In the 5 or so years I had him I never had trouble with him turning mean. I take that back. He turned mean on me one time. That was when I took him outside. It was just like was stated before. It was like a switch. As soon as I set him down, he darted for a tree. I grabed him on his way up. He started hissing at me. As soon as I set him down inside the house he went right back to his adorable self. I would still be cautious though. He has some sharp claws and teeth. I would hate to get nailed by him.