What inspires dogs to roll in yucky stuff is one of the world's unsolved mysteries. If you live with a dog, you've probably seen this situation go down. Spot's nose catches a scent and he ambles off. Suddenly he does a face plant, rolls over, and engages in a writhing shimmying break-dance that causes his entire back to skid along the ground. A look of canine ecstasy fills his face as he stands up, shakes, and repeats.

You come running and shouting in horror. Spot has been doing more than just innocently scratching his back. He's been slathering his torso with the unmistakable essence of something putrid and objectionable—to humans, that is.

A dog's olfactory system is one of the marvels of biology. It dominates a dog's brain much like vision dominates a human brain. A dog's ability to discriminate scents is at least a million times greater than ours. Another feature of the dog's sense of smell is its enormous "dynamic range." L. Wilson Davis, writer, trainer and author of Go Find! Training Your Dog to Track, explains it this way:

"If a skunk sprays your rosebush, you will be unable to smell the roses. Not so for the dog, who can discriminate among a number of subtly different scents all of which have been sprayed by a skunk."

A state-of-the-art nose is what makes your dog crave smelly things—such as stinky garbage, rotten carcasses, and of course poop—which a mere mortal would find unbearably disgusting.

Still, this provides only a partial explanation for this unpleasant habit. In fact, animal behaviorists debate about what purpose this Repulsive Rolling can possibly serve. Conventional wisdom says the behavior is instinctive, a carryover from a dog's wild canine ancestors. Here are some leading ideas:

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