Experimenters put a rat in a small cage that could only be opened from the outside. A second rat introduced into the area would see the situation and start trying to free the caged rat. When the second rat figured out how to rescue the first, the two were observed to do what the researchers called a celebration dance.
More on this can be found at the New York Times, including how the rescuer would share treats with the rescuee.
William Mullen, in the Tribune Newspapers, adds:
Previously, scientists thought that empathy and pro-social behavior to help others were unique to humans... The experiment is being hailed as a new paradigm that will help scientists trace the development of emotions in mammals back through the evolutionary tree.
Well, I'm glad that some scientists are beginning to pay attention to the way animals act. They're using the wrong assumption if they're looking for the "evolution" of emotions, however. Emotions did not evolve; they arise from the spirit, the essence of being, which is fundamentally the same in all living beings. What's different is the expression of emotions, due to physical differences in the bodies, and that is what keeps people thinking humans are so different from the rest of the animal world.