You Really Can’t Compare Human Noses to Dog Noses

Everyone knows that a dog's sense of smell is better than a human's, right? You can see it every time Fido goes tearing through the underbrush, chasing after something only he can smell. But just how much better—is it 30 times better? 200? 4,000? And what do these numbers really mean, anyway?

A new review published in Science suggests that by trying to quantify differences in olfactory ability—turning smell into a competition, essentially—we're asking the wrong kind of question. We've been selling ourselves short, believing humans have an pathetic sense of smell compared to our furry friends. According to the author, John McGann, a neuroscientist at Rutgers University, we can probably discriminate about one trillion different odors. We can track smells, and they can affect our emotions and behavior—ask anyone who awakens to the smell of bacon.
So where'd the myth of the subpar human nose come from? McGann traces it back to Paul Broca, a brain surgeon and anthropologist. Broca was a product of his time and place: namely, 19th-century France, where the Catholic Church was fighting secularization, and didn't look kindly on medical research that implied humans were anything other than unique, ensouled beings created in God's image.

Broca disagreed. As he poked around in the human brain, he concluded that our olfactory bulbs—the part of the brain associated with sense of smell—were relatively smaller than other animals because we had developed free will. Animals were compelled by whatever they smelled; humans, with smaller olfactory bulbs and larger frontal lobes (then understood as related to speech and thought), were not.

"Through a chain of misunderstandings and exaggerations beginning with Broca himself," McGann writes, "this conclusion warped into the modern misapprehension that humans have a poor sense of smell." It's a surprisingly persistent myth. Freud, unsurprisingly, connected smell to sex, suggesting that humankind's supposedly weak sense of smell led to sexual repression—but too much interest in smell could lead to mental disorders.

That kind of thinking has led us to underestimate the human sense of smell. In experiments, humans are on par with dogs and rabbits when it comes to detecting the scent of bananas; we're more sensitive than mice in sniffing out human blood. In an experiment where humans, rats, and spider monkeys sniffed for six different urine odors, humans were three times more sensitive when it came to one of the odorants but the rats and monkeys were more sensitive to the other ones. McGann also points out that the number of neurons in the olfactory bulb is fairly consistent across species; we're similarly equipped to smell, but we're just more sensitive to different scents. (Our sense of smell is not better or worse or just as good as dogs', as lots of headlines are saying.)

Beyond the "who's got the best nose" competition, though, is a deeper point. Smell is much more important than we think—it can evoke memories and emotions, and it can subtly influence our perceptions how we interact with others, including choosing a mate. McGann points to research suggesting that losing your sense of smell can presage memory loss and diseases like Alzheimer's and Parkinson's.

Far from being a feeble, shrunken part of the brain, our sense of smell goes to the heart of who we are as humans.

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