No exchange of hellos or high fives will likely land Fido and Scruffy a spot on a late night talk show, but dogs can and do communicate with one another. Dogs are very expressive and communicative creatures who rely on body language to convey playfulness, affection, passivity, and aggression.
Dogs have a unique language.
As with people, not all dogs communicate the same way. Even within the same breeds, some dogs communicate better than others. A dog who has been socialized will likely communicate better than a rescue dog who has been cooped up in a cage for a long time.
There's no surefire way to know that a specific gesture always means the same thing, but you can still get a general idea of whether a dog is trying to establish dominance or is nervous and apprehensive by reading his body language.
A very playful dog who meets a new pal will likely engage his newfound friend in play by lowering his forequarters while keeping his hindquarters raised. He might paw at his potential new playmate or dance around him and present his hindquarters for sniffing — their version of "Hello! How do you do?" — while smiling. A dog "smiles" when he grimaces without engaging in any other behaviors that might signal a threat for a fight.
Some gestures that signify passivity or playfulness can also be meant as affection. A dog who licks another dog's ear or body, for example, is conveying to that dog that he is deferential to him at that moment, but he's also showing him affection. Gentle pawing or even placing a paw on top of another dog's paw or body is also a sign of affection. The dog is letting the other one know that they are both part of the same pack and his behavior is acknowledging the bond that exists between them.
When a dog stands straight and stiffens his tail while staring at another dog, he is hoping to challenge another dog in a particular situation, whether getting to a food bowl first or gaining control over a toy. Other gestures include mounting the other dog (which is not always sexually motivated!), putting his paws on top of another dog's back, looming over another dog and trying to intimidate with his size and urinating by lifting his leg and clearly marking territory as his.
Though dogs don't fight over who will come out on top in all circumstances, two dogs may challenge each other for control during individual situations.
If a dog is challenged by another who appears to be a possible threat, a passive dog who doesn't want to fight can communicate this to the other by doing something called "pretzeling,"corkscrewing his body into a pretzel or "C" shape while playing with the other dog. In addition to letting the other dog get whatever it was he's after (whether food, a toy, etc.), the passive dog will lower his forequarters, lift one paw and throw his ears back, making himself smaller and allowing the dog to whom he is deferring to loom over him.
A dog defers to another one if he feels vulnerable in that particular situation, or if he just wants peace at that moment. It's a dog's version of, "Hey, I'm not looking for any trouble." Gestures that show deference include a tucked tail, freezing in place, averting eye contact, rolling over and exposing his belly and submissive urination.
Aggression can be a result of a dog being trained to be aggressive or be fear-based. A dog who has been neglected or abused and deprived of humane human contact may display fear-based aggression, for example. Gestures of aggression include eyes staring straight on with no blinking, ears forward and teeth bared. The muscles in an aggressive dog's torso and trunk will tense up and his hackles will raise. Gestures may be the same for a dog who displays fear-based aggression, but when a dog's eyes look forward and his pupils are dilated he is displaying fearfulness and stress. Likewise if his ears are back and pressed against his head and he is panting or breathing hard through clenched teeth and has his tail tucked or low.