I recently got a 5 1/2 month old Mini Aussie puppy. I already have a cocker spaniel who is almost a year and a half old. Although there are plenty of balls and toys around the house, Penny (the cocker) will NOT share with Zelda (Aussie). If I throw a ball for Penny and then for Zelda at the same time, she'll drop hers and go after Zelda's. She even growls at Zelda if she tries to go after something Penny is playing with. Do dogs learn how to share?!