immobility and actually move and act to help the other rat."

To test whether rats could display pro-social behaviour, the team placed a free laboratory rat in an enclosed area with a cagemate trapped in a plastic restrainer.
The free rats became agitated in the presence of a trapped companion, and after several sessions of trial and error learned how to open the restrainer doors to free the trapped rats.

Empathic motivation

To test the motivation of the rats in opening the restrainers, the investigators carried out the same experiment with empty restrainers and those containing objects such as a toy rat. However, the free rats did not open the restrainers in either of these scenarios.

The rats even learned how to open the restrainer in the absence of a food reward or external training. They also opened the restrainers even when they were prevented from socialising with the trapped rat afterwards, ruling out social interaction as a possible motivation. Rather, "they learned how to open the restrainer because they were internally motivated to liberate the trapped rat," said Bartal.

Passing up chocolate chips

In an additional experiment, designed to test how valuable liberation of the trapped cagemate was to the free rat, the researchers placed a free rat in an area with a trapped rat and a food reward of chocolate chips in another closed restrainer.

Instead of opening the food containing restrainer and eating all the food themselves, the free rats not only freed the trapped rat, but in most instances also shared the chocolate with them. "Truly amazing from a rat perspective," remarked Mason. "Imagine a rat seeing food and not eating it!"

"What's nice about this is that this is pretty obvious pro-social behaviour and there is really no two ways about it," said Jeffrey Mogil from McGill University in Montreal in Canada, an expert on rodent empathic behaviour. "It's more than has been shown before by a long shot, and that's very impressive, especially since there's no advanced technology here."