A universal game among most every species is "I'm gonna bite your head" - every litter of puppies, kittens, lion cubs, hyenas, meerkats - every nature documentary that shows youngsters invariably includes a session of this game.

If it's just mouthing and nipping, it's play behavior, and just something to keep an eye on so no pup gets hurt. They're establishing their relationship, only instead of bickering like human childen, they'll do so in puppy form.