Few things are more amusing to dog parents than an upside-down pooch snoozing away on the living room floor without a care in the world. But like everything else about doggie behavior, lying on the back is a lot more complicated than a dog just finding a comfortable position to take a load off.
Despite centuries of domestication and separation, dogs still hold onto some of the primitive behaviors of their wolf ancestors. Howling when a siren wails is one example. Rolling over on their backs is another. In wolf culture, rolling onto the back is a show of deference to the alpha of the pack, much the way a bowed head shows deference among certain people. Rolling over also helps wolves get out of potential danger by peacefully persuading an aggressor to back off.
Some dogs just like to sleep on their backs. Typically, a sound-asleep pooch on his back means he's supremely comfortable around you and he feels safe. In the wild, dogs, like all animals, worry about predators and so protect their bellies by sleeping on them. They also sleep belly-down so that they can get up quickly in case something comes by to harass the pack. A safe, secure-feeling dog is not worried about being belly-up to the world.
Dogs who lie on their backs around others are good communicators, able to use their body language to tell others that everything's all right whenever potentially troublesome situations arise. But consistent and repeated rolling over on the back could be a sign of a phobia, especially if it always accompanies one thing, such as the appearance of the vacuum cleaner. Consider consulting a trainer or behavior specialist if your dog always responds to a certain stimulus by rolling over.
If your dog is exhibiting signs of a phobia, the most obvious step is to not exacerbate the situation. But apart from keeping the vacuum at bay, avoid touching your dog if she goes onto her back in fear. Rolling over in fear is her way of saying "Please don't touch me." It's best to simply walk away if you have spooked her.
If, on the other hand, your dog is happy and safe and just rolls over while you're playing, he could just be hoping for a nice tummy scratch. Dogs, after all, are tactile creatures who love to be petted by their human pack mates. If they trust you, they will usually appreciate a good belly rubbing.
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