A few times lately, 3 in total, Jasper (4.5-5 months) has shown aggression towards other dogs when it comes to food - But not with me, he lets me take it from his mouth. And when I say food, it's food that was never originally designated to him (not that I'd think he'd know this). The first two instances was when he was at a friend's place where they have 2 fourteen week old brothers who have been sharing their food since they were brought home at 6 weeks old. Well Jasper saw the food sitting there and helped himself, in fact scoffed the whole lot. My immediate reaction was to stop him as it was the puppies food, not his, but my friend said it was fine - I guess because he thought it'd teach Jasper and the puppies that they could share food. Well, when one of the pups came over to investigate Jasper chomping on his food (calmly, not aggressively), Jasper lifted his lips and growled at the pup. This happened twice throughout the period that Jasper was there. He shows no aggression to the pups otherwise, just with food. He even took a bone off of one of them from their mouth. They didn't react, but I took it from him straight away as it wasn't his and the fact he took it from the pup's mouth. The third time he's shown food aggression is with my landlord's 12 year old female staffy. We were outside while the staffy, Rosie, was inside and Jasper found an old bone laying around that he thought he'd chew on. Rosie was let outside by my landlords and I'd thrown Jasper's toy over to him previous to Rosie being let out to get him to play with that instead. Rosie trotted over to the toy, noticed Jasper was chewing on a bone (that I later found out was actually hers) and he lifted his lips again and growled. Rosie reacted and growled, barked and jumped at him. It all happened pretty quick, but it was frightening. It was something my landlord was terrified would happen, Rosie has strong jaws and is very large and strong. Jasper remained aggressive but backed up instead of throwing himself at her.
I'd like to be able to get Jasper to listen to me when I tell him to leave something, so these things don't happen. Any other suggestions?
P.S. We're going to his first obedience class tonight, so hopefully can get some advice there too.