Any dog owner (or regular walker in places where dogs can be found) knows very well exactly how much time a dog can spend with its nose in another dog’s rear. (Hint: it’s a lot). But why is this such a vital part of canine ritual?
Dogs are capable of displaying great intelligence. They can be extensively trained and are generally very smart animals. You would assume that a creature of such brain power would not engage with body parts that poo and fart with little warning. With such highly developed olfactory organs it can’t be at all pleasant to have a nose full of manure. What does a dog’s bum have to offer?
Many animals have scent glands in the rear. They produce pheromones as a kind of ID scent and mix it with urine for strength. Dogs are also sensitive enough to tell from the scent of another animal’s waste products if they are sick, much the same as our doctors take samples for testing. It has even been said that dogs can be trained to diagnose cancer! These benefits, although very useful, are not the full picture.
From their heritage in pack hunting and the need for silent communication, dogs have developed an organ at the base of the tail which allows them to generate and manipulate tiny magnetic fields. They also have, in the nose, the other half to this coupling which can detect the changes in the magnetic waves. The positioning of nose-to-tail is valuable as it allows both dogs to be facing forward and maintain alertness in dangerous situations. The whole system is comparable to Morse code, and allows for a great deal of complex messages to be conveyed between animals quickly and quietly.
Sometimes you will find dogs try to sniff at human crotches too, this is simply a sign that they wish they could communicate with you in this way.