There are several reasons a dog might lie on his back, and the reasons vary depending on the situation in which the behavior occurs. When a dog lies on his back during play with another dog, it means something different than when he lies on his back during play with his owner, and when a dog sleeps on his back, there is yet another meaning.
Dog on Dog Play
One of the ways dogs communicate with other dogs is through body language. A dog who is lying on his back during play with another dog might be communicating a number of things. It has long been believed that the dog on his back is being submissive and this is his way of telling the other dog that he isn't a threat. However, more recent research suggests there are other meanings to this behavior.
A study conducted by behavioral researcher Kerri Norman and her colleagues at the University of Lethbridge in Alberta, Canada and University of South Africa in Pretoria, South Africa concludes that a dog rolling over onto his back is not an act of submission; it is a combat tactic. Of the 248 rollovers during play sessions between two dogs that the group studied, none were submissive. The majority of rollovers were defensive moves to avoid neck bites. The dogs, once on their backs, blocked and launched playful bites at their partner too.
The study also concluded that the act of rolling over during play sessions between two dogs is most often used to facilitate play. An example given is that the larger of two dogs is more likely to rollover than the smaller dog to self-handicap himself and make play more fun.
Dog and Owner Play
When you are playing with a dog and giving him attention, he may roll onto his back to tell you he wants a belly rub. According to Dr. Margaret Gruen, a veterinary behaviorist at the North Carolina State University College of Veterinary Medicine in Raleigh, North Carolina, when a dog is comfortable with the person giving him attention, he might roll onto his back to give access to his belly for a rub because he likes the feeling of belly rubs.
Gruen points out that not all dogs like belly rubs. If a dog growls during a belly rub, you should walk away. She also says that pain, illness or behavior disorders could be the reason a dog growls. Consult your veterinarian if this happens.
Dog Sleeping Positions
Dr. Patty Khuly of Sunset Animal Clinic in Miami, Florida estimates 5 to 10 percent of dogs sleep on their backs on a regular basis. It is not a common behavior for wild dogs though because they need to protect their soft underbellies from predators. Khuly suggests that dogs who are furthest removed from their wild cousins in character are more likely to sleep on their backs.
So what does it mean when your dog lies on his back to sleep? It means that he is well-socialized and feels safe, secure and comfortable in his surroundings. It also means that he just plain likes the sleeping position.