Smelling Through The Ages

Before we’re even born, we can smell. It’s one of the most developed senses in newborns, yet it starts to fade away as early as our mid-twenties. (How sad is that?) Our olfactory abilities don’t simply rise and fall with time; they morph—delicately tuning themselves to aid us in different phases of life. So follow your nose and come with us as we chart the development and denouement of our most mysterious sense.

In the womb

You’d think air would be a prerequisite for smelling. But just a 30-week-old fetus can actually catch scents by inhaling amniotic fluid through the mouth and nose. Nostrils and olfactory neurons are formed several weeks earlier, busily forging a pathway to the olfactory cortex, but until the end of the first trimester, the nose is too plugged up with tissue to take anything in. After that, a fetus can smell much of what the mother eats or inhales, to the point where it affects a newborn’s preferences after birth. In fact, mothers who have garlic, for example, have been shown to have babies who prefer it. (The same is true for alcohol and tobacco—yikes.) Scientists think that a fetus’s ability to smell is adaptive—the newborn would enter the world pre-studied up on mom’s smell, which helps with attachment as well as feeding.

Newborn nose

After a baby is born, one of their first jobs is locating the mother’s breast, and research tells us that the nose may be a big help here: one study had hours-old newborns choose between a mother’s breasts: one washed, one unwashed. Over two thirds chose the unwashed breast, supporting the claim that babies lean heavily on their sense of smell for guidance through early life. Score one for mom…your baby likes you even when you don’t shower.

Also on a newborn’s smell résumé: The very cool ability to pick out mom by her scent. Newborns can distinguish between mom’s breast milk versus other breast milk, and they prefer her scent to pretty much anything else. Leaving your baby for a while? Give her a shirt that you’ve recently worn so the sitter can use it to help soothe her in your absence.

Sour milk, anyone?

In the first few years of life, a child’s sense of smell is strong but limited. One- and two-year-olds may be pros at mom-related odors, but they aren’t as great at other odor discrimination. They don’t yet show aversions to what we adults would consider offensive smells (too bad they’re too young to change their own diapers). It’s only at around age three that kids will categorize things like poison or expired food as smelling “bad.” The detection abilities were there, but the associations weren’t.

Although it’s still up for debate, some scientists would say that liking or disliking smells is learned, not hardwired. In other words, we may love the smell of roses only because we notice others loving it. Even adults can be tricked: Rachel Herz, a cognitive neuroscientist and expert on the psychology of smell, ran an experiment where adults were to smell two scents—one labeled “Parmesan cheese” and one labeled “vomit.” People naturally rated the cheese as very pleasant and the vomit as very unpleasant, but surprise: The smells in the experiment were actually the same. Herz believes that our concept of a pleasant or unpleasant odor is a very fickle thing, highly subject to suggestion, context, and culture.

Nose, full-blown

Our olfactory powers plateau around age 8. We may still be growing our library of scent identifications and associations, but the hardware itself isn’t getting any upgrades. Then, around our mid-twenties, our sense of smell actually begins to deteriorate (For tips on how to stave off the decline, see here.)

The one exception is during pregnancy, where copious amounts of estrogen give noses a huge boost (though it may be an unwelcome one…heightened sense of smell and morning sickness often go hand in hand). Some scientists think this sensitivity may help mom eat things that are non-toxic and good for the baby—they even found that pregnant women who had morning sickness were actually less likely to miscarry. Other scientists say the improved sense of smell isn’t helpful to pregnant mothers, just another annoying present from your hormones.

A nose at rest

For most people, sense of smell declines so slowly and gently that many aren’t even aware that anything is different. Our constantly-regenerating olfactory receptors are replaced less quickly than before. Fibers in the olfactory bulb (which send messages from our nose to our brain) begin thinning out. So you can’t blame only your vintage Guerlains for smelling like a pale shadow of their old selves; your fading sense of smell is also to blame.

Once a smelling powerhouse, our noses may become a truly excellent glasses-holder, the better to see grandkids with.

 


Tarah Knaresboro
Tarah Knaresboro

Author

Tarah Knaresboro is a science writer who loves brains, noses, and the fun they enable.



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