Is my rat playing or showing aggression?

Question
I have a male rat named Ringo. He's about 6 months old. Recently, while I've been letting him run around outside of his cage, he runs over to my arm or hand, grabs on with his front paws, shakes it around with his whole body, then kind of hops away. It seems like he's just being playful, but I was just making sure that I shouldn't be worried about him being aggressive. Also, he's usually pretty gentle with me. When I hand feed him, he's very careful not to bite me, however, doesn't seem to show as much consideration for my husband. He nips at my husband while being fed, or just when he's running around, but never has intentionally bit me. Is it possible that Ringo just doesn't like my husband, or maybe just being territorial because they're both male?

Answer
 It sounds like he is being playful.  Even the nipping is probably due to playful behaviour; if a rat is being aggressive they will puff up their fur, bite down hard, and usually not let go.  Their bites go deep and draw blood.  To help stop the playful nipping, make sure your husband washes his hands before handling him (food smells can make them want to taste hands) and if he still nips have him make a loud squeak noise which lets him know in 'rat language' that he is being too rough.  Neutering can also help curb male aggression, though it sounds to me like he is just being playful.  I hope that answers your question, good luck with your rat!