A Dog's Eye (and Ear, and Nose) View
You and your dog may live in the same world, but the world you experience is very different. That's because we have evolved to eat different things and find our food in different ways--even if today, most of our food comes from the grocery store.
Scent: The biggest difference is in our sense of smell. While we tend to be visually-oriented, dogs are smell-oriented. If dogs shared slide shows of their vacations, they would show scent slides of the deer poop they found in Georgia, the fox urine they sniffed in Michigan and the dead seal they rolled on in Alaska. Our dogs must be perplexed that we are so blind when it comes to the rich world of scent.
You're familiar with the incredible feats dogs perform as contraband detectors, search and rescue dogs, hunting dogs, and human trackers. They can tell people apart through scent, and can follow scent trails that are several days old. But did you know dogs are also employed as insect detectors, mold detectors, explosives detectors and even cancer detectors? Dogs can smell cancer cells in the skin, urine (from bladder cancer), and even in the breath of people with lung cancer. If your dog sniffs curiously at a new mole or lump, you'd be smart to have that area checked by a doctor.
Dogs use their sense of smell in social interactions. They get to know each other by first sniffing at each others genitals, anus, mouth corners and ears, all areas that produce a good deal of scent. Of these, the genitals tend to elicit the most interest. When meeting new people, dogs also go for the crotch. Of course, that doesn't mean you have to stand for it!
Dogs also leave powerful scent signals through anal sac secretions. Some of these secretions are forced out whenever the dog defecates, imparting extra, and presumably individualized, scent to the feces (as though it wasn't already smelly enough). When dogs are extremely frightened, they expel their anal sacs to produce a strong musky smell that instantly elicits intense interest from other dogs, perhaps telling them that something terrifying happened there.
Dogs get information about other dogs from the urine they leave. They may even lick some of it up, in order to sample it with their vomeronasal organ, a small organ in the roof of the mouth used to decide if females are in estrus (in heat).
Hearing: Dogs not only beat us when it comes to the sense of smell, but hearing as well. Your dog can hear mid-range noises about four times farther away than you can. He can hear high-pitched sounds you can't hear at any distance. That's why dogs can hear dog whistles and you can't. It's also why they can hear some ultrasonic pest and flea repellers, which can be very annoying to them.
Vision: Finally, a sense where we can beat our dogs! Or can we? We can see fine details our dogs can't see, and our range of color vision is greater. Dogs have color vision like people who are typical red-green colorblind; that is, they can tell blue from yellow but confuse reds, oranges, yellows and greens. Even though dogs do see colors, they don't seem to pay a lot of attention to them. But dogs have the advantage when it comes to seeing in dim light and discerning slight movements. It is this ability to notice our slight facial gestures that we often don't give them credit for, yet it turns out that dogs are accomplished readers of subtle changes of facial expressions and body language.
It's not a question of which species has better senses--dogs have better senses for the world in which they evolved, and people have better senses for the world in which we have evolved. And when we team up, we have the best senses of all!