If you're nice to a rat, she might return the favor.
A new study on Norway rats, the common brown rats who dominate sewers of cities around the world, shows that the rodents will remember acts of kindness by other rats and reciprocate that behavior by being kind in turn. In other words, what goes around comes around in the rat world. The study, published in the journal Biology Letters, tested the reactions of female rats to other "helper rats" who could deliver them food by pressing a lever. Those who delivered tasty bananas were considered high-quality food delivery rats, while those who delivered not-so-tasty carrots were low-quality helpers. Then, the tables were turned: the helper became the eater, and vice-versa.