Which sense do dogs rely on most?

Smell, and the gap is not marginal. A dog has something like 200 to 300 million olfactory receptors against our five or six million, and the part of the brain devoted to processing them is proportionally around forty times larger. Estimates of sensitivity run from ten thousand to a hundred thousand times ours.

The anatomy is built for it. A dog's nose splits inhaled air into two streams, one for breathing and one routed over a large folded sheet of sensory tissue, so it can smell continuously without interrupting the airflow - which is why a tracking dog puffs rapidly rather than taking normal breaths. The two nostrils sample separately, giving directional information, and the wet exterior traps molecules before they arrive.

What that makes possible keeps surprising researchers. Dogs have been trained to detect specific cancers on breath and in urine, to alert to an impending seizure or a diabetic blood sugar crash before the person notices anything, and to distinguish identical twins by scent, which no laboratory instrument routinely manages. They also read time by smell to some degree - a scent left in the morning is measurably weaker by evening, which is one proposed explanation for how a dog knows when its owner is due home.