Experiments suggest rats with a recent drowning memory are quicker to assist their peers struggling to stay afloat. The drive to assist their fellow rats is a form of empathy, researchers say.

To test the rodent's capacity for empathy, researchers from Japan's Kwansei Gakuin University placed dry and wet rats in a cage side by side. The cage, divided by a plastic wall with a easily manipulated door hatch, featured a water-logged side and a dry side.

Dry rats consistently helped their distressed and soaked companions escape to the dry side by opening the door. Dry rats who had previously experienced the discomfort of a cold, wet cage were consistently quicker in offering assistance.

"This suggests that knowing that soaking is distressing enhances the rats' motivation to help their cage mate," lead researcher Nobuya Sato told the New Scientist. "We think this comes from empathy."

Previous studies have arrived at similar findings, but questions have remained whether rats free trapped companions out of empathy or out of a more selfish preference for a playmate. Sato's work showed that rats were disinterested in freeing peers who didn't seem to be in any distress.

"When the rat is in the space without the pool, they don't open it for him. They don't see a problem," explained researcher Peggy Mason, whose work at the University of Chicago inspired the recent experiments in Japan. "We're probably not that different. If somebody looks like they're happy and they're just hanging out, why would you help them?"

But Mason offers a possible alternative explanation for why previously wet rats would be quicker to help. She says their experience in the water-logged chamber may have helped them learn how the door works.

Sato and his colleagues -- whose work was published in the journal Animal Cognition -- plan to address the possibility in future experiments.