Selective aggression

Question
My yellow lab mix, Rex, is about 2 years old now. My boyfriend and I got him from a man who had 4 other dogs. I get the impression that Rex and the other dogs would fight over food because when we got him, he had some scabs on his ears. Despite that, Rex loves other dogs and is only aggressive around other dogs with food.

When we go on vacation, we leave Rex with my mother who also has a yellow lab (Patrick). Whenever Rex and Patrick are around food, Rex will snap at Patrick or bite him.  It concerns me because Patrick is old and doesn't move well enough to get out of the way.

The odd part is that Rex doesn't eat that much. When we put food in his bowl, he sometimes wont eat it until hours later. When we give him a biscuit or other treat, he will take it and leave it somewhere where he can eat it later. Sometimes he'll take one bite and leave the rest for later. This makes it very difficult to separate the dogs when they are eating. Rex will leave his food unattended and when Patrick is even near it, Rex will snap.

It doesn't seem like Rex is aggressive with people.  If he is tearing apart a toy and I need to take it from him to throw it out, he doesn't snap.  If he's chewing on something he shouldn't, like a pen cap, I can take it right out of his mouth and he's fine.

My boyfriend and I want to get another dog, but we don't know how to correct Rex of this behavior. What should we do?

Answer
Actually I think he is pretty predictable with his resource guarding. He guards/covets from other dogs. While not desirable, it's a lot easier to manage and yes manage is the most realistic as it's really more a wrinkle in his brain than any environmental learning.

You can tone things down a bit by putting him on a leash and giving him several really high value treats in quick succession. "Don't run off with it, there's more!" But choosing to leave with something valuable instead of fighting for it is a better choice so it's a judgement call on your part as you know him better.

I would give him way more space-separate rooms if necessary so he can eat without feeling like he has to guard. Give him a period of time to finish it 20 minutes and then it comes up. He will learn to eat now.

Have you read "Mine!" by Jean Donaldson? It's an entire book devoted to guarding behaviors.