I find myself wondering why a rat would choose to liberate a cagemate when they have the opportunity to enjoy a goldmine of chocolates if they would just leave the other rat locked up. Dr. Peggy Mason, a neuroscientist, and psychologists Inbal Ben-Ami Bartal and Jean Decety conducted a study in which they placed pairs of rats in pens with one free to run about and the other placed in a restrainer in the middle. They found that 23 out of 30 rats freed their cagemate but did not bother to open empty restrainers or those containing stuffed rats. In another set-up, the rats still chose to release their cagemate even though there would be no opportunity for social interaction because the opening of the restrainer released the freed animal into a separate pen. Although I am not sure the rats knew this opportunity would be lacking. When given a choice between a restrainer filled with chocolate (a delicacy for rats) or their cagemate, they were found to typically open both and share the chocolate. I honestly don't think that I am strong enough to do the same thing. I would probably choose the cage with the chocolates first, savor a few bites, then go free my cagemate. I mean we are talking about chocolate here.
Some have called this a selfless act or altruism on the part of the liberating rat, which may be crossing the line a bit too far into anthropomorphism. The authors suggest this might be an indication of empathy or pro-social behavior. Others have suggested that they were simply trying to put an end to the caged rat's distress calls. Whatever the motive, it is an interesting study.
Bartal IB-A, Decety J, Mason P. Empathy and pro-social behavior in rats. 334(6061): 1427-1430, 2011.
I really like this article as it reinforces my hobby observations on the African giant rat which exhibit this type of behaviour.