Mild resource guarding is not uncommon among newly arrived foster dogs. It most commonly manifests over a high-value chew, food bowl, or favorite toy, although some dogs also exhibit it around doorways, thresholds, sleeping places, and proximity to the owner. Dogs may guard against other dogs or against humans. The classic pattern is that the dog hunches over his prized possession, lowering his head and forequarters to the ground as if protecting its guarded object with his own body. He may give an approaching interloper a sidelong glance without moving his head, showing the whites of his eyes in a phenomenon dubbed "whale eye" by Sue Sternberg. If tested further, he may growl, snap, or even bite.
Serious cases of resource guarding -- where the dog guards many objects, guards seemingly low-value objects such as empty food bowls, and/or actually bites a person or another pet -- require professional intervention and treatment tailored to the individual dog. Such cases are uncommon, but they do occur, and you should be alert to that possibility when getting to know your new foster dog.
Mild cases, however, may be dealt with at home. A low level of resource guarding directed at other pets in the household is common among newly arrived fosters. Many of these dogs have gone seriously hungry in the past and haven't had many (or any) toys of their own, so they place great value on these new prizes and are afraid to lose them, and they don't have any real relationship with you or other people and pets in the household yet, so they're not as forgiving of intrusions.
Don't punish a dog for growling or forcibly remove his treasures when he warns you not to take them. Punishing him for growling only teaches him not to warn you before biting. Next time, he might go straight to using his teeth. Forcibly snatching his treasures teaches him that you can't be trusted and that he'd better be even more vigilant and ferocious about guarding his things next time, because if he drops his guard for a second you'll yank them away. Again, that road leads straight to a bite for you and a needle for your foster pup -- not a place anyone wants to go.
Mild resource guarding is usually fairly simple to nip in the bud. Once again, management is key. Feed your new foster dog all his meals in the crate and only allow him to have high-value chews when he's being crated for bed or while you go out. This is advisable anyway because it prevents him from being interrupted by other dogs or small children while eating and builds pleasant associations with being in the crate; that it happens to be the best preventative for resource guarding is just a happy accident.
Crating the dog during meals and while he's enjoying chews accomplishes two things. First, it reduces the risk that someone will accidentally provoke the dog into snapping or biting when he's guarding one of his treasures. Second, it gives the dog no opportunity to practice his misbehavior and develop it into a habit. If the foster dog learns that growling makes people back away from the things he wants, he has an incentive to growl more often and start guarding more objects. It's safer to keep him in the crate and just totally ignore any stray growls he makes while in there. Don't punish them, don't move away in response to them, just ignore them as if you didn't hear a thing and continue your normal daily routine.
When you're ready, you can add the Trade Game to your training regimen. The intent of this game is to prevent resource-guarding behavior by teaching the dog that it's not only no problem for people to take his treasures, it's a good thing, because it means he'll get even better treasures in exchange -- and then he'll get the original thing back!
The Trade Game/Let's Make a Deal
For this exercise, you need a low-value toy or treat such as a dry biscuit. It should be something that your foster dog cares about just enough to notice and half-heartedly play with, but not something he likes enough to guard. When your dog displays some interest in the bait toy, say "Gimme" or "Trade" and offer him a small piece of something really nice -- cheese, a little chunk of hot dog, a bit of roast chicken, whatever he loves. When he takes the high-value treat, you take the low-value treat (which he will probably have dropped and completely forgotten about in the meantime).
After he eats the high-value treat, try to get him interested in the bait treat again. Scoot it along the ground, play with it yourself, or toss it from hand to hand and then drop it on the floor in front of him. When he sniffs at it or picks it up, say "Gimme" or "Trade" again. This time, take the bait treat away before you offer the treat. The goal is to teach him that removal of the bait leads to super! awesome! prizes!!, so the order of events is important: the bait treat goes away first, then the prizes come out.
Gradually work up to more valuable objects (empty food bowls, empty Kongs, medium-value toys). You may be able to work up to playing the Trade game with his meals, removing them every so often and offering a treat in exchange before handing back the food bowl. Always keep close watch on your foster pup to see whether he seems to be getting possessive or is enjoying the game and playing it gladly.
If the dog's behavior is more severe than you feel comfortable handling on your own, please consult with your foster coordinator and/or a professional trainer. Resource guarding is one of the main causes of dog bites; it is not an issue to be taken lightly.