Hi Fernando,

 We have 2 dogs Stacia (female 5yo) and Duno (male 6yo). We adopted Stacia

 about 2 years ago, and Duno around 9 months ago, and it seems that Stacia

 has become very territorial.

When we first adopted Stacia, she was very

 skittish about new people that would come into our home. She would run

 behind a table and constantly bark out of fear i'm assuming.

We had guests

 over one time and she barked for 3.5 hours non stop. When we got Duno that

 seemed to change almost instantly. I believe that Duno taught her that new

 people are fine. Duno is a very friendly dog who will greet everyone with excitement and fun. After a few months after adopting Duno, Stacia began to do the same. It was almost immediate that we noticed that if one person is

 paying attention (petting/cuddling) to Duno, Stacia becomes very aggressive to get you to pay attention to her (no matter if she was getting petted and cuddled by someone else).

 One day last week i was in the living room watching TV, and my partner was in his room watching TV, and Duno was in his bed in my bedroom. All of a sudden we hear them growling and barking and fighting in the bedroom. Duno

 ended up with a gash on his arm, and we are seriously trying to figure out why Stacia is having such issues with Duno. Is it because she has the "I  was here first" syndrome? Shouldn't that go away after 9 months?

 PLEASE HELP IF YOU CAN!!!

David

 Hi David,

 Seems to me like you have a combination of issues, the very first that jump to my mind is leadership.  Stacia is really taking over the control of the house and that is why she is trying to rule all what happens there.

I can see an extreme insecurity in Duno's side, which Stacia is totally using in her advantage to dominate him. There are few things that you can do in order to start reverting this situation,

 -walk them together the longest, the better, that helps them build their

 relationship as a pack, a very important part of the activity is to be

 sure they are doing nothing but walking, not sniffing or fooling around

 -- more like a job they need to do together and they need to work as a

 team

 -take away Stacia's authority. By giving her rules and corrections if she

 breaks them, like keeping her in a specific area when you have a guest, or

 when you are eating, she will start taking the follower role rather than

 the leader role.

 -correct her every single time she tries to correct Duno. You can do it as

 an exercise in a provoked situation, have her on a leash and have a

 friend petting Duno (at a safe distance) and correct her every time she

 tries to go after him or just stare at him and reward her (maybe with a

 treat) for every 15 secs she is calm and ignoring the situation.

 -Build Duno's confidence.  Try to teach him the simplest tricks and reward

 him for that like he is graduating from Harvard -- the tricks can be as

 simple as jump on a step and start progressing the difficulty from there.

 Also, set a little treat container by the door as ask your guests to give

 him a treat (without overwhelming him) every time they come by, that will

 start teaching him that new people are actually a good thing.

 I think this few steps should do the trick, please let me know how it goes after 

a week.

 Fernando