Historia Animalium 612a21
"When crocodiles gape the trochilos fly in and clean their teeth, and while they themselves are getting their food the crocodile perceives that he is being benefited and does not harm them, but when he wants them to go he moves his neck so as not to crush them in his teeth. When the tortoise has eaten some of a viper it eats orignaum as well: this has actually been seen. And once when a man had observed a tortoise doing this several times and going back to the viper after each mouthful of orignaum, he began stripping the leaves off the orignaum; and when this had happened the tortoise died."