Dogs spend lots of time with their well-endowed nostrils stubbornly vacuuming the ground or pinned blissfully to the hind end of other dogs. They have about 25 times the area of nasal olfactory epithelium (which carry receptor cells) and have many thousands more cells in the large olfactory region of their brain (mean area of 7000 mm2) than humans (500 mm2). Dogs can differentiate dilutions of 1 part per billion, distinguish T-shirts worn by identical twins, follow odor trails, and are 10,000 times more sensitive than humans to certain odors.
When dogs wiggle their noses and inhale (suction) and exhale (snort) they concentrate odors, pool them into mixtures and expel others. Like wild relatives (wolves, coyotes) dogs gather much information from the symphony of odors left behind. Urine provides critical information about who was around, their reproductive condition, and perhaps their mood. Dogs expel millions of gallons of urine wherever they please (more than 1.5 million gallons along with more than 25 tons of feces per year in New York City) and use it well.
Odors are powerful stimulants. It's said that Sigmund Freud used soup smells to stimulate clients to recall past traumas. Although my late companion dog, Jethro (aka Hoover), enjoyed visiting his veterinarian, he showed fear if he went into an examination room where the previous canine client was afraid. Fear is conveyed via a pungent odor released by the previous dog's anal glands.
Now, what about sniffing other dog's urine? While talking Jethro on his daily walk I conducted a study of his sniffing and urination patterns. To learn about the role of urine in eliciting sniffing and urinating, I moved urine-saturated snow ("yellow snow") from place-to-place during five winters to compare Jethro's responses to his own and other dog's urine. Immediately after Jethro or other known males or females urinated on snow, I scooped up a small clump of the yellow snow in gloves and moved it to different locations. For some reasons passers-by thought I was strange and generally left me alone.
Moving yellow snow was a useful and novel method for discovering that Jethro spent less time sniffing his own urine than that of other males or females. Other researchers have also noted that male dogs (and coyotes and wolves) spend more time sniffing the urine from other males compared to their own urine. Dogs also usually spend more time sniffing urine from females in heat compared to urine from males or reproductively inactive females.
The differences in Jethro's response to the displaced urine from other males or from females are worth noting, especially when considering "scent-marking" behavior. "Scent-marking" is differentiated from "merely urinating" by a number of criteria that include sniffing before urinating followed by directing the stream of urine at urine that is already known to be present or at another target.
When Jethro arrived at displaced urine he infrequently urinated over or sniffed and then immediately urinated over ("scent-marked") his own urine but he sniffed and then immediately scent-marked the displaced urine significantly more when it was from other males than when it was from females.
While domestic dogs are usually not very territorial (despite myths to the contrary), their wild relatives are, and they show similar patterns of scent-marking behavior in territorial defense. I hope this brief foray into the olfactory world of dogs removes the mystery of dog sniffing, and gives some idea of what the dog's nose tells the dog's brain. You can easily repeat this simple experiment (and risk being called weird). The hidden tales of yellow snow are quite revealing about the artistry of how dogs make sense of scents.