Dogs Can Smell Time
It can be easy to assume that dogs have a poor sense of time - they are often just as excited to see you after 2 minutes as they are after several hours. However, despite their extremely excitable nature, dogs actually have an acute sense of time that is driven by their sense of smell. After all, my dog knows to wait by my side door every evening between 6 and 7, because she knows that's when I usually get home from work. My fiance tells me she even waits there when I'm out of town, before dejectedly returning to the living room when I don't walk in by 7:30 or so.
As time goes on, the air in a room or house moves. Hot air rises, and usually travels up the walls of a room. Therefore, air travels up the wall, moves to the center of the room, then drops as it cools. As this motion is repeated, the smells that the air carries travel in the same pattern throughout a room. If, like a dog, you can smell these underlying odors, you can get a good sense of the time of day based on how the room smells. Morning smells different from evening. Likewise, a person's smell fades as time passes after they left, so a dog can tell how long you've been gone, or who was in the house while they weren't, and when. Weaker smells are older, stronger smells are newer. This is how tracking dogs find a scent - they follow the smell from its weakest point to its strongest point.
The way a dog perceives smell changes in air currents is similar to the way a summer day smells different than a winter day to a human, just on a much more minute scale. Weather and seasons, of course, then smell even more intensely different to dogs than they do to humans, which is why your dog might get nervous before a thunder storm and not when it starts raining.
So what exactly makes up for the differences between a dog's sense of smell and a human's? To begin with, dogs have "stereo olfaction," meaning their nostrils each interpret smells independently of one another. This helps them pinpoint the source of a smell's origin, and means your dog can tell what time you got out of bed just as well as it can tell what time you walked out the front door. Dogs also have hundreds of millions of more cells that transmit smells to their brains, which can account for the increased acuity of their sense of smell. They also have many more types of smell receptors, meaning they can differentiate between smells that human noses may lump together and interpret as a single odor.
