Dogs are very social animals and are expert communicators with one another.   Their evolution required this over the millennia to ensure the dog pack would function smoothly and could successfully hunt. We’d love to be able to speak and understand their language; however, our understanding of canine communication is stunningly limited.  It’s often hard for us to know what another dog is saying to our dog and so we may have no idea why our dog responds in a certain way - we call our dog’s behavior “unpredictable”.  Our dog would disagree.  

 Although much of dog communication is undecipherable  by us, we are aware of some of the methods dogs use to communicate. These include at least the following:

•  scent markers (including pheromones, urine, feces and anal gland secretions, and undoubtedly others we are unaware of),
• vocalization (such as barking, whining, yelping, howling, growling, grumbling and general muttering),
• visual signals (such as body postures; appearance of facial features such as eyes, ears, and mouth; appearance of other body features such as tail and hair coat),
• Body movements (fast or slow; face-to-face or indirect; closeness of physical contact; use of the mouth for licking, snapping, or biting).

Of these, scent is probably the most important for communication between dogs.  A large part of the canine brain is devoted to the sense of smell (compared to the very small portion of the human brain which has this function).  The first thing dogs do when they meet peacefully is give each other a thorough sniffing over,  and the most important and enjoyable part of most dog’s daily walk is sniffing everything possible, and maybe leaving behind a few scent marks of their own (pee-mail?).    Unfortunately, this major aspect of dog communication is probably the least understood by humans since it is completely outside of our own experience.   Who really knows what information dogs are picking up from sniffing one another?  We can only guess.