New experiments show rats demonstrating compassion and helping other rodents. It is a trait some scientists thought was reserved only for humans and higher primates.

And it is certainly not the sneaky, selfish rap that goes with calling someone a dirty rat.

In repeated tests, rats freed another trapped rat in their cage, even when yummy chocolate served as a tempting distraction. Twenty-three of the 30 rats opened the trap by pushing in a door. The rats could have gobbled the chocolate before freeing their partners, but often didn't, choosing to help and share the goodies.

"Basically they told us (freeing another rat) is as important as eating chocolate," said study author Peggy Mason of the University of Chicago. "That's a very striking thing."

In some cases, the rats first took the chocolate chips out of a container, but didn't eat them, then freed the other rat and shared "almost as if they were serving them chocolate," Mason said. The research is reported in Thursday's journal Science.

Also, females 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 behaviour than males, she said.

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

Jeff Mogil at McGill University in Montreal, who was not part of the study, said it was a tad surprising but even more convincing.

"It's a very, very obvious demonstration of the phenomena," Mogil said. Both scientists said social empathy is probably a characteristic that is important in the evolution of animals.

Mason joked that if rats can be so caring and helpful "there's a sense of optimism. It's something we could be."