My biggest BP Pierce hisses either when I try to give him f/t rats (he refuses to eat those), or when he wants out of his tank and I take too long to get lid off (lamp is not strap onto the screen so I have to move that, unlock tank, untwist twist tie (extra lock for no reason other than boredom), unsnap the clips, then slide lid off. He is a strong bugger so it is necessary lol).