The sweet smell of sweat

Home Technologist 04 Smell The sweet smell of sweat

Everyone knows that animals use odours to communicate. Now a growing body of research suggests that humans do, too.

An illustration depicting various elements of smell, including a human face and a dog face with prominent noses, smoke from a car and a cigarette, and cloud-like shapes in different colours depicting scents.

Taking a long, deep whiff of someone’s dirty laundry may not sound like the ideal way to start a hot date. But if the latest evidence is anything to go by, the time has come to reconsider that strategy. The primal smell of a potential partner’s body odour may be just the trick to finding out how compatible you really are.

This discovery has not been lost on those touting “pheromone perfumes” on the Internet, or those behind the growing popularity of “pheromone parties”. At these events, partygoers each sleep in a t-shirt for three nights before handing it in, unwashed, at the door. The idea is that, guided by their noses rather than their eyes, attendees sniffing these sweaty garments will choose suitable matches.

Whilst it has long been known that many animals, including mammals such as rodents, rely heavily on their sense of smell for mating strategies, the idea that humans can do the same inspires scepticism. Yet a growing body of evidence is beginning to reveal just how sensitive humans are to chemical cues released in body odour.

“I’ve been doing research in smells since the early 70s and I’ve seen a remarkable change,” says Charles Wysocki of the Monell Chemical Senses Center in Philadelphia. “A lot more information is becoming available to show that humans are processing odours even though they may not consciously be aware of it.”

FRAGRANT FACTS
By Emilie Fedders

Illustration of a face of a person smokingMunchies
Binge eating after smoking marijuana – a.k.a. the munchies – are a scientific fact: a cannabis receptor in the brain heightens the sense of smell, which in turn triggers food cravings.

Drawing depicting bad breathBad breath
A scientific review found that in 15 per cent of cases the cause is not just germs in the mouth; it can include inflammation of the airways or stomach diseases.

Drawing of a death skull symbol with odourSmell of death
A large American study found a correlation between decreased ability to smell and a threefold increased risk of dying within five years.

Drawing of a woman's head in profileFemale skills
Women are better than men at detecting odours, peaking around the age of 15 and remaining at a significantly higher level all through life, according to a 2014 review.

Pie diagram split in three33 per cent
A 2013 study estimates that one third of the 400 receptor proteins found in the membranes of our olfactory neurons (located in the nose) vary from person to person. This may explain why different people enjoy different smells.

Unknown
The number of fragrances a human can distinguish is still uncertain. A 2014 study (quoted in Technologist 01) claimed “over one trillion”, but its methodology has since been criticized. Other researchers have found that people are very good at differentiating smells, but bad at identifying them.

Smell or smartphones?

Whether to find a mate, warn others of danger or identify prey, animals use smells to communicate a wealth of information to others of the same species. Humans, on the other hand, have long underestimated the importance of body odour. A recent survey of more than 7,000 young people around the world found that half would rather give up their sense of smell than their smartphones.

It is no surprise then that our olfactory sense has been undervalued compared to other senses that seem to be more essential to human survival. After all, we pay little attention to odours unless we suddenly encounter a particularly wretched one. And people find it much harder to identify fragrances – putting a name to them or detecting exactly where they come from – than they do sounds and sights.

The idea that humans may have pheromones – small chemicals or compounds that trigger a social response in other members of the same species – has come into favour largely thanks to research led by biologist Claus Wedekind at the University of Bern in 1995. His experiments were the precursors to pheromone parties: female students were asked to rate the smell of sweaty t-shirts that men had worn to bed.

Wedekind’s “sweaty t-shirt” study found that women preferred the smell of garments worn by men whose immune systems were significantly different from their own, as measured by a set of molecules called the Major Histocompatibility Complex (MHC).

Mating with someone with a different immune system could be evolutionarily advantageous for adapting to selection pressures in the environment, such as pathogens. The finding led the authors to conclude that humans had evolved a way of sniffing out a good mate.

Now it seems that sniffing out an ideal immune system may just be the beginning. “As humans we tend to think we’re rational and logical, yet we’re subjected to the same primitive animal-like signals as the rest of the animal kingdom,” says Liron Rozenkrantz of the Weizmann Institute in Israel.

“Our decisions, behaviours and opinions are influenced by factors we’re not aware of, yet that have major effects on our mood, hormonal state and brain activity.”

An evolutionary advantage

Some of the strongest evidence comes from studies of fear and anxiety. Researchers at the Christian-Albrechts-University in Kiel, Germany, found that the smell of sweat collected from students anxiously awaiting exam results could be communicated to other students, unlike the smell of sweat they produced when they exercised. The findings led the authors to conclude that in humans, as in other animals, “the smell of alarm pheromones seems to be evolutionarily advantageous.”

This idea was confirmed in 2008 by Lilianne Mujica-Parodi of New York’s Stony Brook University, whose lab investigates the neuronal basis of stress.

Rats are known to be able to smell the fear of other rats – for example if they are placed in a cage in which another rat had recently been exposed to a predator. To test the idea in humans, Mujica-Parodi’s team collected sweat from volunteers as they jumped out of a plane during a sky dive – a stressful situation if there ever was one – as well as the sweat they produced when running on a treadmill.

When they presented these different sweat samples to other volunteers in an fMRI scanner, the researchers observed an activation in the amygdalae, the brain centres involved in emotion. They also found that sniffling the skydive-sweat sample primed participants to pay more attention to images of threatening faces than neutral ones; they saw no difference when the volunteers were presented with the perspiration sample that came from exercise.

“When individuals are anxious or fearful, their body odour changes and that body odour can communicate information to other individuals who smell it,” concludes Wysocki.

Crucially, the volunteers could not consciously tell the two sweat samples apart. “It may be at a subconscious level but it does alter their brain activities. The smell of fear may make people more vigilant of their environment.”

Vale of tears

Nor is perspiration the only body fluid that contains such communicative chemicals. In 2011 the Weizmann Institute’s Noam Sobel asked female volunteers to watch sad movies, then collected the tears they shed on small cotton pads. As a control they used a saline solution that they had dropped down the women’s faces. Men could not consciously detect any difference between the smell of tears of sadness and the saline solution. Yet the chemicals contained in the tears seemed to affect them on an unconscious level.

When the men were shown photographs of the women’s faces, they rated them as less attractive after smelling real tears than after smelling the control solution. The researchers also found that sniffing the tear-soaked pads lowered the men’s testosterone levels. In a third test, the men were asked to watch sexually arousing films while in an fMRI scanner; they had less brain activity associated with arousal when they sniffed the tear-soaked cotton pads.

The idea that a woman’s tears could turn off a man has clear evolutionary advantages, but there may be other as yet undiscovered chemical signals conveyed through tears, says Rozenkrantz. It may also explain why women tend to cry more during menstruation. “Since intercourse in that phase of the menstrual cycle cannot lead to pregnancy, making their mate less sexually aroused at that time might be evolutionarily preferable,” she says.

Lower testosterone levels make men less aggressive, so such a chemical signal could be an effective protective mechanism for women. The idea has yet to be tested, but the same mechanism may also apply to infants who, after all, make up the critical mass of humans who shed emotional tears.“If children’s tears have a similar effect as that of women, we can see this effect as a potential defence mechanism, lowering the aggressiveness of adults around them,” she says.

Sure to score

Many people would like to believe that humans produce pheromones that they can just spritz on to attract a mate. “Lots of websites try to sell people the magic bullet that they can either put on their body directly or in their perfume or aftershave,” notes Wysocki. “They think that if they apply it and then go to a singles bar, they’re sure to score”.

These so-called releaser pheromones (see box below) were identified over 50 years ago thanks to the behaviour of moths. When the male detected a chemical that had been released by the female, it would start flying upwind to find her. The idea that such attractants exist in humans remains sketchy. “There really isn’t any good biomedical evidence,” says Wysocki.

If human use smells to convey subtle information to each other without even realising it, why do they go to such lengths to mask their natural body odours? Actually, says Wysocki, our best efforts are not that successful. His lab collected almost 50 substances that go into deodorants and used them to cover up body odour. “About a third of those substances were quite effective in reducing the impact of male body odour in the nose of men,” he says.

But in women, they were completely ineffective. Even more surprising, whilst the scent of a man was able to make it through those deodorants, other pungent smells – for example farmyard odours – did not. “So it’s not just biological odour, it’s human odour.”

The conclusion is not that it’s a good idea to leave your deodorant at the door. But as you sit down to that dinner date, take a deep breath through your nose and be prepared for your emotions, behaviours and opinions to be influenced by signals you are not even aware of.

WHAT ARE PHEROMONES?

The definition, and the question of whether humans have them, remains controversial. According to Charles Wysocki of the Monell Chemical Senses Center in Philadelphia, they can be divided into four subtypes:

Primer pheromones typically alter the endocrine or neuroendocrine system. In humans, the best-known example is the way women’s menstrual cycles change when they live together. Chemical signals from lactating women have also been shown to affect the menstrual cycles of other women. Some research also suggests that a chemical in male underarm sweat can alter the hormones that regulate women’s cycles.

Modulator pheromones alter the moods and emotions of other people. Some studies have shown that certain chemicals released by breast-feeding women can increase the sexual desire of other women. The findings that the smell of fear can elicit a response in people may also fit into this category.

Signaller pheromones provide information about the signaller – for instance, women indicating when they are fertile. One study of exotic dancers found that they earned more tips during the fertile phase of their cycle.

Releaser pheromones induce a response in behaviour. Babies are known to make their way towards a breast pad that has been worn by their lactating mother, though this is the only real evidence that humans rely on smells to attract others. Disappointing as it may be for some, there is so far no evidence that you can use these chemicals to make someone find you attractive.

By Catherine de Lange

More on the science of smell: Master of fragrances; Magic moleculesHumans, dogs – and now e-noses

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