Dogs play because they are practicing behaviors they need to survive in the wild. The definition of surviving is being able to live long enough to pass on your genes (and have them survive).
Most species stop playing after adolescence because they are now using those behaviors and no longer need to practice. Dogs are one of the few species who continue to play past adolescence/sexual maturity. Humans are another lifelong playful species. It's probably why we get along so well with dogs!
Play occurs in 15-30 minute increments. If it gets beyond that time frame, arousal levels often rise and the dogs will need a break. Most dogs will take a break on their own. A break can be as simple as changing play styles, changing playmates, or taking a literal break or nap.
Play can be broken into 4 categories: The 4 F's of Play
F1: Fight
Rough and tumble play styles. Face biting, wrestling, and chest bumping
F2: Flight
Includes both chasing and being chased
F3: Food
Rip and shake with toys, stalking, and tug
F4: Fornication (Sex)
Mounting, pre-mounting and courtship
Dogs use all 4 F's during play, but most have their favorite F. Dogs that play with each other often will have a "short hand" many times. The F's are present but might be less exaggerated. All are normal parts of play and all are required in order to fit the definition of survival.
How do you know if it's actually play or something else? Watch the body language of the other dog. Are the dogs taking turns? Is the other dog returning to play if the first dog is held still briefly (this is called a consent test)? Are their bodies staying loose and wiggly? If the answer is yes to all of those questions, then it is play.
It is possible for play to turn into something more than play very easily. All our staff at Positive Pooches are trained in dog body language and have the ability to notice when play is no longer play. Freezing, bullying, and chasing are just a few reasons why we would step in and break up play.
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Resources on play and body language:
Calming Signals - Turid Rugaas
Dog Body Language - Brenda Aloff