My two boxers 9 and 4 months have been pushing their MuZzles together with their mouths open and growling while walking Is this a form of playing

Yep, that is play behavior. Thank you for not leaping to the conclusion that something growly has to be aggression. This is not.

If they are the same sex, fighting could occur when the younger dog is about nine to eighteen months. At the ages cited in your question, fighting is unlikely. If they are opposite sex, fighting is unlikely, although it’s not unknown for a teenage male boxer to become a bully. If one of your dogs shows that tendency, direct a specific question about that to me or another knowledgeable dog person.

This is not play; it is territory competition. It is neither healthy nor unhealthy, it is just the way dogs express competitive instinct. Another Quora user asked why racing Greyhounds wear muzzles. This is the same behavior being played out in a different context.

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