Do you smell that?
I have a good nose for scents.
I think it’s something I inherited from my mother, a woman who can sniff out mysterious odors like a bloodhound. When I was growing up it was not uncommon for her to freeze mid-conversation, tilt her head back, and ask, with voice hushed and nostrils flared, “Do you smell that?” My sister and I never could, but we liked to tiptoe behind her on the hunt. She’d prowl from room to room, wafting air to nose silently, as if the offending odor was an animal we had to be careful not to spook.
Of the entire household, my father had the worst sense of smell. Whereas my mother could pinpoint the precise location of an errant sliver of orange peel from two rooms away, Dad was better at detecting more distinct aromas. He was able to determine, for instance, when the cat left a puddle on his side of the bedspread (the visual clue may have helped).
Since then, my sense of smell has matured, and I’ve noticed the gender-specific olfactory divide in my own marriage. This difference between male and female noses seems especially apparent when it comes to food: leftovers in particular.
Most people use scent as a first line of defense against possible food spoilage. When bacteria and fungi start colonizing the tupperware-clad remnants of meals past, the telltale signs are usually smelly. Microorganisms snip the connections that hold our food together, breaking it into bite-sized bacterial pieces, and releasing odiferous compounds in the process. Though these tidy clean-up crews can turn a piece of leftover lasagna into a microbial metropolis, like any booming population, they leave behind the garbage of their growth. And this is what can make us sick.
Humans have learned to associate the smells of decaying food with the hazards of microbial waste; we generally avoid eating anything that causes our noses to wrinkle. Well, most of us do.
Shortly into our relationship, I discovered that my husband has never met a leftover that didn’t smell fine to him. He considers my periodic purges of the fridge a waste of good food; I draw the line at leftovers that hint at malodor. (There’s a reason why married men live longer than their single peers.)
Although women usually outperform men in tests of odor detection and identity, on the mammalian scale of scent sensitivity, humans rank near the bottom. We’re not the worst though: dolphins and whales are believed to have no sense of smell at all.
Differences in mammalian nasal architecture contribute to this wide spectrum of olfactory abilities. In all smelling mammals, odor receptors (which detect and respond to specific scents- like citrus or urine, for example) are part of specialized nasal tissues that line the inside of the nose.
Dogs have about 100 square centimeters of this smell-sensing tissue (about the size of a dollar bill); humans have only 10 (that’s less area than a stick of gum). Clearly, not all noses are created equally.
But, it’s not just a matter of olfactory receptor real estate: even mice have more sensitive noses than we do. In fact, the nasal tissue of dogs, cats, rats, and mice is densely packed with an assortment of odor receptors. Humans also have the genes that encode these receptors, but many have mutated or lost sections, and the remnants are no longer functional.
If the genes that encode odor receptors were cars, dogs would have a parking garage full of well-oiled, finely tuned models. We’d have a more modest lot of vehicles, but most would have flat tires or broken-down engines.
Why have we lost so many of the genes necessary for a good sense of smell? And what are other mammals smelling that we’re not?
Germs. Pathogens, to be precise. Diseases. Using just their noses, mice can detect potentially harmful sicknesses. It’s a clever tool for avoiding illness, and can be helpful in selecting a healthy mate. In fact, mice are known to use smell to steer clear of prospective paramours harboring parasites, though until this summer, no one understood how.
In May, Swiss scientists reported in Nature the discovery of a new family of mouse smell receptors that not only senses microorganisms, but also the odorants produced by infection-fighting immune systems.
Though the ability to sniff out sickness and bacteria via these new health-sensing odor receptors was first described in mice, there’s mounting evidence that dogs have similar talents.
In a 2006 study in Northern California, 5 dogs were trained to identify breath samples from cancer patients. The clinic then presented the dogs with 169 different samples (86 from patients diagnosed with lung or breast cancer, 83 from healthy volunteers) and tested their ability to distinguish between the two groups. The dogs were 99 percent accurate.
It’s likely that sensitivity to smell helps certain animals avoid dangerous contamination (which can come in the form of bodily sickness or spoiled food).
Fortunately, humans have developed another way to determine food safety: trichromatic vision. In primates (us included), the loss of odor receptors coincides with the capacity to see in full color. In other words, what we’ve lost in smell, we’ve made up in the ability to detect blue-hued bits of mold. Our canine counterparts have not.
As humans, our sense of smell may be dulled, but, paired with superior vision, it’s sufficient to alert us to potential hazards lurking in our tupperware.
It’s not uncommon for human males, however, to have some degree of color blindness. After all, more than 95% of all variations in human color vision occur in the eyes of men. According to my husband, he can fully differentiate between colors. Based on his leftover preferences, I have my doubts.