Resource guarding is a common but very unwelcome behavior. Dogs are not big sharers; they typically like to keep what they consider a valued item. Some dogs never consider anything to be that valuable; others may consider only one particular item to be worth trying to keep and then there are the dogs who think that everything is theirs and theirs only. Recently I watched a video of Caesar Milan working with a dog who had a resource guarding issue with food. Being the conventional, harsh trainer that he is; he addressed the situation with dominance and in return, he was bitten pretty badly. Ah, the old way of training, no thinking, just acting. Not smart.
Often guarding can sneak up on us; if a dog has never had an object that they thought worthy of guarding it may have never come up before. If your dog growls at you for the first time it can be scary, saddening and confusing. Your once loved dog has growled; a growl often changes everything. But I am going to tell you that a growl sometimes brings things into perspective; and this is that our dogs are indeed dogs, not little furry people. A growl is simply a communication; a way for your dog to tell you how they feel. So they love whatever they have, enough to tell you that you are not allowed to have it.
If your dog has growled, displayed frozen posture or lip curled at you because of a possession then you have a guarding issue. You have been given the information needed to now deal with the behavior. First you need to consider what your dog was guarding. Dogs can guard everything from you, their owner to their food, a bone, all the toys or just one specific, special toy. For this example I am going to use a bone. Many people think that the solution is to take the bone away. That indeed gets rid of the need to guard but it does nothing to deal with the guarding issue.
What you have to convince your dog of is that 'humans' or the 'people' around him don't want the bone. But if we do want to touch the bone or hold the bone that he will be rewarded for allowing us to do so. I always start this training from the get go; that way it is something already dealt with when my dogs mature. First you work with something that is of lesser value and teach the drop command. Teaching the dog that if they drop objects when asked you will reward them and then give the object back. Once you have a nice drop then you need to work on the item which is guarded.
For bones I start with holding the clean bone and only allow the dog to chew it while I hold it. Then you can implement the drop command. When the dog lets go of the bone you offer a really meaty morsel of food as a reward and allow them to continue chewing the bone then. The big secret is to teach your dog that people around the guarded object is a great thing. Always work in baby steps, never proceeding closer to the object until you have a calm and relaxed response from your dog.
Once you have the dog releasing the bone nicely you will want to put the bone on the ground. Now what you must offer here is a very clean, meat free bone. You are going to approach your dog and toss meat to him before he displays a warning. So that means that you are staying far enough away not to elicit a warning. You are changing how he feels about your approach entirely. As you approach, meat is tossed; that is a great thing in his eyes. He has to drop the bone to pick up the meat so this is also making him let go of the bone in your presence.
Next is to add the drop command as you approach, when he drops you toss the meat to him and move on. You are going to work at getting closer and closer until you are bending down like you are going to take the bone but not. Not yet. You want to do lots and lots of practice runs before actually touching the bone. When you finally do touch the bone after asking for a drop, immediately give several pieces of raw meat and give the bone back. You will continue this until you can walk up, ask for a drop, pick up the bone, reward and give the bone back without any stiffness at all from your dog. You will continue this work for a good year. Make the association to you walking around your dog when he has a bone a wonderful thing.
Sure there are dogs who are just too guarded over their objects to offer up a big meaty bone to them, perhaps ever. But you should still work on things that are a bit lesser of value like a meatless bone. Our initial response when a dog growls is to simply take the item away; this shows the dog that they were indeed correct and do have to guard their stuff. Association is the root of the cure. Change the association of your presence to the bone and you will have rehabilitated your object guarder.
Also implementing a solid leader/follower relationship is essential.