Trained is probably an overstatement. However, I have a large diamond x jungle carpet that operates on the "bite first ask questions later" approach. Hey, if it's not food (say for instance it's only my hand) he can always spit it back out. He's not aggressive, just wants to make sure he doesn't miss a meal. When I enter his cage for non feeding purposes, I give him a couple of gentle taps on the snout and chin with a hook and he goes out of bite mode. I don't think he's trained, it just gives him time to recognize that it's not dinner time.