A dog's sense of smell is one of the means by which he experiences life and the world around him. For his ancestors, a keen sense of smell was necessary for survival. Now it aids not just theirs but even human survival -- consider the work of search and rescue dogs. Lost dogs can and do sniff their way home.
Dogs' noses have more than 40 times as many scent receptors as humans have. Depending on the breed and the scent in question, scientists estimate that their sense of smell is between 40 and 1 million times stronger than humans'. It is believed that one-third of the canine brain is dedicated to interpreting scent data. The shape of the canine nose, the mucus within and the act of sniffing itself all help dogs take in scent molecules. Dogs have something called a Jacobson's organ or vomeronasal organ, which humans do not have. Located in the roof of the mouth, it is sensitive to odorless chemicals like pheromones.
The ultimate purpose of your dog's sense of smell is survival. It warns him of nearby presences; it alerts him to the scents other animals leave behind in urine markers and pheremones, including those in heat; it alerts him to natural prey that has been or may still be in the area. By domesticating canines, people co-opted their keener sense of smell to find food for human subsistence.
Dogs who find their way home do so by retracing their steps. Obviously, scent is part of this process. Trailing and tracking dogs home in on and follow a specific scent, ignoring all others. This scent might be that of a human, of a fellow pet at home, or even their own scent. Once they recognize it and pick it up, all they have to do is follow. Surprisingly, a part of this process is memory. According to WebMD, dogs are exceptionally good at making mental maps and knowing precisely where they are in space and time. This sharp memory combines with their sense of smell to help them get home.
Obviously, a scent trail will not last forever. Time, weather conditions and even environment affect the presence of scent molecules. According to founder Missing Pet Partnership founder Kat Albrecht, who has spent eighteen years training, observing, working with and learning from search dogs nationwide, a scent trail may last long enough to be followed for thre to four weeks in ideal conditions. Ideal conditions for a scent trail are cool, damp areas with heavy vegetation and no wind. In urban environments, such as city streets, the chances of a scent trail remaining longer than four days are slim. A heavy downpour, a scent trail on a sidewalk will gather into gutters and puddles -- ostensibly a dog could follow a puddle patchwork. Light rain does not completely wash away scent molecules. In fact, water helps scent survive longer.
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