Tied for the oldest sense

 kw: book reviews, nonfiction, science, olfaction, nose, sense of smell

It is fascinating to watch a motile bacterium such as E. coli in motion. It trundles along, its rear-mounted flagella spinning to propel it in a mostly straight line. If it bumps into something it will back up some distance, tumble, and then move off in a new direction. Frequently, after a short distance, it may reverse course or tumble again to pick a new direction.

The latter action hints at what is going on. How does it pick a direction to go; what is it trying to reach? Of course, like all living things, it is searching for food. It is following a chemical gradient by sensing a chemical of interest in the water around it. If, as it moves along, it senses a stronger concentration, it keeps moving. If the concentration is decreasing it reverses course or tumbles to try a new direction. Its chemical sense can be called either "taste" or "smell" and is one of the two oldest senses. The other is touch. Bumping into something, or alternatively, approaching closely enough for cilia on the cell to touch the something, coupled with the chemical sense telling it, "this isn't what you are looking for," triggers actions such as backing up and/or tumbling. Touch is the other "oldest sense." The two seem to go together, and they work together to guide the cell to a possible source of food.

Strictly speaking, smell is thought to relate to chemical cues carried in the air, so the bacterium, being in a watery medium, must be using taste rather than smell. But at the most basic level these are actually the same. Chemical substances that are smelled first enter a watery layer over the sensory nerves, where they are detected.

For air-dwelling creatures, smell is a more long-range sense. Chemical substances travel through the air faster than they do through water, although both aerial and fluid currents can bring them from far away. But diffusion in still air is faster than in still water. Furthermore, when two senses work together, smell precedes touch, while taste follows contact.

The title of Jonas Olofsson's book, The Forgotten Sense: The New Science of Smell and the Extraordinary Power of the Nose led me to think, "Why didn't he call it The Neglected Sense?" No matter. He reveals the neglect that smelling has undergone through the centuries since it was placed by Aristotle at the bottom of the list of useful senses, an error compounded when Paul Broca divided animals into "osmatic" and "anosmatic": those like the dog for which smell was primary and those like humans (he thought) for whom smell was definitely not worth much. I guess that neither Broca or old Ari stopped to consider how he would detect a bad lot of wine or olive oil if he plugged his nose. Tasting without smelling could be a risky business!

I was most fascinated by an analysis to which the author refers, "Human and Animal Olfactory Capabilities Compared," by Matthias Laska in the 2017 Springer Handbook of Odor. Humans and a number of medium-sized and smaller animals were tested for their sensitivity to a few dozen scented substances. Animals tested included rats, dogs, vampire bats, a couple of monkey species…twenty species in all. The only animal with a nose more sensitive than ours was the dog!

Earlier studies that compared the size of an animal's olfactory bulbs to its brain were misleading because it is the absolute size of the bulb that matters. The roughly 60 mm³ volume of human olfactory bulbs is a tiny fraction of the brain's volume, about 1/20,000th. In relative terms, the olfactory bulb of a mouse is enormous, about 1/16th of the total brain. However, its actual size is less than half that for a human: 25 mm³. An ordinary dog (not the tiny breeds or pug-nosed ones) has an olfactory bulb six times larger than a human, which gives it a huge advantage, as was shown clearly by the sensitivity tests. I wonder if they could give a similar test to an elephant, with an olfactory bulb volume of 11,000 mm³ ?

Technical issues aside, a big section of the book relates the emotional effects that smells can mediate, such as a whiff of salty air evoking a favorite memory of a seaside vacation, or the comfy aroma of a morning coffee and bowl of blueberries. 

This also introduces the subject of smell gone wrong: COVID-19 introduced millions around the world to a life without smells, at least temporarily. An interesting consequence of the sudden loss of smell for many people was that they began to wonder how they could tell if their own smell was offensive to others! Another was the loss of interest in food, because most of what we call the taste of many foods is actually a combination of smell and taste. We can taste but five qualities: sweet, salt, savory (umami), sour, and bitter; we can smell thousands or hundreds of thousands of different qualities. So many that we can seldom describe any of them to someone else.

Loss of smell is called anosmia. In some ways, a distorted sense of smell, parosmia, can be even worse. Imagine one day finding that your morning cuppa smells like rotting onions! This can also be caused by viral infections, but there are other causes, including a hard bump on the head. Some people with parosmia can't stand to eat favorite foods, though some are able to eat enough to stay healthy by putting on a nose clip. There is a long section describing ways of desensitizing and retraining the sense of smell. Sadly none of the methods is effective in all cases, but it can be a lifesaver for many.

A side note: There is a reversible distortion of taste I have experienced, caused by an Asian spice called Tiger Claw, related to Star Anise. The seed pods are used whole to flavor soup. They aren't supposed to be ingested. Biting into one causes a shift of taste, such that water tastes like battery acid and nothing seems edible; it lasts several hours. I have numerous Chinese friends, and I wound up mostly fasting during a potluck lunch…

The author is a scientist of the sense of smell. He first wrote the book in Swedish, then translated it into English himself. His and his colleagues' work just might elevate our understanding of the sense of smell, from a "neglected" sense to one that is equally essential. And in time perhaps we'll attain added vocabulary to help us describe our favorite (or otherwise) aromas.