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Ricardo Bilton has emailed me to ask for my take on the following -
"If the history of the dog is one of humans breeding out unfavorable traits for the sake of more preferable ones, then why haven't dogs been selected to not sniff each other's rears when they meet each other? To me that seems like a pretty major thing that breeders would want well-behaved dogs to stop doing.
I have two hypotheses. Is it possible that dogs who do not possess the inclination to sniff other dogs' rear ends also lack the most basic canine social inclinations? (I.e. butt sniffing is linked to general affability.) Breeding these dogs, then would result in dogs that are just not the greatest social animals.
The second option is that perhaps people have historically not cared too much about it."
I think that both of these are probably right, in their way, but that the first is the key to understanding the persistence of this "disgusting" (to some) behavior. In "Dog Sense/In Defence of Dogs" I argue that one essential function of mutual sniffing is to gain/renew information about what the other dog smells like. The dog's primary sense organ is its nose, not its eyes, and so dogs are probably much more likely to remember other dogs by what they smell like than what they look like. However, not all encounters between dogs get as far as sniffing, so vision must also be important, particularly in identifying dogs that may not be safe to sniff.
But while vision has the advantage of allowing circumspection, odor has the advantage that it can be left behind, both in urine-marks and, in the case of the contents of the anal sacs, on feces. Dogs can therefore get a great deal of information about the other dogs in their area from the scent-marks they have left behind - hence many dogs' obsession with sniffing every landmark they pass. However, they will get a lot more information about these scent-marks if they can also recall sniffing the dog that deposited them - a process given the term scent-matching by Professor Morris Gosling, back in 1982.
Scent-matching requires regular contact between dogs, because each dog's characteristic odor changes from month to month. This is not just caused by changes in diet, although these must play a part, but also by changes in the bugs that inhabit the scent-producing glands - it's often not appreciated that most mammalian scents are not produced by the mammal itself, but by micro-organisms on the skin and in the glands themselves, feeding on largely odorless precursors provided by their host.
Scent-matching probably also has a role to play in the dog's courtship behavior, specifically if it allows a bitch to distinguish between, and then select the best of, her potential suitors. While bitches may not be given such options as often as they used to (though it's not unknown for pedigree bitches to reject their owner's choice) such behavior must have played a major part in the evolution of the dog and so was unlikely to disappear from their repertoire.
