You have two noses. You may not know it.
Plus a roundup of octopuses that taste through their arms and spiders that hear through their legs
Humans have a second nose.
There is a saying in Sanskrit – ghranena ardhabhojanam. Half of eating is in the aroma. This is something that most of us have forgotten. Flavor is not simply taste. Modern science has demonstrated that if you block the branch of the facial nerve originating in the taste buds, the flavor of common foods such as ketchup and chocolate drops by 50%.
In fact there are “two noses”. When we sniff something then we smell it by orthonasal olfaction. But when volatile molecules in our mouth go to the back of our throat, we smell by retronasal olfaction.
Most animals don’t possess the “second nose”. A dog is very good at sniffing, but it wouldn’t be able to decipher the aroma of an excellent meal. And the reason for this a missing bone; most animals have a bony traverse lamina that humans and some other apes and monkeys lost.
Look at the two green arrows (emerging from the back of the throat and in the nose) in the figure below and our two types of smelling becomes abundantly clear.
Why don’t we realize this? This is because most of flavor perception is an integrated sensation combining taste, aroma, and other sensations in the mouth. The results of this are quite remarkable. Most of us underestimate the effect of smell on our lives until the point we lose this sense. Unless they are remarkably odious or pleasant, odors jump across from conscious to unconscious. We can all become smell-blind over time.
We knew very little how smell works until recently.
The sense of smell was, until recently, very poorly understood. And by recently, I mean until 1991.
Dr. Linda Buck, was a postdoc in Prof. Richard Axel’s lab at Columbia University who took on the hardest project possible— figuring out how smell works. In a landmark paper in Cell in 1991, the two scientists discovered the family of receptors that are responsible for the sense of smell.
Only thirteen years later Buck and Axel won the Nobel Prize for their landmark work. The 1994 Prize citation says it all—
The sense of smell long remained the most enigmatic of our senses. The basic principles for recognizing and remembering about 10,000 different odors were not understood. This year’s Nobel Laureates in Physiology or Medicine have solved this problem and in a series of pioneering studies clarified how our olfactory system works. They discovered a large gene family, comprised of some 1,000 different genes (three per cent of our genes) that give rise to an equivalent number of olfactory receptor types. These receptors are located on the olfactory receptor cells, which occupy a small area in the upper part of the nasal epithelium and detect the inhaled odorant molecules.
How smell works (and why it is so complicated).
Let me try to explain how smell works in simple terms. Each smell is caused by a chemical substance referred to as an odorant. There are special kinds of cells in the nose known as odorant receptors. We detect smell by inhaling air that contains odorant molecules that attach to the receptors inside the nose. Each odorant receptor can recognize very specific odorant molecules that result in different smells. These receptors send electrical signals straight into the olfactory bulb of the brain. An entire repertoire of signals is combined together and then the brain constructs smell.
Odor perception is remarkably subtle. I don’t think people realize just how subtle it is. People understand how vision and hearing work better than they understand smell. Hearing and vision are tied to the physical properties of sound and light, while smell is tied to the chemical properties of substances— and chemical space is massive in comparison. Most scents are composed of many odorants; the aroma of chocolate is made up of hundreds of them.
The degree of specialization of cells is remarkable. There are three receptors for vision and at least four hundred for scent.
Because there are so many olfactory receptors, a mutation in one can lead to a different perception. For example, many people find that cilantro (dhaniya) tastes soapy because of a genetic variation close to the OR6A2 gene for a specific olfactory receptor.
Smell is a fickle sense.
If smell results from chemical compound which are called odorant molecules, you wouldn’t be off base to think that changing a chemical structure would change the smell. Unfortunately, smell isn’t predictable from chemical structure yet, because the “coding” is in the brain. Molecules which are very similar in structure can give very different smells.
Humans have fewer olfactory receptors than many other animals supporting the general observation that we have a generally poor sense of smell. However, the sensitivity of smell may not have to do with the actual number of functional receptors. Smell is, after all constructed in the brain. So, while people may not have as many olfactory receptors, we have a large olfactory brain. Often, we are not are not aware of the sense of smell and how it integrates with pleasant (or unpleasant) experiences. Smells make us hungry or lose our appetite.
The olfactory brain is highly plastic and can learn. Sommeliers (expert in wines and wine pairings) can often recognize very distinct aromas in a wine that they describe using colorful language. Fragrance specialists also have a keen nose.
Different people can smell the same substance differently based on how their receptors work so it is a subjective sensation. French perfumer Cristophe Laudamiel dipped strips soaked in a substance called sulfurol. No one was sure what it was because they were not familiar with it. When shown a picture of milk, the sniffers thought it was milk. When shown a picture of ham, they thought it was ham. They had never smelled sulfurol, but the pictures trained them to associate the smell with objects that they knew.
You can change the association of a smell with an object. You can recode your preferences. Fish eaters can tolerate or even salivate at a fishy smell, which is downright obnoxious to those who don’t like it. Martini olives or vinegar are not intrinsically pleasant smells.
In fact taking this to an extreme, in the 1980s, a company swapped the labeling of two fruit-flavored drinks: strawberry drinks were labeled as raspberry and vice versa. The company expected a backlash, but as A.S. Barwich details in her new book Smellosophy, hardly anyone noticed. The two smells are very easily confused when considered in isolation!
Octopuses taste through their arms. Spiders hear through their legs
In the past few days I’ve read new scientific research that describes how octopuses taste through their arms and spiders that hear through their legs.
Octopuses can taste with their arms using special cells that they have (and these cells are not found in any other animals). This amazing discovery made the cover of Cell- one of the most prestigious scientific journals in the world. This is so freaking cool.
Ogre-faced spiders, which have the largest eyes of any spiders, can hear prey even though they don't have ears. They hear sounds up to 2 meters away through their legs. If you’re like me, then you are wondering why these spiders have such big eyes that make them look kind of like owls from this angle. And it’s because like owls, they’re completely nocturnal animals. This was published in Current Biology.
I am so grateful to be living in a world that is more mysterious and awe-inspiring than anything anyone could have imagined. And I want to end this newsletter on this positive thought.
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