5yr old sister rivalry

Question
My boyfriend and I have recently adopted (2) 5yr old Pugs that are suppose to be sisters named Misty and Trixie (they may be a mix because their noses are out a little further than normal pugs, but they have all the other obvious features of Pugs)which are both neutered. We take them out on walks about 3-4 times a day to go to the bathroom since we don't have a fenced in yard.  We noticed that Misty will be real 'bossy' to Trixie when outside (not when they are inside).  If there is a squirrel or other dog in sight, Misty will lunge at Trixie and it sounds very vicious and aggressive, but she does not bite her and Trixie just brushes it off.  No skin has ever been broken.  It is almost like she is saying "leave it alone, its mine!" or something.  When they are by a tree that a squirrel had gone up, Trixie will look up at the tree and then Misty will charge and lunge at her too. Otherwise, they want to give kisses to anybody they see when they are out. I am not sure what to call this behavior and if I would need regular training, or a specialized training. Please help.


Answer
What you're seeing is called 'displaced aggression.' Misty wants desperately to go after the squirrel or other dog, or whatever, but she cannot do so because of the leash, so she goes after Trixie because she's the next closest thing. The same thing can happen between dogs that are behind a fence. They will attack each other because the fence is in the way of what they really want to get to.

You should probably consult a local behaviorist that can help you handle the issue. Trying to correct Misty could make the behavior worse if done incorrectly, and it could even make Trixie aggressive as well. If you can, I would recommend that you walk them with another person, so that you aren't having to handle both leashes and both dogs at the same time. If you're walking both dogs by yourself, if you try to correct Misty, Trixie could interpret it as a correction to her as well, and could become frustrated because there's no reason for her to be corrected, and as a result, she could lash out at you or Misty, and things could get much worse. Does that make sense?