In a recent study, psychologists sought to find out if a rat would assist another rat that was trapped in a confined box. They tested it in various situations including ones where the rats were not united after being freed. They found that the the rescue would be done quickly regardless of the reward. They also found that, when the savior was given chocolate pieces, the saving rat would keep at least one piece of chocolate around for the rescued rat.
The conclusion? Rats are empathetic and that there is reward in doing good things.
http://www.washingtonpost.com/national/health-science/a-new-model-of-empathy-the-rat/2011/12/08/gIQAAx0jfO_story.html