When a dog enters the scent cone and detects odor, how does he follow it back to its source?
A dog’s sense of smell is so fine-tuned that he can distinguish which nostril is experiencing more odor at any given time. So when his left nostril smells more odor than the right, he will turn left. Then if he leaves the scent cone and the right nostril detects more odor than the left, he will turn right.
To observers, the dog’s initial head snap (when he first detects odor) is often followed by bracketing back and forth. Eventually, he’ll narrow down the precise location of the source.