I have two male Australian Shepherds and while I know their aggression levels are supposed to be lower the vet still warns about the possibility of them getting into these kinds of fights. I have never had to deal with this problem to be honest, my dogs have never been in a real fight and the times they do goof around if either one yelps out either from pain or surprise the other one will immediately jump off and look at me for confirmation at he isn’t in trouble. But I took a whole lot of time and effort to extinguish any and all aggressive behavior that they demonstrate. They look to me as an absolute leader and they do not overstep their boundaries. When they do I address the situation promptly.

Fighting is a very serious problem. If they are actually causing injury then this going on downhill fast and needs to be dealt with now, the longer you wait the harder it will be overall. The obvious first step is to get them to behave while being monitored in a controlled room. It needs to be a room so that you can get to them quickly, if it’s outside they can avoid you easier and it’s harder to appear dominate. I do not believe that this type of behavior can be fixed through positive or negative reinforcement. It could have been prevented through those means but I think the problem is too far gone for that method. I think the best choice for you is to designate some time everyday to get them together in this controlled room and you have to literally play the role as the alpha. It’s completely common for dogs to want to display dominance; it’s when that dominance they display starts to turn into an alpha male complex. Then you get these fights that result in injury.

On a side note, I see a lot of comments throughout this forum that talk about displaying dominance towards the dogs to let them know you are in charge. But I rarely see anyone explain how to do that. Let me give you a brief overview of what I think the appropriate way to display dominance would be. A rule of thumb when attempting to display dominance: you aren’t trying to put the d*** in dictator, if you overdo it your dog will either begin to resent you or it may lead to other problems. (With your dogs fighting so much it would probably lead to them acting aggressive towards you, so be sure to just stress the problems that need to be stressed. None of this I have to walk into the room first or some of the other ridiculousness I’ve read online). Really all dominance is, is how you present yourself. Your dogs already recognize you as the primary caregiver, what they are fighting about now is who is in charge. You need to look like the leader, stand up straight, look them directly in the eye, and speak with firm strong voice. (I know some people say not to look animals in the eye but that’s only some animals and they only see it as a threat because it displays dominance which is what you want to accomplish). Now when they fight… that’s a sensitive problem that needs to handled carefully. One of the ways that I display dominance over my dogs is doing what I call a “firm hand” (I really just use it when we play wrestle). And it’s what I recommend you do as well. It is when you match whatever strength the dog is trying to output, don’t over power him if you match the strength they exhibit it lets them know you aren’t trying hurt them you’re trying to stop them . For instance, if I were to see my dogs fighting the first thing I would do is yell “HEY” in the strongest voice you can master up. While you do this walk over to them quickly (run if you’re outside) make your presence known. This should all take about 5 to 10 seconds do not hesitate and do not pick favorites. When you get to them you hold them both down. (I would usually never recommend any use of violence but they are refusing to pay you attention you might need to give them a little slap, understand though that dogs have a high pain threshold and they are currently beating each other up so don’t be afraid of hurting them whatever slap you deliver probably won’t hurt near as bad as that eye injury, but don’t hit more than once and the punishment goes to both, no favoritism. The object is to throw them off guard and pin them down). After they are both subdued, see how the aggressive behavior develops. Do they calm down? Does it get worse? Is there a lot of growling and obvious tension? If the aggressive attitude subsides for both dogs then you can “test the water” , but if an aggressive attitude is still present I suggest muzzling them as a punishment. This not only punishes them but it also lets them be in the same room with no worry of them fighting forcing them to get along. If you use it after they fight they will hopefully start to associate the two and through you acting consistently with dominance and punishment it should hopefully start to subside. (By the way when it comes to animals it is better to openly express how you feel. For instance the fighting pets would cause a lot of stress, anxiety, and frustration. Let it out, your dog already knows something is going through your head. He may not understand a word you say but by the time you’re done talking, yelling or whatever they will know how you feel. They probably will be less prone to fighting if they see just how much it bothers you.)

However the fighting is only part of the problem. It’s the biggest part but another important thing to keep in mind is aggression control to help prevent this from regression. In the case of my dogs I established early in the relationship what was community (toys, bones, pig ears whatever) and what was regulated (meals). I always make sure that they each get the meal that I set out for them, no more no less (and believe me they did try and get around that). Because I’ve reinforced this since I’ve gotten them I don’t have to worry about them stealing each other’s food. Now as far as toys and such go, they used to growl at each other when one came near and the other didn’t want him stealing his toy. While it was cute when they were puppies I knew that prolonged reinforcement of that aggressive possessive behavior would not end well. So what I would do was every time one growled at the other, I would walk over, take the toy, and throw it in the other room so they had to race to get it. Several months later and here they are sharing their most desired treats and playing in harmony. And my last little piece of advice in trying to get your dogs to recognize you as the leader is go rough house with them a little, make some games that force them to cooperate. I rough house with dogs all the time and most times it is me vs. the two of them. It’s a great way to remind them who’s boss without showing aggression and it keeps them on the same side.

Those are some serious behavioral problem, I hope this helped. Just remember, you’re the boss, what you say is law, and try to come up with some bonding exercises that make them less individualistic and more like a duo.

P.S. I know some people think it is better to pick the dominate dog and actually show favoritism but personally I think that it sets you up for regression, and any time the dynamic changes between the two they would probably end up fighting (where if they recognized you as the alpha there would be no dynamic change). And with prolonged favoritism from a primary caregiver might leave the impression that the dog is the alpha, and you don’t want that. I would guess that a full blown alpha male complex with an untamed ego in aggressive dogs would probably result in the weaker dog getting seriously injured possibly killed. These are natural instincts that you are dealing with and if you want see an example of where this can lead, look in nature, animals fight and some die just to be alpha… it can get pretty ugly. There is a reason we "domesticate" animals. And you found out first hand how fast the dynamic can change seeing as how you said that this is an all of the sudden kind of behavior. Your dogs are the around the same age so they probably demonstrate very similar aggression levels. And I have noticed that small dogs are not afraid to fight up a weight class or ten. Be careful with whatever you decide to do.