Increasingly Aggressive German Shepherd/Lab Mix
We've had the best dog in the whole world in every way possible . . . until recently. We brought home a Lab/Golden Retriever mix about 7 months ago, when the puppy was about 2 months old. Wonderful dog in every way! However, in the last few months the older dog (just turned 13 this month) is becoming increasingly aggressive toward the puppy, who is now just a whisker bigger than her. They do a lot together: go to the Dog Run, swim in the local river, play in the plastic pool, run in the forest, etc. But the older dog is barking more and more at the younger, sometimes incessantly. The barking may start for nothing more than the puppy walking into her field of visions. Now and then she snaps at her, and on a few occasions seemed to lunge at her. She's never been that way to anyone or any dog; she' always been very friendly.
Thaw's causing such aggressive behavior? I suppose the puppy can be a little annoying at times ("Let's play!") but never aggressive. We trained her to realized that the older dog is the lead dog and we attempt to give equal time and attention to both. Also, the older dog is still very active and in very good shape. Could the cause be her bread, part German Shepherd? Her age (onset of dementia, painful arthritis)?
You help is greatly appreciated.