Remy is a 13 month old, neutered male Aussie. We brought Remy into our house to be a playmate for our 12 month old, neutered, female Sheltie. They bonded immediately. They are crazy about each other and will not even go outside unless they are together. If one is in their kennel the other will pace back and forth in front and whine until they are together again.
Both have been to training classes and generally mind quite well. The issue is that play often gets VERY aggressive—it is almost like a switch gets flipped and Remy will become overly aggressive with the much smaller Sheltie (who will not back down even though the Aussie has 20+ lbs on her). It sounds like 2 wild dogs trying to kill each other and does not look much better than that. We literally have to pull them apart physically. This usually happens over food or toys or if the Sheltie just pushes Remy too far.
We do not want to limit their play together but we are afraid that the smaller dog will get hurt at some point. We use loud noises and a harsh tones to get his attention and that will not break it up. We have tried a dog whistle and a pop can with pennies to startle to no avail.
He is a very gentle dog for the most part and does not get aggressive with our other much older Lab mix (10 year old female)—in fact he is very gentle and passive with her. Any thoughts?