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To piggy back off of Judy, my female Rottie became more dominant in play at about the same age. We doubled down on leadership and also stopped having her play with dogs that were not interested or a good match. We try to never set the dogs up to need to self advocate against any variables we can control.
Rotties are supposed to be wary and assertive toward strangers and anyone or anything they perceive as a threat. If both dogs are the same age it is natural for one to be looking to make decisions for the crew especially if leadership at home could use a booster. Have you read the leadership article? Here is the link for the article and also the Canine Culture video is really good!
https://www.youtube.com/watch?v=owjyJh2NlfE&t=1s
https://dogtraining.world/knowledge-base/introduction-to-leadership/
