An agonistic pucker, also known as a warning snarl, is a distance increasing behavior.  It can be seen in young puppies, and in adult dogs.  They are a dog’s way of signaling displeasure and the desire to increase distance or stop interacting.  Failure to heed an agonistic pucker during a warning, could result in the escalation of threats and/or the initiation of a fight.

41 Day-Old Puppy Being Pinned Does an Agonistic Pucker to Express Her Displeasure and Desire for Greater Distance Between Herself and the Pup Who Has Her Pinned.

16 Month-Old Dog is Doing a Shoulder Roll/Play Bow. The Agonistic Pucker Definitely Suggests His Desire for Increased Distance, Even Though The Play Bow Says This Is Still Play.