The Ecology of Humans – Smell & Taste

Smell & Taste

In mammals, the senses of olfaction (smell) and gustation (taste) converge in the mind-brain to produce a unified experience. Smell predominates the sensory input for tasting. Taste is 80% smell, with other senses contributing to the experience.

Smell is the only sense that bypasses logical neural processing and plugs directly into the mind-brain’s emotional centers. Smell alone is immediate.

Another factor for olfaction dominance comes with the confluence of taste and smell via the retronasal route. Air reaches the smell receptors by 2 routes: the orthonasal route when breathing in, and the retronasal route when breathing out. The mouth takes the credit, but flavor is mostly retronasal smell.

The influences of smell are so profound that simply imagining a smell can affect what you taste. ~ American psychologist Lawrence Rosenblum

One naturally exhales after swallowing. A thin layer of food lingers on the pharyngeal walls. Flavor sampling continues with breathing after a mouthful: aftertaste.

Taste
All of life is a dispute over taste and tasting. ~ German philosopher Friedrich Nietzsche

The taste receptors are taste buds. Most taste buds are on the tongue, but a few reside on the roof of the mouth (soft palate), pharynx, the inside of the cheeks, and the epiglottis. As people age fewer taste buds populate the sides and roof of the mouth, leaving taste buds mostly on the tongue, which themselves become less sensitive.

An adult has ~10,000 taste buds, but 10 to 100 million olfactory receptors. There 7 categories of odorant receptors and 7 basic tastes which allow distinguishing 1 trillion distinct scents.

Most taste buds sit on papillae: raised protrusions on the tongue’s surface. There are 4 types of papillae on our tongues: fungiform, filiform, foliate, and circumvallate. One – the cone-shaped filiform papillae – does not have taste buds.

Mushroom-shaped fungiform papillae are concentrated at the tip of the tongue and along the sides, scattered among filiform papillae. Cone-shaped filiform papillae are the most numerous papillae on the tongue: a toughened epithelial cell with mechanical and protective functions. Filiform papillae are constructed via keratinization, the same process that grows fingernails and mammal hooves. Foliate papillae populate the side of the base (back) of the tongue. Only a few circumvallate papillae populate the back of the tongue: 10 to 14 for most people.

A taste bud is a bundled group of 50–150 columnar taste cells. Taste cells within a bud have exposed tips that form a tiny taste pore. Taste cells extend microvilli (tiny hairs) through this pore. The microvilli bear the taste receptors.

Interwoven among the taste cells in a taste bud is a network of taste nerves. Taste cells depolarize when stimulated by chemical binding to their receptors. This depolarization is transmitted to taste nerve fibers, resulting in an action potential that is transmitted to the brain.

An action potential is a quick excitation and release of the electrical membrane potential of a cell. Excitable cells in plants and animals use action potentials for communication. Animal neurons, muscle cells, and endocrine cells employ action potentials. Action potential is a primary mechanism for neural cell-to-cell communication. In other cell types, action potentials mainly activate intracellular processes.

Taste reception is rapidly adaptive: an initial strong response diminishes within a few seconds to a much lower amplitude.

The tongue is tough neighborhood: subject to friction and intense heat which destroys taste buds. Taste buds are in a constant state of cell renewal. Receptor cells are replaced by division and differentiation of neighboring basal cells. A taste bud has a typical lifespan of 5–20 days.

Behavioral studies, mostly with mice, indicate that different taste receptor cells are sensitive to different tastes. Taste receptor cell specialization is not well understood.

One myth about taste sensation is that different regions of the tongue are sensitive to different tastes. American psychologist Edwin Boring hypothesized the notion from misunderstanding a translation of a 1901 German paper that showed minute differences in sensitivities at different areas of the tongue.

Taste signals pass through the brain stem and trigger digestive reflexes: saliva flows in the mouth, gastric juices flow into the stomach. Saliva abets taste reception by carrying the object being tasted.

Humans are capable of 7 different taste sensations: sweet, starch (carbohydrate), sour, salty, savory (umami), fat (oleogustus), and bitter. Though water is tasteless, as Aristotle observed, humans can ‘taste’ water in the mouth, as can insects, amphibians, and other mammals.

Salty and sour tastes are sensed directly through ion channels, though sour is detected only when protons are transported across the receptor membrane. Sour arises from acids.

Alkali metal ions trigger a salty taste, but the further from sodium a substance is in the periodic table the less salty the taste. Table salt is sodium chloride: an ion compound needed – in small doses – by all known living creatures.

Sweet and bitter taste work through G protein-coupled receptors. The various sugars found in organic compounds taste sweet. Alcohol has an admixture taste based upon a sweet foundation. Alkaloids, such as caffeine, trigger bitter.

The savory (umami) sensation is activated by the amino acid glutamate, which plays a principal role in neural activation.

An esterified fatty acid is a fat combined with an alcohol or acid to form an ester. Lipids are esterified. A non-esterified fatty acid occurs free of any such combination. Non-esterified fatty acids have a unique taste sensation.

There is some overlap between sour and oleogustus for short-chain fatty acids, and between umami and longer-chain fats, but the taste of fat is distinct.

Carbohydrates uniquely activate taste receptors in a positive way. Carbs instantly facilitate better motor control and physical performance. This is biologically logical, as carbohydrates comprise the bulk of nutrition in a healthy diet.

Lining the lower airways to mammal lungs are cells that can sense bitter molecules and send signals to regulate breathing. Speculation for this adaptation varies: from preventing ingesting plant toxins, which are often bitter, to a bacterial defense. Bacteria produce bitter compounds. Minute quantities of bitter substances can be detected, whereas sweet and salty require higher concentrations to register.

Human taste is oddly entrained and deceived. Hot soup raises an alarm by its reception, yet the capsaicin in chili peppers, triggering those same receptors, get a much milder response. Peppermint cools by its minty methanol seizing up cold receptors.

Tellurium is a mildly toxic metalloid, chemically related to selenium and sulfur. Tellurium is rare on Earth; much more common in the cosmos. Contact with the tiniest bit of tellurium results in a reek like pungent garlic which can last for weeks.

The veracity of taste sensations varies. Bitter is the taste of poison. Savory is the taste of protein, most notably essential amino acids. Especially crucial to survival, bitter and savory taste sensations are relatively reliable. In contrast, sour and sweet are readily fleeced.

Miraculin is a glycoprotein found in miracle fruit. Miraculin strips sourness from foods without altering their taste overtones. Though miraculin is not sweet, it bonds to the taste buds for sweet, putting them on alert for acidic tastes. This is invoked by stray hydrogen ions (H+).

Apple cider vinegar and a bit of miraculin tastes like apple cider. The effect lasts as long as the protein binds to the tongue, which can be up to an hour. Miraculin is commercially employed as a sugar substitute.

Italian physicist Alessandro Volta invented the battery in 1799. The term volt, which signifies electric potential, is named in honor of Volta.

Volta had a chain of volunteers each pinch the tongue of one neighbor. The two people on each end then put their fingers on opposite battery leads, thus inducing a current flow. Instantly, up and down the line, every person tasted each other’s fingers as sour.

Salty taste is also triggered by a flow of charges, but only of certain elements. Sodium most strongly triggers a salty reflex, but so too does potassium, sodium’s chemical cousin.

Both elements appear in nature in ionic form. It is the charge, not the chemical per se, that triggers the taste of salt.

Sodium and potassium ions are instrumental in sending nerve signals and facilitating muscle contractions. Other physiologically important ions, including calcium and magnesium, also have a vaguely salty taste.

But then, other ions of less or no physiological use come off as salty, including lithium and ammonium. Whether sodium and potassium taste salty or sour depends upon what else they are paired with.

Potassium chloride tastes bitter in low concentrations, but surprisingly turns salty in high concentrations. A significant does of potassium can shut the sweet taste buds down.

Gymnema is an herb found in the tropical forests of central and south India. It is quite high in potassium. Chewing gymnema leaves completely suppresses the sweet sensation. After chewing the herb sugar tastes like sand.

Gymnema has been a natural treatment of diabetes for thousands of years. Effective for up to 2 hours, it can fight sugar cravings.

The human sense of taste, as an evolutionary product of the environment, is a marvel in its natural setting, but a poor judge of the chemical elements in forms not normally encountered. Beryllium is a toxic, insoluble metal with tiny atoms. Pure beryllium does not naturally exist and is a relatively rare element throughout the universe. It bears no chemical resemblance to the complex sugar molecule. But the taste buds readily mistake beryllium for sugar.

Taste aversions are inborn. A newborn baby grimaces at bitter, while sweet, indicative of energy-rich nutrients, brings contentment. While the sense of taste is well-developed at birth, taste preferences change through life.

Individual animals vary considerably in taste sensitivity. 25% of people are highly sensitive tasters. Taste sensitivity is heritable, reflecting differences in fungiform papillae abundance, and hence the number of taste buds.

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Animal species have different taste capacities and live in their own world of taste. For instance, cats lack sweet receptors. This modified taste sense lends to cats’ evolutionary niche as carnivores. Plants, with a relatively high sugar content, lack appeal, while savory meat is tremendously tasty.

Most birds lack much of a sweet tooth. Hummingbirds are a notable exception, as nectar is as indispensable to them as essential amino acids are to humans. Hummingbird acuity to sweetness evolved from savory taste receptors, affording the necessary accuracy to detect sugar content.

 The Senses of Taste

One cannibal said to another while they were eating a clown: “Does this taste funny to you?”

Beyond the overwhelming contribution of smell, how something tastes, and how appealing that taste is, is determined by a confluence of perceptive factors, beginning with how hungry you are.

A food’s appearance strongly influences its flavor. Presented with 4 glasses of fruit drinks (cherry, orange, lime, and grape), you would probably be able to correctly identify all of them. But if you could not see the drinks, chances of correctly identifying them drops to 20%.

Sound influences taste. The pleasantness of consuming fruits, chips, crackers, and other stiff foodstuffs is summoned in part by how it sounds when we chow down. Freshness snaps crisply.

Taste and touch go hand in hand. The texture of a food has much to do with how it tastes. Hence apple sauce may not taste much like an apple.

Taste is itself influenced by a food’s temperature. Taste sensitivity is at its peak when a food is close to the same temperature as the tongue (22–37 ºC). Restaurant dinners are sent back more often for being too cold than for any other reason. Fine restaurants know this and test the temperature of their dishes to determine the optimal delay between when a food leaves the heat and arrives at your table. For expert tasters, a very few degrees make a telling difference in how something tastes.

The tactile sense of taste is affected by the irritants inherent in spicy foods. Such pungent spices do not affect the taste buds. Instead, they tickle the sensitive touch fibers on the mucous membranes of the inner mouth. Wasabi also triggers the trigeminal nerve at the back of the throat and in the nasal passages, which is a considered a touch receptor.

Diet and body condition affect taste. The obese do not taste fat nearly as well as the lean, and generally have less sensitive sweet and salty tastes. Being less sensitive to the most satisfying tastes is itself a formula for increased consumption.

The final arbiter of taste is the mind. Expectation shapes taste.

Subjects given a glass of wine before dinner and told it was from California thought that the wine and food eaten afterwards tasted better than those who received the same wine, but were told it came from North Dakota, a state not known for top-quality vineyards; likewise, price. The same wine with a higher price tastes better than when low-priced. You can’t cut-rate taste.

Smell

Smell is a potent wizard that transports you across thousands of miles and all the years you have lived. ~ Helen Keller

Olfaction is of odorants. An animal with a keen sense of smell is macrosmatic.

Insects and other invertebrates smell with their antennae, and through pores on their body, chiefly the mouth and legs. For vertebrates, odorants are detected by specialized receptor cells in the nasal cavity.

Bacteria do not have noses, but they do have the biochemical traction for olfaction. Bacteria respond to odors. Certain chemicals evoke an urge to get a move on.

The scent of ammonia provokes the soil-dwelling bacterium Bacillus licheniformis to form a biofilm. A colony can get to a nutrient source and feast faster than they could by going solo. The nitrogen in ammonia (NH3) is the draw. Nitrogen is essential for producing proteins and nucleic acids.

Bacteria respond to other gases, including oxygen and carbon monoxide. Bacteria migrate to an area with an oxygen concentration optimal for growth. Those 2 gases produce no scent (that we know of). Ammonia does.

That bacteria can “smell” may seem an overstatement. But if olfaction is defined as sensing a volatile molecule, that is exactly what bacteria do.

As insects primarily communicate chemically, insect mind-brains have a strong discrimination of odorants. Olfaction sensitivity in many insect species, particularly eusocial insects, is highly tuned to the pheromones used for communication. But insects employ olfaction for many purposes. Butterflies follow scent to nutrition. Bees can identify their hive-mates by smell. A beehive has its own scent: home sweet home.

The aerial environment usually presents a variety of odors at any one time. Tracking a scent to its source is tricky.

When multiple odors mix, insects and other animals perceive them as a perceptual unit, not as individual components. Nevertheless, insects can distinguish concurrent scents from closely spaced sources based on millisecond differences in their arrival. This lets an insect suss an odor to its source.

Honeybees learn which odors matter. Relevant scents become more salient. This aids the odor-segregation process.

Though aquatic, fish have a keen sense of smell. Many fish use their sense of smell to find food and identify mates. Lobsters have an amazing ability to follow small scent signals in turbulent water.

Catfish can identify other individual catfish by smell. Abetted by olfaction catfish maintain a social hierarchy.

Salmon smell their way back to their spawning grounds to breed; so do sea turtles.

Many bird species lack a good sense of smell, but some seabirds follow their nose. Petrels feed on zooplankton, foraging by scent. Zooplankton release dimethyl sulfide (DMS) after grazing on phytoplankton (single-celled aquatic plants). Petrels birds can detect DMS up to 4 km away. Birds in the same order that don’t feed on zooplankton show no interest in DMS if they can detect it.

Vultures and kiwi have a keen sense of smell. Their lifestyle necessitates it.

Many vertebrates – most reptiles and mammals, but not humans – have 2 distinct olfactory systems: a main olfactory system, and an accessory system. The main olfactory system of air-breathing animals detects volatile chemicals, while accessory olfaction detects fluid-phase chemicals. For water-dwellers, the odorants are in the drink.

The accessory olfactory system is primarily employed to detect pheromones, though pheromones can also be detected by the main olfactory system. A pheromone is a secreted or excreted hormone intended as a conspecific communication signal of some sort.

Early mammals evolved larger brains to better process smells. The fossil record shows a dramatic increase in the size of the olfactory bulbs in mammalian brains that process scents around 190 million years ago. Concurrently reptilian predators worked the day shift. A keener sense of smell helped mammals hunt at night.

Like vision, olfaction can be improved by sniffing in stereo. Moles have stereo-smell which they use for hunting and navigation. Other mammal families likely have this ability.

Odorant receptor genes form the most prolific gene family in the genome of many animals. The mouse genome has 1,200 such genes. For a balanced sense of smell to function, genetic expression must be selected in proper proportion to other odorant receptors.

The probability of a certain gene being chosen depends upon how many olfactory sensory neurons in total produce that receptor type. What is known is that number of regulatory elements for each receptor type gene cluster is high: for a mouse, as many as 150. What is not known is how genetic expression is determined, nor how the regulatory elements interrelate to foster expression, nor how production of different receptor types is apportioned.

For vertebrates, smells are picked up by olfactory receptors in the olfactory epithelium. How well one can smell depends upon the nasal real estate dedicated to the task, and the density of olfactory nerves at work.

 Human Olfaction

Human olfaction begins at the epithelium in the roof of the nasal passage, where hair-like cilia connected to receptor cells transmit odorant detection to nerve (mitral) cells in the olfactory bulb, where initial scent processing is performed. These filtered olfactory signals are then sent to various parts of the brain associated with emotion, behavior, and memory.

Olfactory receptor cells wear out rather quickly, and so continually regenerate, with total turnover every 2 months.

Like dogs, humans can track scents using the difference in odor intensity between the 2 nostrils. This is not to say that human sense of smell is up to snuff to that of dogs.

Dogs have 50 times the number of olfactory receptors cells in their noses, which sit on long snouts situated close to the ground. Whereas humans have about 10 cm2 dedicated to olfaction reception, some dogs have 170 cm2. Further, the nose of a dog is 100 times more densely innervated (packed with nerves) than a person.

7 categories of human-detectable odors have been suggested, corresponding to the 7 types of smell receptors in the nasal cavity: camphor-like, musky, floral, minty, ethereal (dry-cleaning fluid is exemplary), pungent (vinegar-like), and putrid. But this bears no relation to the complexity of odorant receptor production, nor even sufficiently characterizes the subtleties of human olfaction.

Excepting some hunter-gatherer tribes, humans generally struggle to explicitly name specific smells. Even experts are readily fooled by an odor’s context. Human ability to vividly imagine a smell is hardly worth a snort.

Olfaction has long been considered the least significant of the human senses. Biological anthropologists have suggested that vision supplanted olfaction as humans became upright. ~ Dutch psycholinguist Asifa Majid & Swedish linguist Nicole Kruspe

The limited terminology for scents and tastes suggests that these are not the strong suits of human sensory perception: contrast these vernaculars to the vocabularies associated with sight and sound. It may instead be that smell simply does not carry the cultural currency that vision and audition do: that we do not commonly converse of odors so specifically as to merit a verbose nomenclature.

Though dogs may have better sniffers, humans are able to sift through billions of different odors. We use scent to detect fear, stress, and sickness, and to assist in selecting mates.

There’s a true underappreciation for the way we use our sense of smell that contributes quite significantly to our overall well-being. ~ English physiologist Johannes Reisert

Women pack 40% more olfactory receptors in their heads than men, giving them a superior sense of smell.

Individuals have decidedly different senses of smell, owing to genetic and lifestyle differences.

Health determines smell sensitivity. An early sign of dementia and Alzheimer’s disease is loss of smell (anosmia; hyposmia is diminished or deficient sense of smell).

 Vibration versus Shape

There are competing explanations of how smell works. The most widely accepted is the shape hypothesis of olfaction, which proposes that a smell derives from a molecule’s shape. Alternately, the vibration theory of olfaction posits that a molecule’s smell character comes from its vibrational frequency in the infrared range.

The vibration theory was first proposed in 1937 by Malcolm Dyson. A 1947 paper by Yale University researchers Walter Miles and Lloyd Beck concluded that “smell is not a chemical sense,” but a detection of infrared radiation.

The Yale experiments, on honeybee and cockroach participants, put the insects in boxes with windows of 2 types, which both looked alike, and of the same color.

One window type was a block of thallium bromo-iodide, a salt crystal, which allowed passage of infrared rays, but was chemically impenetrable to the chosen odorants: honey for the bees, clove oil for the roaches. The other window type blocked both chemical and infrared transmissions.

Bees flocked to the infrared-passing window that allowed the honeyed vibrations past, but the full-blocking window gave the bees no indication of honey outside.

Roaches wiggled their antennae at clove oil, either when inside the box (24%) or outside the infrared-passing-window (26%). Clove outside the blocking window mitigated the wiggling whiffing; only 15% of the roaches jiggled their antennae when no odor was present in the box, or with the clove oil outside a box with full-blocking window.

In 1949, English physiologist R.W. Moncrieff, cribbing from American chemist Linus Pauling’s notion of shape-based molecular interactions, proposed olfaction by molecular lock-and-key fit between wafting odorants and olfactory receptors.

The shape theory remains mainstream for both fragrance chemists and academic molecular biologists. But the shape hypothesis fails to explain why similarly shaped molecules have distinct smells; something which vibrational theory readily explains. Further, shape-hypothesis experiments have often not supported the claimed conclusions, demonstrating defective logic by the researchers involved.

A proposed mechanism for the vibrational theory was further elaborated in 1954 by Canadian chemist Robert Wright, who surmised that particular chemical bonds in olfactory receptors were affected by specific vibrational frequencies.

Lebanese biophysicist Luca Turin picked up the scent with a 1996 paper suggesting molecular vibrations as an important determinant of smell. Turin suggested quantum electron tunneling as essential: that olfactory mechanics are reliant upon extra-dimensional dynamics. In this scenario, olfaction works more like a “swipe card” than the lock-and-key mechanism proposed in the shape hypothesis.

As Turin pointed out, traditional bio-receptors using a lock-and-key mechanism are provoked by agonists, which increase the duration that a receptor is in an active state, while antagonists increase inactive state time. In other words, some ligands (functional groups) tend to activate a receptor, while some deactivate.

A 2003 paper by Japanese shape-theory researchers (Yuki Oka et al) purported to show “antagonism-based modulation of receptor codes for odorants.” The experiments showed “inhibited” responses by odorants of slightly different chemical combinations and concentrations. But agonists were not explored. Oka’s line of reasoning for supporting shape theory was more sophisticated but similar to a 1956 paper by Moncrieff that purported to support shape theory by observing “olfactory fatigue” sets in, and that “rest time” is needed so “the nerves may recover from their refractory state.” The evidence supposedly supporting shape theory in both instances is orthogonal to proving molecular shape as the olfactory mechanism; not at all the logical inductive proof required. In short, there is only spurious logic for results that do not support the shape hypothesis.

Research published in 2013 by Turin and others supported the vibrational theory. They deuterated odorants: replaced protium hydrogen with deuterium. That left the shape of the molecule unaltered, but doubled atomic mass, thus altering vibrational frequencies.

The chosen odorant was musk, a common perfume ingredient. Musk is a large molecule, comprising 15–18 carbon atoms and 28 or more hydrogens.

The results were that the numerous deuterated musks of diverse structures smelled (to humans) strikingly different from the parent (hydrogen-based) compounds, but similar to each other, “even to naïve subjects.”

Only sulfur and boron hydrides (respectively known as thiols and boranes) smell sulfurous to us, despite having no chemical properties in common. What they do have in common is a selfsame vibrational frequency.

Flies have radically different olfactory systems than humans, so much so that their sense of smell would be divergent from ours if the shape hypothesis were true. Yet flies trained to avoid boranes then avoid thiols and vice versa. This suggests that they are detecting a vibration at the same frequency, just as people do.

Numerous studies have been conducted to suss the nature of smell. The facts are dispositive.

In organic chemistry, a functional group is the specific group of atoms within a molecule involved in characteristic chemical reactions for that molecular structure, regardless of the size of the molecule the functional group is part of. Hiding a molecule’s functional group does not mask the group’s characteristic odor, which it would if olfaction was shape dependent.

Selfsame shaped molecules with distinct molecular vibrations smell differently, as shown from tests on insects, fish, and humans. Very small molecules with similar shape, which should not be distinctive if olfaction were based upon shape, have quite distinctive odors.

A remarkable feature of olfaction, and perhaps the hardest one to explain by shape-based molecular recognition, is the ability to detect the presence of functional groups in odorants, irrespective of molecular context. ~ Luca Turin et al

The isotopes of molecules smell discernibly different despite having identical shape. Conversely, distinctly shaped molecules with similar molecular vibrations smell alike. Olfaction works via molecular vibration.

From structural changes in chemistry to molecular signaling, all dynamical processes in life have to do with molecular vibrations. ~ Indian American physical chemist Ara Apkarian

 Body Odor

An animal’s body odor, human bodies included, is influenced by genetic inheritance, gender, health, medication, lifestyle, and diet. Major histocompatibility complex molecules, which play a role in the immune system, affect body odor.

Among other things, dietary toxins are exuded onto the skin. Sweat alone is lightly scented.

Body odor takes work, though not human effort. Corynebacterium live on human skin and are plentiful in the armpits where sweat collects. They metabolize the lipids in sweat, raising a stink in the process. Beyond providing raw ingredients, people have nothing to do with how smelly their sweat is. Corynebacterium are common in Nature – soil, water, plants, and food – and are mostly innocuous, though a ne’er-do-well in the genus causes diphtheria.

Lipases are essential to digestion of dietary lipids into most organisms. Genetic encoding for lipases exists even in certain viruses.

The industrious armpit bacteria break down the lipids into smaller molecular by-products, including butyric acid. Butyric acid is found in butter, parmesan cheese, and vomit. Butyric acid is a product of anaerobic fermentation that also occurs in the colon. Butyric acid has an unpleasant odor and an acrid taste, with a sweetish aftertaste similar to ether.

Mammals with good scent detection, such as dogs, can smell butyric acid at a concentration of 10 parts per billion (ppb). Humans can detect butyric acid in concentrations above 10 parts per million (ppm).

 Deodorant

Deodorants fall into 2 functional categories: maskers and preventers. Maskers attempt to mask body odor by perfume fragrances or natural essential oils. These deodorants are activated by the moisture of sweat.

Deodorants with antiperspirant agents are classified as drugs by the FDA. Antiperspirants attempt to stop or diminish perspiration.

Aluminum compounds are common in antiperspirants. Their primary function is to plug the sweating process. Aluminum complexes react with the electrolytes in the sweat to form a gel plug at the sweat gland duct. Aluminum salts interact with the keratin fibrils in sweat ducts to form a physical plug, keeping sweat from reaching the skin’s surface. Aluminum salts are also slightly astringent, causing skin pores to contract, thus limiting sweat flow.

Aluminum is one of the few abundant elements that plays no positive role for living cells. Aluminum is a neurotoxin. Aluminum can adversely affect the blood-brain barrier, have epigenetic effects, and cause DNA damage.

The FDA has acknowledged that aluminum can be absorbed through the skin. Yet the FDA has done nothing to prevent the use of aluminum in deodorants.

 Pheromones

Scent tells a wolf quite a tale: where an animal went, how fast it was traveling, and how long ago. For a wolf, the scent of an animal’s urine tells the sex and whether the animal is in good health. Poor health is a promising sign: the prospect for outrunning the prey is improved.

Despite having a comparatively poor sense of smell, olfaction remains a powerful sense in humans, often subliminal in its tug to guiding behavior. Pheromones, the prevalent communication form between insects, and a major facet in the lives of many animals, plays a significant role in human relations as well.

Much of the allure between the sexes is chemistry. Literally. Both men and women are affected by the scent of a potential mate. There is a built-in biological propensity to prefer a mate with a different, and hence complementary, immune system. This is detected by the scent of major histocompatibility complex (MHC). All cells in the body have these protein-processed marker molecules. Widening the gene pool in this way potentially confers enhanced protection to offspring.

This is a strong subconscious attraction, and so practically undeniable. Both women and salmon succumb to this lure.

A woman’s preference for men varies during her menstrual cycle. A woman’s sense of smell is keenest near ovulation.

During ovulation, a coupling with a masculine man has a stronger allure. Masculinity signals includes facial symmetry (an indicator of health), muscle tone, a more masculine voice, and dominant behaviors.

A woman’s behavioral signals subconsciously change during ovulation: a higher-pitched voice, desire to appear more attractive, an urge to flirt. Less masculine men become less attractive, though the cause may not be consciously recognized.

Hormonal birth control disrupts this natural biorhythm and alters MHC. The desires of women on the pill during ovulation are unchanged. Their preference for partners with different immunities disappears.

Birth control pills also affect a woman’s libido and mood, and may affect long-term relationships. A woman who partners with a less masculine man while on the pill may feel dissatisfaction coming off the pill. The urge to stray may have a say with a less manly partner. This does not happen to a woman with a masculine sexual partner.

Men do not exhibit shifting interest in an ovulating woman taking birth control pills, as the cues are not detectable.

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Despite relatively limited faculty (compared to other animals), human sense of smell is obviously important to our lives. Its diminishment affects appetite and general enjoyment of life. We each have our own scent signature and can tell our health condition by it. Humans can smell whether they are related to another person. This is part of the innate sense against incest.

Human smell develops in the womb. A few days after birth, an infant can recognize its mother by the scent of her nipples and underarms. The distinctive odor of siblings is well known by the age of 5 years.