The long-held and perpetuated belief that dogs have a sense of smell superior to their trusty humans is total bunk, according to a review recently published in the journal Science. This according to Rutgers University sensory neuroscientist Dr. John McGann, who sniffed out the truth by reviewing years of prior olfactory-related research and comparing the brain's response to smelly stimuli in mice, dogs and humans.
"When we started working with people, it was impressive how excellent the human sense of smell really is, and I started closely reading the many years of previous work on the subject," he explains in an email interview.
So, where did the idea of canines as superior smellers come from? McGann traced it back to the works of Paul Broca, a 19th-century-era neuroanatomist. Broca dubbed humans as "nonsmellers," without any sort of sensory testing to back it up. "He believed that the evolutionary enlargement of the human frontal lobe gave human beings free will at the expense of the olfactory system," McGann writes in Science.
Broca also noted that other mammals feature olfactory bulbs that are much larger in proportion to their brains than those in human beings. But this doesn't make humans any worse for the wear at smelling. In fact, primates and humans have an olfactory bulb making up 0.01 percent of the brain by volume, which compared with 2 percent of the average mouse brain can seem a little paltry.
"The human bulb is a very small percentage of the human brain, which is part of the origin of the myth that humans are bad smellers, but it's actually quite large in absolute terms, much bigger than a mouse, say," McGann explains. "It is also richly interconnected with many other brain regions that are larger and do more powerful computations in humans."
That's not to say that certain species don't excel at identifying specific types of scents, however. Dogs are better at discerning between urines (like those found on the stereotypical fire hydrant) and humans excel at differentiating between more pleasing aromas, like fine wines, and more critically important scents, such as volatile chemicals.
"When an appropriate range of odors is tested, humans outperform laboratory rodents and dogs in detecting some odors while being less sensitive to other odors," McGann writes in the paper, noting that humans are even capable of following scent trails in the great outdoors. "It is difficult to make generalizations, but I think it's fair to say that though the human olfactory system is different than the rodent or dog olfactory system; it is not worse than they are," he adds in the interview.
In addition to being more powerful than you previously thought, the human sense of smell is actually more important, too. Smells can cause us to recall specific associated memories, and can also elicit related behavioral and emotional responses. Personal odor can also relay information about anxiety, stress levels, even reproductive status – all things we once thought were only communicated by other animals, according to McGann. And an improperly functioning olfactory system can be a symptom of neurodegenerative diseases like Alzheimer's or Parkinson's disease.
"Smell is underappreciated in our daily lives. Most of the flavor of food is actually its smell reaching the nose by going up through the back of the throat. We are also beginning to understand that humans communicate information about our genetics, diet, and emotional state through individualized body odors that influence the behavior of other people, often subconsciously," he explains.
"Loss of smell can thus be very distressing for people, which is one of the reasons I wrote this article -- to increase popular awareness of the importance of smell in humans and hopefully encourage more emphasis on the development of olfactory medicine," he says.