Dogs tend to live in the moment, accepting each treat, snuggle, or toss of the tennis ball as the whole reason for their existence with a charming singlemindedness. Yet, their understanding of time can be complex. Many dogs are able to anticipate predictable events accurately, which is why they leap on the couch to look out the window when the kids are just about to come home from school or adults are nearly home from work. Even more dogs appear to know, from their own stomach’s rumblings, that the dinner hour fast approaches. And according to researcher and author Alexandra Horowitz, “Dogs smell time.”

What does it mean to smell time? Horowitz writes in her new book “Being a Dog: Following the Dog Into a World of Smell”, that their powerful noses allow them to perceive the passage of time. This is not mystical or other worldly. It’s just that dogs can understand much about the past because of the extreme sensitivity of their noses.

Odors change over time, sometimes predictably. When you leave the house to go to work each day, the smell of you in the house decreases with each hour of your absence, and your dog can detect the difference. Perhaps your dog has learned through repetition that when the smell of you has weakened to a specific degree, you come home. In other words, the strength of your odor predicts the time of your return.

This degree of scent discrimination is not hard for most dogs. Many dogs can, for example, tell which way to follow a scent trail by heading from where it is weakest (oldest) to where it is strongest (most recent) when the difference is miniscule. Stronger odors are often newer and weaker ones are older. That means that when dogs smell weak odors, they are perceiving events of the past. Because dogs can detect both new and old odors, they are perceiving events and substances across intervals of time.

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Each day, even in the same place, smells help dogs understand the passage of time. As air heats up over the course of the day, air currents change and move around in space, taking the molecules responsible for odors with them. Dogs, with their sensitive noses and large olfactory lobes are able to sense the movement and presence of chemicals people barely sense if at all.

Though we humans may detect daily patterns in light or even sound, our ability to smell clues about the passage of time, is barely worth a mention. Yet, dogs detect odors that reveal past happenings to them in complex ways we are only beginning to understand.