Humans have a centuries-old reputation as poor smellers. Though we can see more colors than the average mammal, our noses are simply no match for the questing snouts of rabbits and hounds. Sure, the aromas of coffee and pie are great. But intelligent humans outgrew the need to sniff our way through life. Or so the thinking went.
In a review published Thursday in the journal Science, John McGann, a professor of psychology at Rutgers University in New Jersey, argued that this is a flawed perception dating back to the 19th century. He blamed pioneering French anatomist Paul Broca, who wrote that, given the comparatively small olfactory organs in the primate brain, “it is no longer the sense of smell that guides the animal.” As for smelling in apes, humans included, “All that exceeded the needs of this humble function became useless.”
Broca was hunting for the part of the brain that gave humans free will, McGann said, to separate us from animals. At the time, too, the Catholic Church in France was criticizing Broca's work at the Faculty of Medicine in Paris.
“He’s under pressure for humans to be special,” McGann said. “He’s under pressure for humans to be different.”
To Broca, that meant highlighting the large frontal lobes of the human brain. The olfactory bulbs, two lumps of nerve endings in the brain that process scent, were trivially small in comparison. Human olfactory bulbs account for just 0.01 percent of the brain’s volume. (In mice, the fraction is 200 times larger.) Broca, then, divided mammals into smellers and non-smellers. He included humans among the latter along with dolphins, which lack olfactory bulbs entirely.
“He thought of smell as this very animalistic, bodily, earthy thing. Smell makes animals have sex,” McGann said. The depiction of primate smell as a humble function stuck. “It’s taught to this day, all because Broca thought we had to have free will.”
It's high time to end the myth, McGann said. Humans have just about the same number of neurons in their olfactory bulbs as mice, capybaras and star-nosed moles. Scientists who study human olfaction would argue, “Of course the human sense of smell is excellent!” as McGann told The Washington Post.
Beyond that expert group, though, people are generally unaware how potent this sense is, said Johannes Reisert, who studies olfactory receptors at the Monell Chemical Senses Center in Philadelphia, in an email. McGann, he said, makes a “correct case that our sense of smell actually is pretty good in doing what we need it to do.”
For years, McGann studied the olfactory abilities of the mouse. To bridge his work between mice and people, he began to hunt for scents that people couldn’t tell apart. Instead, he found that “it’s pretty hard to find two odors that people can’t discriminate.”
Meanwhile, behavioral experiments began to demonstrate the true power of human smelling. One of the most dramatic involved blindfolding psychology students and having them sniff like dogs.
Passersby at the University of California at Berkeley campus at just the right moment a few years ago would have spotted something peculiar: Students, wearing thick gloves, blindfolds and earplugs, on their knees and elbows with their noses pressed to the grass in the manner of a bloodhound. They had been tasked with following a 30-foot string soaked in chocolate oil through a field.
Twenty-one of the 32 subjects were able to sniff their way along the string, using their nostrils to follow the oil as it twisted through the grass. If the experimenters plugged the subjects’ noses, though, all of the students failed the test. As scientists reported in the journal Nature Neuroscience, this was the first experimental evidence that humans could track by scent alone.
What limited studies exist suggest that sensitivity to smell depends on the odor, McGann said. Humans are about as sensitive as dogs at detecting amyl acetate, a chemical with a banana odor. We're better than mice at detecting a smelly compound in human blood. And one widely cited 2014 estimate, though later critiqued, suggested that humans can detect 1 trillion different scents.
“It's fair to say that there has not been exhaustive cross-species testing of sensitivity to different odorants,” wrote Alexandra Horowitz, an expert in canine cognition at Barnard College in New York, in an email. “Those that have been tested often find dogs able to detect at lower quantities, and do sometimes find humans surpassing or equal to non-humans (as on amyl acetate).”
We're great at food smells, she pointed out, though that's often lumped in with taste. But Horowitz would not go as far as to say that humans are particularly good smellers among mammals, only that the human sense “is really not poor.”
Complicating the question is that there's no real metric by which to judge variety. When it comes to sound perception, there are high and low frequencies. But where on an numerical axis would you place the chemical cocktails responsible for the fragrances of bananas, human blood and coffee? (Down that path lies madness, or descriptors normally reserved for wine.)
Some canine feats appear almost super-heroic. Horowitz cited the “incredible detection abilities evidenced every day by search-and-rescue, narcotics- and explosives-detection dogs, as well as everything from bedbug detection dogs to dogs who can find the scat of orcas in Puget Sound.”
McGann argued that dogs may be better at sniffing urine but humans could, perhaps, be better at smelling wines. “There’s a terrible shortage of data supporting that humans can’t do things,” he said. “That’s an experimental question that hasn’t been done.”
(Not only can dogs identify other dogs by the urine left on a hydrant, Horowitz countered, “they can tell from the concentration of the urine left one step apart which direction the dog who left the urine was headed — because the direction is indicated in the slightly greater concentration of the more recent mark.")
Beyond questions of sniffing superiority, though, are the understudied health effects of human smell. A sniff can be more powerful than we consciously understand.
“If you’re really anxious, there’s a smell that seems to go with it,” McGann said. People who sense someone else's anxious odor may themselves become stressed, as a 2009 study suggested: “Smelling the feelings of others could be termed as an incorporation of the chemical expressions and thus the feelings of others.”
Reisert called smell the “ugly duckling” sense after hearing and vision. The “loss of olfaction is broadly seen as benign,” he said. “It is not!” From aging, diseases like upper respiratory infections or trauma such as car accidents, it is possible to lose the sense of smell, a condition called anosmia.
This disconnection from a world of scents affects not only diet but emotional health, McGann said. “The medical world just kind of shrugs,” he said. “I’m hoping that by raising the profile of smell, maybe we’ll get more traction for studying it.”