Is the human sense of smell just as good as a dog's?
The long-held belief that our noses are weak is dispelled by new research in journal Science
When we see dogs sniff the ground and follow a scent, it gives the impression their sense of smell far surpasses our own. But research suggests that that may not be the case.
In fact, if you tried it, you might find that, on your hands and knees, you could track whatever your four-legged friend is tracking.
That's the conclusion from neuroscientist John McGann, of Rutgers University-New Brunswick, who conducted a meta-analysis of more than 1,000 scientific papers on humans' sense of smell — or olfactory ability.
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The idea that human sense of smell is magnitudes inferior to that of dogs, and even mice and rats, dates back to 1879, McGann reported in the journal Science. That year, renowned anthropologist and brain surgeon Dr. Paul Broca wrote that our olfactory bulb was quite a bit smaller than the brain, which meant humans had free will and didn't need to rely on smell to survive. It's an idea that seems to have stubbornly taken root, even though several studies have proven that our olfactory bulb isn't small at all, particularly compared to that of a mouse.
In fact, when McGann examined the two together, he was surprised at how large a human one was.
"I went and got a human brain from a med school … put it next to a mouse brain and went 'Holy cow, this human stuff is way bigger than we thought!' McGann said. "The olfactory bulb was too big to get under the microscope. We had a lot of trouble getting them in the same photo."
So why is our olfactory bulb persistently maligned even though the evidence over the past 138 years has proved that it's pretty impressive?
"Part of the challenge has been confirmation bias, and part of the challenge has been, well, it's difficult to compare a dog's nose to a human's nose to a mouse's nose," McGann said.
When we inhale, through our noses, molecules enter, triggering olfactory sensory neurons, which in turn send the message to the brain. There are hundreds of receptors, each of which can fire independently or in groups, helping us to identify a particular smell.
Mice have about 1,000 of these receptors, while we have 400. Somehow, McGann said, people have been hung up on that number.
However, what decodes those smells are called glomeruli, and it's believed we have far more than mice or dogs. So this may help us sense up to one trillion odours, significantly more than the 10,000 once believed and comparable to dogs and mice.
Dogs vs. humans
A 2006 study by Jess Porter and Noam Sobel that was published in Nature Neuroscience suggested we actually can track a scent like a dog.
In the study, the researchers blindfolded 32 human participants and had them track a 10-metre piece of twine that was dipped in chocolate essence. The participants also wore earmuffs, kneepads and thick gloves to prevent them from using any of their other senses. Two-thirds of the participants tracked the scent and even zigzagged across the path as a dog does.
"Humans have a really, really good sense of smell," Sobel told CBC News. "Amongst researchers there's very little doubt about that."
However, he said, the question is to what extent do we rely on it? Sobel believes we rely on it more than we might think.
Studies of our olfactory senses have discovered an array of fascinating things, said Sobel, who heads the Olfaction Research Group at the Weizmann Institute of Science in Israel.
One study from the institute found that a woman's tears can reduce testosterone; a study from Denise Chen from Rice University suggested that if you take sweat from someone in a fearful situation and expose it to others, it changes brain activity.
But Sobel said that not everyone agrees.
"There are no olfactory scientists who will say 'Oh humans have a horrible sense of smell,'" Sobel said. "What many people will argue against is that it's a critical part of our behaviour. That's where the question is really open."
But why aren't we constantly being bombarded by smells? Most of the time we don't notice odours. That's due to adaptation: we're good at detecting changing smells and filter out constant smells.
"You ever notice if you go on vacation and then come home and your house smells funny?" McGann said. "Your house always smelled like that, just you didn't notice it because you live there."
So are we as good as dogs? It's an open-ended question, McGann said. It can be said that we are as good as, but not the same as, our canine companions.
"To say that the human is not bad compared to a dog or rat is not the same as saying the same," McGann said. "All of our noses are tuned kind of differently. So it's not surprising, in retrospect, that there are some odours that we're more sensitive to than dogs, and there are some odours that dogs are more sensitive to than humans."