Dog aggression doesn't have a gender, but it's true that un-neutered males can be more aggressive with other males around a female dog in heat. And an un-spayed female dog can show defensive behavior when surrounded by males dogs who have picked up her scent. With un-neutered male dogs, that female scent is an invitation to mate and when there is competition, they can go into a fighting mode. Remember, aggression is not just a physical state; it's also mental. There are fights because there's only room for one at the top. This is why it is very important not to bring un-neutered dogs to a dog park. When someone goes to neuter an aggressive dog, it's helpful to create a calm-submissive state of mind before handing over to the vet. Neutering prevents signals from this part of his body and will ease aggression around other dogs.