Each of our three dogs has, to one degree or another, exhibited food aggression. If you're not a dog person, let me explain: it's an aggressive (snarling, growling or - at worst - biting) stance that a dog will take over his or her food bowl to warn others, be they human, canine or feline, to back off and stay away.
So here's what I'm wondering: I've never seen a dog exhibit water-bowl aggression. Even our Miss Polly, who is by far the worst of the lot with the food thing, has no concerns whatsoever about sharing her water-trough with the others. Doesn't even bat an eyelash. I've seen her and Dozer face-down in the water bowl at the same exact time. (He always lifts out and drips all over the top of her head. It's a mess.) Do you suppose dogs have some rudimentary instinctual scarcity model for food that doesn't apply to (generally abundant) water?