There is no delicate way to ask. This issue is as sensitive as a dog’s sense of smell. To address why dogs sniff rear ends, it is important to understand their sense of smell and their communication techniques.
Do dogs really have a good sense of smell?
Like humans, dogs have five basic senses: taste, touch, hearing, sight, and smell. Of these senses, smell takes the lead in the canine world. A dog’s sense of smell is far more advanced than ours. When compared to humans, dogs have a smelling ability that is about 100,000 times more sensitive. The human nose contains about 5 million olfactory receptors that detect aromas, while a dog’s nose has 150 million scent receptors. Dogs devote about 30% of their brain mass to the detection and identification of odors, while humans use a mere 5% for olfactory purposes.
"Dogs devote about 30% of their brain mass to the detection and identification of odors, while humans use a mere 5% for olfactory purposes."
Plus, dogs have an additional tool to enhance their sense of smell. Dogs have a special organ called Jacobson’s organ (also known as the vomeronasal organ), which is located inside the nasal cavity and opens into the roof of the mouth, right behind the upper incisors. This amazing organ serves as a secondary olfactory system and detects specific chemicals by using nerves that lead directly to the brain. Unlike olfactory cells in the nose, the odor receptors in Jacobson’s organ do not respond to ordinary smells. In fact, these nerve cells respond to a range of substances that often have no odor at all; they work to detect “undetectable” odors.
Jacobson’s organ communicates with the part of the brain that deals with mating. By identifying pheromones, Jacobson’s organ provides male and female dogs with the information they need to determine if a member of the opposite sex is available for mating. In addition, this organ enhances the sense of smell that newborn puppies need to find their mother’s milk. Jacobson’s organ allows puppies to identify their mother from dogs using their sense of smell. Puppies placed between two nursing mothers will move to the one that gave birth to them.
The two separate parts of the dog’s odor detection system, the nose and Jacobson’s organ, work together to provide delicate sensibilities that neither system could achieve alone. When a dog curls his lips and flares his nostrils (similar to horses), he opens up Jacobson’s organ, increases the exposure of his nasal cavity to aromatic molecules, and becomes a remarkably efficient smelling machine. This may also happen through panting and lapping at the same time.
Do dogs use smell to communicate?
When two people meet, they evaluate body language, facial expressions, and tone of voice to quickly assess each other. With this information, they may choose to shake hands or hug, share a casual verbal greeting, cry with joy, or totally ignore each other.
Dogs may not verbalize, shake hands, or hug like humans, but they do assess each other and gather lots of information from body language. When two dogs meet, they usually walk in circles while scrutinizing demeanor and posture. Are the ears back? Is the tail wagging? Are the hackles raised?
Dogs have an advantage over people because they can use their keen sense of smell, along with a visual assessment, to get vital information about a new canine acquaintance. Biochemical compounds emitted by dogs are a mode of chemical communication. The aromas tell a dog what his new-found friend likes to eat and what sort of mood he is in. By simply smelling a companion, a dog can determine whether they are male or female, happy or aggressive, healthy or ill. Dogs can get a general idea about each other with a quick sniff and can get more detailed information by getting up close and personal.
So why do dogs smell each other’s rear ends?
What perplexes many pet owners is why dogs sniff this particular part of the anatomy. Why rear ends? Why not ears or feet? The answer is anatomical. Inside the rectum are two small sacs called anal glands which secrete a noxious-smelling substance into the rectum through a pair of tiny openings. The glands are emptied naturally when the rectal sphincter muscles contract during a bowel movement. Pet owners are unaware of this occurrence since the odor of the anal glands is masked by the odor of the dog’s stool; however, dogs can tell the difference.
Dogs sniff rear ends as a greeting and obtain vital information from the anal secretions. Is this dog a friend or foe? Is he going to be a good “date”? Is he aggressive? Is he feeling ill? And because the odor is unique to every dog and serves as a form of identification, two dogs can quickly determine if they have met before.
The very act of sniffing rear ends can establish the foundation of a canine relationship. A more assertive dog will usually initiate the sniffing, while the less assertive dog waits his turn. A submissive dog may end the sniffing first and retreat. A dominant dog may growl to end the smelling session. Some dogs like to limit the information they give out and will simply sit down and clamp their tails over their rectums, reducing the odor they emit.
Dogs also have a good scent memory. They can identify dogs they have not seen for years and who was the dominant member of the pair simply by smell. When dogs belonging to the same family are separated for a while, they use their sense of smell to find out what has happened during that time: changes in odors may convey where the dog went, what he ate, and what he did.
In addition to chemical communication, there is another purpose to sniffing rear ends. Dogs smell rear ends as a calming mechanism. Performing this innate ritual soothes them and serves as a stress reliever.
