As I pondered the eternal mystery of why dogs roll in disgusting gunk, I read Lyall Watson's "Jacobson's Organ and the Remarkable Nature of Smell", written in 2001, which is rather a good and lively read, if rather lacking in pictures or scratch 'n sniff bits.
Here's a sketch of a human embyro from Gray's anatomy showing the vomernonasal organ (aka Jacobson's organ) as the two openings in the developing nose (the blue is the nasal septum that will divide the nostrils and create the nose bridge.)
This is best for picking up the slow, thick molecules that don't hit your nose first off nor diffuse rapidly in air, such as musk, pheromones, and the immune system of your partner. Bloodhounds may have their extreme stamina due to equal development of the olfactory bulbs and of the vomeronasal organ, allowing the dog to switch back and forth to avoid nasal fatigue while on a trail.
I learned that you can throw a pebble into a river, have a dog go in and fetch the right one with your scent back to you. This is the foundation of "find it!" command for scent discrimination in utility dog trials, as well as obedience. Teaching a dog to pick things that have your scent on them can also make for cute magic tricks in which the dog picks the right card. Of course, you can skip the training and just daub peanut butter or bacon grease on the cards.
I cannot discuss this book in too much detail, because I think "sex" was used more often than "smell" in this book, and I'm not in the racy blog biz. He also hammers home his opinion about how smelly and stinky humans are to humans themselves and how distasteful human smell is. Well, Wilbrodog begs to disagree.
"Humans smell better when they don't bathe. Palms, face, and other areas that are extremely stinky are good to smell. MMM. People smell like food, sorta... but not in THAT way. Now when are you giving me that walk?"
The book takes time out from endless pheromone discussions to mention that the inhabitants of Madagascar, in a few thousand years of deciphering a land full of flora and fauna that are found nowhere else have managed to compile an impressive herbal lore. The author thinks that our noses guide us to what may be useful.
Ironically, when I was out on a walk with Wilbrodog, he started digging in a pile of leaves trying to eat something. It turned out to the leaves themselves. I checked, and the leaves actually have a mild, pleasant odor that reminds me slightly of peanuts or tea. For all I know, though, I am not smelling something else that is highly unpleasant to others. I confirmed that this was his target by picking up a branch and watching Wilbrodog try and eat as much of it as he could. Here's a photograph of it coated with dog drool. Since my nose said it also has leaf mold on it, I threw it out.
Native herbalists all over the world also depend on observing animal behavior for a clue to plants' properties, not just direct sampling and praying to survive. After all, if you see a cat going crazy after eating catnip, you know there's a reason for it. And in fact, catnip tea has been used as a remedy for centuries. Madagascar medicine seems less mysterious when you remember that they could observe wild animals and also feed the stuff to dogs, too. Woof!
One critter I particularly wanted to know more about is the Crested Rat (Lophiomys imhausi). Other sources describe it as being skunk-like in smell; however, according to Lyall Watson, the physiological effect is different from skunk spray. Instead of wanting to gag and vomit, you feel dry-mouthed, uneasy, and repulsed without noticing any overt smell-- suggesting it releases pheromones that act as a stimulant of some kind, activating the fear response. Dogs supposedly have died from eating Lophiomys. It is not eaten by people, period.
Back to Mr. Nosey. Why does he roll in icky leaf-laden mud that smells like the most nauseating, rotten gutter leavings in the world? Could it be for hunting down deer while disguised as dung, like wolves do? Or because it smells like perfume? Because it's a handy mudpack and anti-itch lotion? One thing is for sure, Mr. Nosey isn't rolling in it because it smells edible, or he'd be eating it, too.
Many animals roll in mud to repel insects or throw dirt on themselves, such as elephant, horses, cows, etc. After all, if you're ever looking to lay low and hide out from mosquitoes, mud and charcoal is good olfactory camoflague. Even birds will engage in dust baths and anting, which serve definite grooming purposes. Also, some animals will urinate or otherwise mark other animals as being part of their harem or troops. Dogs and other carnivores also like to roll in smelly things as well-- including skunks, live or dead.
Some dogs learn after their first skunk encounter, while others never learn. Hmm. Maybe some dogs also suffer from selective anosmia, and can only smell the great part of skunk smell.
Speaking of which, a neurobiologist wishes to disagree with the idea that humans have a poor sense of smell. We have a goodly number of olfactory genes; over 900. However, dogs have 30% more olfactory genes, and rats have 66% more. And while our nose is bigger than the great apes', allowing more surface area for olfactory uptake, it cannot begin to compare to most snout sizes of four-legged animals. So while we can certainly sniff how good or bad dinner is, we still can't track it down from a mile away.
But we can practice Kodo.