Many geckos perform breeding with agressional displays. The male usually bites the neck of the female to make her more submissive during breeding. Even in docile domesticated geckos, males will want to react aggressively towards other geckos. It is a natural procedure, but if you see the gecko with a bunch of little sores or cuts you should probably remove it from the cage. The line between hormones and just plain aggression is hard to see, but if you feel that your gecko is truly threatened. You should deffinitely remove it.