Is there an evolutionary purpose to armpit perspiration?

Given that most Americans seem to do fine using anti-perspirant, it got me wondering: is there an evolutionary purpose to underarm perspiration? I can't imagine it acts to attract the opposite sex.

Or is it just likely an unfortunate side effect of whole-body evaporative cooling via sweat?

There may be an evolutionary purpose to underarm perspiration, as alluded to in the "hidden hint" link of @DevashishDas, and also hinted at by the answer by @Chris. The apocrine sweat glands of our armpits produce steroids. One group of steroids secreted by the axillary apocrine glands are sex pheromones. Although we may not consciously notice the chemical odors of sex pheromones, they do likely affect our behaviors.

For example, Wedekind et al. showed that females showed distinct preferences for certain male odors. Women tended to prefer the odors of males that were genetically different at the major histocompatibility complex (MHC) loci, a group of genes associated with our immune response system. Greater genetic variation of MHC genes generally translates to greater ability to combat disease. Females that select males that are genetically different at the MHC would produce offspring with greater genetic variability at the complex, and therefore more likely to have greater resistance to disease or ability to fight disease.

These results suggest that the pheromones secreted in underarm perspiration is a sexually selected trait in humans. If so, underarm perspiration would have an evolutionary (fitness) advantage.

First: Not everything what happens in biology is necessarily directly selected for. Sweating is of course selected for, so our body can maintain its normal body temperature in hot climates.

There is a difference between sweat glands under our armpits and for example on the forearm. Under our armpits we have the so called "apocrine sweat glands" while most of the rest of our body is covered with "eccrine sweat glands". They are not only different in size, but produce different types of sweat. Eccrine sweat glands produce sweat, that is made mostly of water and salt, while the apocrine sweat glands sweat contains additionally lipids, proteins and also steroids. Decomposition of the sweat by bacteria which live on our skin produces the typical sweaty odor. The sweat glands also differ in their activation (see here for more details), so can the apocrine glands be triggered by stress.

According to this source ("A short history of sweat gland biology") is roughly comparable, although the total numbers vary quite a lot between the single studies. Assuming that the amount of sweat is comparable, than another factor comes into play: The different evaporation from our arms and from under our armpits. The last form a kind of cavity where the sweat can not easily evaporate and will then stay. It might also play a role, that they can be activated by stress, which will happen in situations which are uncomfortable for us and make the situation worse. A paper which also looks very interesting in this context (to which I unfortunately have no access) is: "The evolution of sweat glands."

Armpit sweat does not have a strong intrinsic odor. What you smell is the waste of the microorganisms which colonize your armpits. It is a similar mechanism responsible for foot odor and bad breath. All external bodily surfaces have particular microbes which prefer different areas. This also includes the intestinal, oral and nasal regions.

If you charted these microbes on our body, their colonies would read like a map of microbial nations. Different lipids, sugars, proteins on individuals promote different types of microbes and different metabolic pathways giving individuals different scents.

Our ability to perceive a wide range of microbial odors might have an evolutionary pressure given the importance of detecting microbes in spoiled food, etc. I hypothesize our ability to detect body odor is inherited from detecting microorganisms in general.

Surely as our olfactory sense has diminished over evolution, these microbial odors are being taken out of some ancient context.

Armpit body odor research

We have done a series of clinical trials on armpit microbial transplants in order to solve armpit body odor. We simply replace the malodorous armpit microbiome with a healthy non-odorous microbiome. The donor is usually a close family member. He/she stops washing the armpits for 3-4 days, so we have a large amount of donor bacteria. The donor bacteria are also screening using molecular techniques to verify if the ‘good’ microbes are there. The acceptor washes the armpits thoroughly for 1 week, using soaps, antibacterial washes, and sometimes topical antibiotics. The microbiome is transferred as fresh as possible. We have tested this on about 18 people thus far. Results look very promising – we found significant improvements in armpit body odor for minimum a month. Preliminary findings were reported in Experimental Dermatology.

In recent years, skin microbiome transplantation and manipulations have been tried for a number of skin disorders. My friends and me have written a review summarizing the current state of the art. Here we discuss the challenges, tentative outcomes and future directions. You can find the open access paper in Computational and Structural Biotechnology Journal.

The Science of Sweat

From nerves to exercise, this is sweat's cool factor.

Sweat gets a bad rap. We blame it for stink. We accuse it of staining clothes and ruining white tees, and we react in disgust when it appears. But this ill will toward perspiration is misguided. Truth is, we need sweat &mdash we just also need the right antiperspirant like Degree Men Black + White Deodorant to fight it and protect our clothing from white marks and yellow discolorations.

When your body starts to heat up, whether it's because of exercise, work, or outside temperature, your brain reacts by releasing sweat from the more than 2.5 million eccrine glands spread out across nearly all of your body, pouring liquid through pores to lower your body temperature. But when sweat simply drips off you and hits the floor, it can't lower your body temperature. To reap the cooling effect of sweat, though, that salty liquid must evaporate off the skin and turn into a gas, says William Byrnes, a sweat expert at the University of Colorado.

Cooling sweat isn't the only type of sweat. Humans also have apocrine glands, primarily in the armpit and groin. These glands also act as scent glands&mdashin animals, musky sweat can help attract both males and females, Byrnes says. The milky fluid from this sweat contains more nutrients, which makes it more attractive to the bacteria Staphylococcus hominis that largely resides in the armpit and groin. When these bacteria and sweat interact, we get body odor.

Like eccrine glands, these glands get activated during exercise, but apocrine glands also come alive when we get emotional, nervous, or excited. That means the most smell-inducing (and white tee-ruining) activities may not be running up a hill or playing basketball, but rather going in for a first kiss or giving an all-staff presentation.

Sweat isn't triggered by heart rate or movement, but by receptors in the hypothalamus area of the brain. Individuals living in hot and humid environments will adapt to the weather, just as people aerobically trained will sweat more and sooner. The body welcomes "adaptions that help us with heat regulation," Byrnes explains.

Because the stink of sweat doesn't come from the odorless, colorless liquid the body produces in an effort to cool the skin, but rather from contact with bacteria present on the body, we have to combat it from all angles. With this bacteria concentrated in the armpit, applying a powerful deodorant to that specific area, instead of rubbing it all over, proves the most effective way to combat B.O.

The most smell-inducing activities are less running up a hill and more going in for a first kiss or giving an all-staff presentation.

To take odor protection one step further, researchers the world over continue to study how to limit this bacteria, and companies work to mask the smell with pleasant fragrances. Degree, for example, uses tiny capsules of scent meant to break down throughout a day for freshness.

But smell definitely isn't the only problem with sweat. The yellowing of our shirts, another unpleasant side effect, is again caused not by sweat alone but by acidic chemicals in some antiperspirants as they react with sweat. Thankfully, deodorants like Degree Men Black + White Deodorant don't cause those reactions, preventing staining on your white shirts. And not only will Degree not cause those yellow stains, it'll also save your darker clothing from the unattractive white marks left by some other brands.

The next time you want to curse your perspiration, just remember: Sweat offers the body-altering benefit of keeping us from overheating, while all these ill effects like stink and pit stains come from bacteria. The moral? Leave sweat alone and instead blame bacteria&mdashand get yourself a great deodorant to fight it.

Why Do We Have Underarm Hair?

As of 2015, there are few heavily debated beauty topics left. There are models of all sizes and shapes, we now choose any haircut we like regardless of age, and even complicated makeup techniques are easier than ever. But there's still one big discussion that has been at the forefront of the beauty world for the past few years: female body hair. Namely, letting it grow instead of shaving or waxing it off.

With the recentcontest in China that's gone viral, it's clear that female underarm hair is actually coming back into style. But before you shake your head at the prospect of tossing your razor, listen to what a dermatologist and an esthetician have to say.

"Underarm hair has a few functions," explains Marta Camkiran, esthetician at Haven Spa. "It reduces friction between the upper and lower arm during vigorous labor or motion, covers exposed parts of the body with vital arteries, and facilitates the release of sex pheromones." So if you prefer a hairless look to your underarms, that is absolutely your choice, but keep in mind that smooth skin can occasionally come at a price.

"When you shave, you can get irritation, folliculitis, rashes, inflammation, and even infections from dirty razors," says Dr. Mona Gohara, associate clinical professor of dermatology at Yale University. "We do it because it's a societal norm, but there really isn't a biological reason to remove that hair." So, why do we bother shaving in the first place?

"There's this false association that hairlessness equals cleanliness, but that's not actually true as long as you're clean," says Gohara. "Men wear deodorant, have underarm hair, and don't smell &mdash there's no biological reason women can't do the same." Provided you keep your underarms clean and treat the hair similarly to the hair on your head, washing it every day, bacteria will not accumulate and therefore won't cause any bad odors.

Of course, if you want to keep your underarms smooth, you should wax and shave to your heart's content &mdash just don't do it only because you think growing out the hair will suddenly make you dirty. As long as you keep it all clean, you'll be fine either way.

Sure, you don&rsquot have to exfoliate, but you should to avoid pesky, painful ingrown hairs. A loofah or exfoliating body scrub will do the trick to remove dead skin cells and bacteria (along with any deodorant gunk) to help you achieve the smoothest shave without razor burn.

You can cut it dry, but Whitely recommends to do it in the shower. Hot water softens the hair and reduces the risk of pulled hair or nicks, he says. Shave towards the end of your time in the shower and use shaving gel for added moisture to prevent irritation.

What Is the Purpose of Armpit Hair? (with pictures)

Though some subscribe to creationist beliefs, it is fairly common scientific belief that Homo sapiens evolved from earlier homonids that had copious amounts of body hair, not just in their armpits and pubic regions but all over every inch of skin. Over many millennia, less and less hair seems to be the path that natural selection has taken, though some believe the hair that remains serves some purposes. Armpit hair, it is believed, not only nurtures a heated, musty nest of mate-attracting pheromones, but also offers protection from chafing. Women who regularly shave their armpits and other regions in 2011, however, do not seem to mind its absence.

Though it could easily be considered conjecture, some scientists believe that armpit hair has continued to be part of the human package mostly due to pheromones. The apocrine glands of the armpits are a major producer of these secretions, which many animals and humans emit to attract mates. More hair present results in a more protein-rich environment for chemical communication via pheromones. It is actually the bacteria of the armpit hair that breaks down sweat into something stinky, since pheromones are considered odorless.

Several perfume and cologne manufacturers claim to include human pheromones in their recipes to give an added appeal. Some scientists dispute whether it is possible to distill these odorless hormones, stating that many of these concoctions contain synthetic versions of the body's natural hormones. Regardless of the scent attraction debate, some speculate that oil-rich armpit hair is useful for lessening the amount of friction that is produced between between the upper arm and torso during movement. Others, however, insist that the bristly hairs actually add to the friction and do not provide any lubrication at all.

Researchers started suspecting and investigating the existence of hormonal pheromones more than 100 years ago. It was not until 1959 however, that it was named as such, by two scientists, the Swiss entomologist Martin Luscher and the German chemist Peter Karlson. The word "pheremone," means in Greek, "carrier of excitement." The discovery, however, was made with termites instead of humans and their armpit hair.

According to the National Academy of Sciences, pheromone research has largely focused on insect attraction, primarily in the pest management field rather than the pheromone content of human armpit hair. In 1980 for instance, 4,000,000,000 beetles were lured by pheromones to traps in Norway and Sweden to stem an infestation that threatened crops. The technology has spread worldwide, though an understanding of human pheromones is still largely elusive in 2011.

Race and Body Odor

I’m currently reading Nicholas Wade’s A Troublesome Inheritance: Genes, Race and Human History, and it’s an outstanding read. He succinctly puts the science of racial differences so it’s easy for the lay person to understand. I’ve come across a part in the book where he talks about race and body odor. In the past, I’ve gotten into discussions on how and why different races have different body odors. My article Gene Expression by Race is great to get up to speed on the differences I’ve already written on in regards to the races of man.

Robert Lindsay has a good post on race and body odor, but I thought I’d add more information to how and why these differences exist.

East Asians have thicker hair than do Africans and Europeans. The gene, called ‘EDAR‘, is present in both Africans and Europeans, however a different type of this allele is widespread in East Asian populations (Han Chinese, 93 percent Japan and Thailand about 70 percent and 60 to 90 percent in ‘Native’ Americans [which makes sense since we know that they branched off of Siberians around 10kya]). This allele is called “EDAR-V370A” since the V (valine) and A (alanine) switched on the �th codon” (Wade, 2015: 88).

East Asians who carry this allele have thick and shiny hair. However, correlation is not causation, blah blah blah. So researchers genetically modified a strain of mice whose EDAR gene was converted into the form that East Asians carry. The results were shocking. The mice with the East Asian EDAR variant had thicker fur, and more eccrine glands in their footpads. Sweat glands come in two forms–eccrine glands which secrete water to cool the body and apocrine glands which secrete proteins and hormones (Wade, 2015: 89). Looking at the Chinese, we can see that they do carry significantly more eccrine glands!! Moreover, the mice also had smaller breasts. If East Asians had this EDAR variant then, logically speaking, they would have smaller breasts and what do you know: they do! This is, most likely, the reason why East Asians have smaller breasts than Africans and Europeans.

Another prominent effect of the EDAR variant that East Asians carry is the proclivity for shoveled teeth. When viewed from the back, East Asian teeth look ‘shovel-shaped’. The reason that this single gene is able to effect a lot of the phenotype is because this gene is active early in development. “EDAR has a great influence on the body because it’s switched on so early in embryonic development and helps shape organs such as the skin, teeth, hair and breasts” (Wade, 2015: 89).

Why does this singular gene have so many effects? One posited reason is thick hair and small breasts were admired by men (and thick hair for women) and this is what drove the selection (sexual selection). Another possibility, says Wade “is that many or all of the effects of EDAR-V370A were advantageous at one time or another, and that natural selection favored each in turn” (Wade, 2015: 90).

Turning our attention to Africans, they have more eccrine sweat glands, and, in comparison to East Asians, Africans have wider pores in their skin. Due to this, blacks have a stronger scent when sweating than East Asians. Conversely, East Asians have smaller pores.

I no longer have access to the source of this next quote, however, I have parts of the text saved:

There are fewer apocrine glands in Orientals and Native American Indians than in Blacks and Whites. Apocrine glands excrete fat and protein along with water (Poirier et al p 567).

The amount of chloride excreted by sweat glands varies by race: Blacks have more chloride in sweat than do whites. Acclimatized Whites excrete less chloride than unacclimatized whites– a useful adaptation (text 452). Water loss can be considerable: in extreme temperatures young males can loose 4 liters per hour. Thus, human ancestors in tropics must have always had ready access to water (see Overfield for many details).

So since apocrine sweat glands excrete more fats and proteins along with water, this explains why the two races differ in smell in comparison to East Asians. It also makes sense that ‘Native’ Americans would have fewer apocrine glands than Caucasians and Africans since they split off of East Asians around 10-15 kya. You can also see that blacks have more chloride in their sweat. Now, I’ve never encountered this myself, but I’ve seen numerous people say that blacks smell worse than whites or East Asians. This does have a biological basis, and it has to do with pore size, pore type and amount of chloride excreted by the sweat glands.

Of course, like with a lot of traits (not all), whites fall in the middle.

“Almost all Europeans have and all Africans have the wet earwax allele of the ABCC11 gene. The sharp differentiation of the two alleles implies a strong selection pressure.” (Wade, 2015: 90) The function of earwax is to prevent bugs from flying in the ear. Obviously, if the wax is wet (like Africans and Caucasians), the bug won’t be able to get too far into the ear before it gets stopped by the wet earwax. And as luck would have it, the two alleles of the ABCC11 gene are involved in the apocrine sweat glands (Wade, 2015: 90).

The apocrine sweat glands, unlike the eccrine glands, are restricted to the nipples, eyelids, armpits and other special areas on the body. These glands make slightly oil secretions, the specialty of which is to secrete earwax. The glands are odorless, but begin to cause a smell after bacteria begins decomposing dead skin cells(Wade, 2015: 90-91).

East Asians, having the dry earwax allele produce fewer excretions from their apocrine glands and, thusly, have less body odor. “Among people spending many months in confined spaces to escape the cold, lack of body odor would have been an attractive trait and one perhaps favored by sexual selection.” (Wade, 2015: 91) I love this!! It makes so much sense. East Asians were already selected for smaller sweat glands which, along with their dry earwax, produce less of a strong smell when they are perspiring.

Wade further goes into the earwax/body odor relationship and states that the dry earwax allele is almost universal in north China “but yields to the wet allele toward the south.” (Wade, 2015: 91) Most, but not all East Asians have the dry earwax allele, as well as the EDAR-V370A allele.

For a final point, it is assumed that all races have roughly the same skin structure. HOWEVER, morphological differences exist between the races. To quote Ruche and Cesarini (1992):

Under the microscope skin structure is roughly the same in all races, but morphological differences exist, particularly within the epidermis, with potential practical consequences. In comparison with white skin, the black skin stratum corneum is equal in thickness but more compact: about twenty cell layers are observed in blacks versus sixteen layers in whites. The lipid content of black epidermis is also somewhat higher, and this perhaps explains the greater cellular cohesion, hence the difficulty in stripping off the black horny layer. These findings could also explain a slightly inferior permeability of black skin to certain chemicals. The hair of blacks in naturally more brittle and more susceptible to breakage and spontaneous knotting than that of whites. The kinky or wooly form of black hair, the weak intercellular cohesion between cortical cells and the specific hair grooming practices among black people account for these effects. The higher electrical resistance of black skin suggests that the black epidermis would be less hydrated than white epidermis. Anatomically, the amount of sweat glands in black and white skins is identical and varies with climatic changes but not with racial factors. Likewise, sweating is thought to be similar in both races, taking into account the contradictory results from studies, but black subjects withstand humid heat better while whites cope better with dry heat.

So racial factors have no bearing on this, but climatic changes do. Generally, Africans come from humid climates nearer to the equator while Europeans come from cooler places farther from the equator. So we can say that, technically, there is a racial variation between the two.

Excessive sweating is a more common problem for Caucasians and Africans, who tend to have more hair follicles to which the apocrine glands are attached. East Asian people appear to have less and smaller apocrine glands, which explains why they might not need to use deodorants as often as populations of Africa and Europe (see paragraph below). As a matter of fact the deodorant/antiperspirant market in Asia is much smaller than in the western world. Surely, though, there may be many exceptions since body odour is obviously influenced by many factors, for example by one’s personal diet (spicy asiatic food etc.).

Below this section, the article talks about earwax and body odor, which I have covered above.

Differing allele frequencies between the races of man produce differing phenotypes based on where that groups’ ancestors evolved. Changes in certain alleles and not in others clearly led to differences in phenotype that did not occur because the environment was different between the races. These pheno and genotypic differences *prove the existence of race*, along with modern-day genomic testing. The fact that the races differ, albeit subtly, on numerous traits proves the existence of race, population, whatever you want to call it. It doesn’t change race’s reality.

If you have not read it, buy it. You’ll be a better and sharper race-realist as Nicholas Wade is an outstanding researcher and can explain complex concepts very simply.

Why do armpits stink? Study on bacteria that causes pungent body odor could help design effective deodorants

(Getty Images)

A bacteria residing in the armpits is one of the main culprits behind the stinky body odor in humans. In a new study, scientists have discovered the microbe's secret weapon which helps it produce the pungent smell. These findings pave the way for effective deodorants or antiperspirants.

The bacteria is Staphylococcus hominis. It comes armed with an enzyme that produces the odor. "We’ve discovered how the odor is produced,” Prof Gavin Thomas, from the University of York and one of the authors of the study, told The Guardian. “What we really want to understand now is why," he added. The research is a collaboration between the University of York and a consumer goods manufacturer named Unilever. Human armpits host different kinds of microbes, some of whom are responsible for generating pungent-smelling molecules. Earlier, York scientists revealed that Staphylococcus hominis is majorly involved in this. It eats a compound produced by human sweat glands, called Cys-Gly-3M3SH, and releases the foul-smelling thioalcohols as a byproduct. But how exactly the microbe carries out this conversion was unknown until now.

To find out more, the team experimented on another armpit resident which reportedly does not produce any odor: Staphylococcus aureus. They got the organism to make the hominis' enzyme. Their results showed that the innocent bacteria began producing the foul-smelling thioalcohols. “Our noses are extremely good at detecting these thioalcohols at extremely low thresholds, which is why they are really important for body odor. They have a very characteristic cheesy, oniony smell that you would recognize. They are incredibly pungent," Thomas told The Guardian.

(University of York)

“Solving the structure of this ‘BO enzyme’ [ Body odor] has allowed us to pinpoint the molecular step inside certain bacteria that makes the odor molecules. This is a key advancement in understanding how body odor works and will enable the development of targeted inhibitors that stop BO production at the source without disrupting the armpit microbiome," Dr Michelle Rudden from the University of York’s Department of Biology, and co-author, said in a statement.

The researches trace the human origins of body odor to the primate ancestors. The bacteria colonized primates and produced the stinky compound, even before they made human armpits their home. Researchers suspect that this could suggest that body odor may have something to do with societal communication. "This discovery raises important questions about the role of odor production in the evolution of modern humans," the experts wrote in their study. “This research was a real eye-opener. It was fascinating to discover that a key odor-forming enzyme exists in only a select few armpit bacteria – and evolved there tens of millions of years ago," Unilever co-author Dr Gordon James said.

“All we can say is this is not a new process. BO was definitely around while humans were evolving,” Thomas told The Guardian. “It’s not impossible to imagine these were important in the evolution of humans. Before we started using deodorants and antiperspirants, in the last 50 to 100 years, everyone definitely smelled.”

The study is published in Scientific Reports.

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But that’s not all! The hairs prevent skin-on-skin friction that can sometimes be painful or uncomfortable and can even lead to the formation of skin tags. That hair also serves as a protective cover for the arteries that traverse through your underarms. Basically, your body knows what’s up and is simply trying to protect you.

Arguably, the biggest benefit of letting your armpit hair grow out is that you don’t have to deal with the hassle of shaving it all the time! Just like any other part of your body, you’ve just got to make sure you’re taking the time to keep the area clean and groomed.

But what if you want to get rid of it? Like we said, we’re not here to tell you what to do with your body. Growing out your armpit hair does have some setbacks. The biggest is probably having more difficulty applying deodorant and dealing with a potentially stronger odor in between washes. (Protip: a spray-on deodorant may make the process easier.)

If you do want to shave your armpits, Caruana recommends waxing over other hair removal methods.

“Waxing helps prevent ingrown hairs versus shaving. It also thins the hair out so it’s finer and there’s less of it when it grows back in,” she advises. “You also don’t have to do anything to it until your next wax four to six weeks later.”

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There are a few products that will help with in-between waxing appointments.

The first is exfoliating pads — try Pixi By Petra’s Glow Peel Pads ($22) — which will buff away the top layer of skin cells to keep your underarms bump-free. The second is a softening oil to apply to new growth. Caruana says she’s a huge fan of FUR Oil ($46) because “it’s all-natural and keeps hair and skin soft in order to prevent ingrown hairs. It also helps with chaffing.”

The bottom line? Do what you want when it comes to shaving, but know that armpit hair does have its evolutionary benefits and that no matter what you’ve gotta keep the area groomed.

Explainer: The bacteria behind your B.O.

Relax. Everybody sweats. But what causes that distinctive sweaty smell? Blame your bacteria.

Dandamanwasch/iStock/Getty Images Plus adapted by L. Steenblik Hwang

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There are some aspects of being human that just aren’t very glamorous. One of them, without question, is our body odor. Most people sweat when it gets hot outside or we exercise. But that reek emanating from our armpits and private parts? That’s not from a hearty workout. In fact, it’s not from us at all. Our distinct funk comes thanks to bacteria living on our skin.

Bacteria take innocent, non-smelly chemicals and turn them into our human stank, a recent study shows. The results suggest that while our body odor might be unappreciated now, in the past it may have been part of an individual’s allure.

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Our armpits sport glands — groups of cells that produce secretions — called apocrine (APP-oh-kreen) glands. These are found only in our armpits, between our legs and inside our ears. They secrete a substance that might be mistaken for sweat. But it’s not that salty water that seeps out, all over our bodies, from other eccrine [EK-kreen] glands. The thick secretion released by apocrine glands is instead full of fatty chemicals called lipids.

If you take a whiff of your underarm, you might think this secretion stinks. Scientists have been trying to figure out the source of our signature scent. They have put forward many different molecules as the source of body odor, notes Gavin Thomas. He’s a microbiologist — a biologist who specializes in one-celled life — at the University of York in England.

Scientists used to think that hormones might cause our sweaty smell. But “it doesn’t look as if we make those in the underarm,” Thomas says. Then scientists thought our sweaty smell might come from pheromones (FAIR-oh-moans), chemicals that affect the behavior of other animals. But those didn’t seem to matter much either.

In fact, the thick secretions from our apocrine glands don’t smell very much on their own. This is where the bacteria come in, says Thomas. “Body odor is the consequence of bacteria in our underarms.”

Bacteria are real stinkers

Bacteria coat our skin. Few have stinky side effects. Staphylocci (STAF-ee-loh-KOCK-ee), or staph for short, are a group of bacteria that live all over the body. “But we found [this] particular species,” Thomas reports, “which only appears to grow in the underarm and other places where you have these apocrine glands.” It’s Staphylococcus hominis (STAF-ee-loh-KOK-us HOM-in-iss).

Thomas looked at the diet of S. hominis while he was working with other scientists at the University of York and at the company Unilever (which produces body products such as deodorant). This germ takes up residence in your pits because it loves to dine on a chemical from the apocrine glands. Its favorite dish is called S-Cys-Gly-3M3SH. S. hominis pulls it in through molecules — called transporters — in its outer membrane.

The molecule has no smell on its own. But by the time S. hominis is done with it, the chemical has been transformed into something called 3M3SH. This is a type of sulfurous molecule called a thioalcohol (Thy-oh-AL-koh-hol). The alcohol part ensures that the chemical escapes easily into the air. And if it’s got sulfur in its name, that hints it’s likely to stink.

What does 3M3SH smell like? Thomas gave a group of non-scientists in a local pub a whiff. Then he and asked them what they had smelled. “When people smell thioalcohol they said ‘sweat,’” he says. “Which is really good!” It means that the chemical is definitely a component of the body odor we know and loathe.

Thomas and his colleagues published their findings in 2018 in the journal eLife.

Other staph bacteria also have transporters that can suck up the odorless precursor from our skin. But only S. hominis can make the stink. That means that these microbes probably have an extra molecule — one other staph bacteria don’t make — to chop up the precursor inside S. hominis. Thomas and his group are now working to figure out exactly what that molecule is and how it works.

And there’s still more to the story

3M3SH is definitely a part of our distinctive sweaty scent. But it’s not working alone. “I’ve never smelled someone and thought ‘Oh, that’s the molecule,’” says Thomas. “It’s always going to be a complex of smells. If you smell somebody’s underarm it’s going to be a cocktail [of scents].” The other ingredients in that cocktail, though, vary from person to person. And some of them still await discovery.

B.O., it seems, is a partnership between our apocrine glands and our bacteria. We produce 3M3SH, which has no smell. It serves no purpose, except to act as a delicious snack for the bacteria that turn it into the stink in our sweat.

That means that our bodies may have evolved to produce chemical precursors, just so the bacteria could gobble them up and make us stink. If true, why would our bodies aid bacteria to make these smells. After all, we now spend so much time trying to make those smells disappear.

In fact, Thomas says, those odors may have mattered far more in the past. People are very sensitive to the stink of sweat. Our noses can sense 3M3SH at only two or three parts per billion. That’s two molecules of the chemical per billion of molecules of air, or the equivalent of two drops of ink in a 4.6-meter (15-foot) diameter backyard swimming pool.

What’s more, our apocrine glands don’t become active until we hit puberty. In other species, smells like these are involved in findings mates and communicating with other members of a group.

“So it doesn’t take a huge leap of imagination to think 10,000 years ago maybe smell had a much more important function,” Thomas says. Until a century ago, he says, “We all smelled. We had a distinct smell. Then we decided to shower all the time and use a lot of deodorant.”

His research has made Thomas a little more appreciative of our natural fragrance. “It makes you think it’s not such a bad thing. It’s probably quite an ancient process.”

Power Words

apocrine gland A sweat gland in mammalian skin that secrete oil compounds in people. Humans have apocrine glands in their armpits and private areas. Other animals, such as horses, have apocrine glands all over the body and use their secretions to keep themselves cool.

bacteria (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).

behavior The way something, often a person or other organism, acts towards others, or conducts itself.

B.O. Short for body odor. It&rsquos the pungent scent that usually is associated with sweaty underarms.

cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall.

chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

colleague Someone who works with another a co-worker or team member.

component Something that is part of something else (such as pieces that go on an electronic circuit board or ingredients that go into a cookie recipe).

diet The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (verb) To adopt a specific food-intake plan for the purpose of controlling body weight.

eccrine glands These are sweat glands in mammalian skin that mainly secrete salty water. In mammals such as mice and cats, eccrine glands are on the pads of the feet. Their sweat helps these animals keep their grip on slippery surfaces. In humans, eccrine glands occur all over the body, and pump out sweat to help us cool our bodies.

gland A cell, a group of cells or an organ that produces and discharges a substance (or &ldquosecretion&rdquo) for use elsewhere in the body or in a body cavity, or for elimination from the body.

hormone (in zoology and medicine) A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body. (in botany) A chemical that serves as a signaling compound that tells cells of a plant when and how to develop, or when to grow old and die.

lipid A type of fat.

membrane A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.

microbiology The study of microorganisms, principally bacteria, fungi and viruses. Scientists who study microbes and the infections they can cause or ways that they can interact with their environment are known as microbiologists.

microbe Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

pheromone A molecule or specific mix of molecules that makes other members of the same species change their behavior or development. Pheromones drift through the air and send messages to other animals, saying such things as &ldquodanger&rdquo or &ldquoI'm looking for a mate.&rdquo

precursor A substance from which some later thing is made. It may be a compound that will change into something else as a result of some chemical or biological reaction.

puberty A developmental period in humans and other primates when the body undergoes hormonal changes that will result in the maturation of reproductive organs.

secrete (noun: secretion) The natural release of some liquid substance &mdash such as hormones, an oil or saliva &mdash often by an organ of the body.

secretion A liquid substance &mdash perhaps a hormone, an oil or saliva &mdash released by the body, often by an organ, such as the eye, pancreas or skin.

side effects Unintended problems or harm caused by a procedure or treatment.

species A group of similar organisms capable of producing offspring that can survive and reproduce.

staph Short for Staphylococcus. It's a family of bacteria that is responsible for a number of serious human infections. One member of the family also contributes to the distinctly pungent smell of human sweat.

transporters (in cellular biology) The structures on biological membranes through which ions and small molecules can enter cells.


About Bethany Brookshire

Bethany Brookshire was a longtime staff writer at Science News for Students. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.

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