Your skin is teeming with bacteria. There are billions of them, living on the dry parched landscapes of your forearms, and the wet, humid forests of your nose. On your feet alone, every square centimetre has around half a million bacteria. These microbes are more than just passengers, hitching a ride on your bodies. They also affect how you smell.
Skin bacteria are our own natural perfumers. They convert chemicals on our skin into those that can easily rise into the air, and different species produce different scents. Without these microbes, we wouldn’t be able to smell each other’s sweat at all. But we’re not the only ones who can sniff these bacterial chemicals. Mosquitoes can too. Niels Verhulst from Wageningen University and Research Centre has just found that the bacteria on our skin can affect our odds of being bitten by a malarial mosquito.
We all know that as we type on our keyboards or click our mice, we leave behind fingerprints that could be used to deduce our identities. But these prints aren’t the only remnants of our presence. Bacteria from our skins also linger on the things we touch and they could act as a sort of living fingerprint.
The thriving community of bacteria and other microscopic passengers on our skin has many traits of interest to a forensic scientist. For a start, they are remarkably personal in their membership and stable over time. Just 13% of the bacteria on my palm also live on yours, and even identical twins can harbour very different microbes. Even after we give our hands a good wash, our individual communities eventually bounce back with a similar mix of members. Skin bacteria are also easy to dislodge. As we go about our daily lives, we leave a trail of our own personal bacteria on the things we touch, which can stick around for days or weeks.
Noah Fierer from the University of Colorado has found some preliminary evidence that these dislodged microbes could help to identify an individual. He showed that bacteria swabbed from keyboards and mice matched the microbes on their owner’s skin more closely than those from other people. It’s a promising technique, but David Foran, director of Michigan State University’s Forensic Biology Laboratory, says that it’s “utility in a forensic context is doubtful”. It’s unlikely to ever meet the high standards of certainty needed for a criminal investigation, although that probably won’t stop it from appearing in a future episode of CSI.
What part of the body do you listen with? The ear is the obvious answer, but it’s only part of the story – your skin is also involved. When we listen to someone else speaking, our brain combines the sounds that our ears pick up with the sight of the speaker’s lips and face, and subtle changes in air movements over our skin. Only by melding our senses of hearing, vision and touch do we get a full impression of what we’re listening to.
When we speak, many of the sounds we make (such as the English “p” or “t”) involve small puffs of air. These are known as “aspirations”. We can’t hear them, but they can greatly affect the sounds we perceive. For example, syllables like “ba” and “da” are simply versions of “pa” and “ta” without the aspirated puffs.
If you looked at the airflow produced by a puff, you’d see a distinctive pattern – a burst of high pressure at the start, followed by a short round of turbulence. This pressure signature is readily detected by our skin, and it can be easily faked by clever researchers like Bryan Gick and Donald Derrick from the University of British Columbia.
Gick and Derrick used an air compressor to blow small puffs of air, like those made during aspirated speech, onto the skin of blindfolded volunteers. At the same time, they heard recordings of different syllables – either “pa”, “ba”, “ta” or “da” – all of which had been standardised so they lasted the same time, were equally loud, and had the same frequency.
Gick and Derrick found that the fake puffs of air could fool the volunteers into “hearing” a different syllable to the one that was actually played. They were more likely to mishear “ba” as “pa”, and to think that a “da” was a “ta”. They were also more likely to correctly identify “pa” and “ta” sounds when they were paired with the inaudible puffs.
This deceptively simple experiment shows that our brain considers the tactile information picked up from our skin when it deciphers the sounds we’re listening to. Even parts of our body that are relatively insensitive to touch can provide valuable clues. Gick and Derrick found that their fake air puffs worked if they were blown onto the sensitive skin on the back of the hand, which often pick up air currents that we ourselves create when we speak. But the trick also worked on the back of the neck, which is much less sensitive and unaffected by our own spoken breaths.
While many studies have shown that we hear speech more accurately when it’s paired with visual info from a speaker’s face, this study clearly shows that touch is important too. In some ways, the integration of hearing and touch isn’t surprising – both senses involve detecting the movement of molecules vibrating in the world around us. Gick and Derrick suggest that their result might prove useful in designing aids for people who are hard of hearing.
Reference: Nature doi:10.1038/nature08572
More on perception:
In the early days of the last US elections, Hillary Clinton’s campaign was accused of deliberately darkening Barack Obama’s skin in a TV ad. The implication was that by highlighting Obama’s “blackness”, Clinton’s camp was trying to exploit negative associations that voters might have with darker skin. But you don’t need editing software to do that – a fascinating new study suggest that people literally change the way they see a mixed-race politician, depending on whether the candidate represents their own political views.
Liberal American students tend to think that lighter photos of Barack Obama are more typical of him, while conservatives think he’s best represented by darker photos. You can see this effect even after adjusting for any racial prejudices, be they hidden or overt, and even with a person less famous than Obama. And regardless of political views, people who associated Obama with lightened photos were most likely to vote for him.
Eugene Caruso from the University of Chicago, who led the study, thinks that this effect is the result of two biases: the positive associations of white and lightness among some Western cultures; and the tendency to view people of the same group (political or otherwise) more favourably than those of another group. He says, “Group membership provides a lens through which people generate representations of reality.”
Caruso asked 221 students about their political ideologies and then showed them three photos of Obama and three of John McCain. On the grounds that some photos can capture the “true essence” of a politician better than others, the students were asked to rate how well each photo represented each man. But unbeknownst to them, two of each set of pictures had been altered with Photoshop, so that the subject’s skin tone was either lighter or darker.
When it came to McCain, the students’ political leanings had no bearing on their choice of photos. For Obama, it was a different matter – liberal students were more likely to pick the lightened photo as the one that represented him best. Conservative students were more than twice as likely to associate him with the darkened photo. These biases were reflected in the students’ votes. Whether liberal or conservative, the more people associated Obama with the lightened photo, the more they were likely to vote for him.
Of course, this effect could simply be down to racism – people who harbour prejudices against Blacks would be more likely to associate Obama with a darker photo and less likely to vote for him. But Caruso accounted for that – he repeated the experiment with 49 people a week before the last election and specifically evaluated the recruits’ attitudes on race.
Each of them filled in a questionnaire called the Attitudes Toward Blacks scale, which asked them whether they agreed with statements such as “Generally, Blacks are not as smart as Whites.” Obviously, people can lie on these questionnaires, so each volunteer also did an “implicit association test” (IAT), designed to reveal any hidden prejudices (try one here).
The same patterns emerged as before and this time, Caruso found that they remained even after adjusting for racial attitudes, both hidden and explicit. A week after the election, Caruso caught up with his recruits and confirmed that those who thought the lightened photos represented Obama were actually more likely to have voted for him. Those who linked him to the darkened photo were more likely to have voted for McCain. Amazingly, the photo effect turned out to be a better indicator of voting choice than the scores on either of the two prejudice tests.
Barack Obama’s fame is perhaps a bit of a distractor since people can judge him on his policies, personality and more than just his skin colour. But Caruso found the same effect using a non-celebrity. He asked 102 students about their views on important educational issues and then showed them three photos of a man allegedly running for a position in the US Department of Education. He told them that the mystery politician either agreed or disagreed with most of their stances.
But Caruso kept two important things from the recruits. First, the ‘politician’ was actually Jarome Iginla, a mixed-race ice hockey player whose father was a Black Nigerian and whose mother was a White American. None of the students twigged to this. Secondly, as before, some of the photos had been doctored so that Iginla’s skin tone was either lighter or darker.
The same trend emerged. Caruso found that students who were told that the politician supported their views were more than twice as likely to pick the lightened photo as the most representative one. Those who thought that Iginla disagreed with them were more likely to associate him with the darkened photo. And across the board, people who picked the lightened photo were most likely to vote for him.
Across all three experiments, the way that American students literally see a mixed-race politician depends on whether they agree with his views. If they felt aligned with a candidate, they tended to mentally lighten his skin, and Caruso suggests that this might reflect subconscious associations of white with good, and black with bad. Sadly, the study provides no information about the ethnicity of the students involved – it would be very interesting to see if the same bias in perception applies to viewers who are themselves Black.
Reference: PNAS doi:10.1073/pnas.0905362106
More on politics:
It’s a diverse melting-pot of different groups, with hundreds of different cultures living together in harmony, many sticking to their own preferred areas. No, not London, New York or any other cosmopolitan city; I’m talking about your skin. It may all look the same to you, but to the bacteria living on it, it’s an entire realm of diverse habitats.
From a microscopic perspective, the hairy, moist surface of your armpits is worlds apart from the smooth, dry skin of your forearms. Even though they are separated by mere inches, these patches of skin are as different to their microscopic residents as rainforests and deserts are to us.
The true diversity of the bacteria on our bodies has just been revealed by Elizabeth Grice from the National Human Genome Research Institute, who has done a thorough survey of our “skin microbiome”. She recruited 10 healthy volunteers and took swabs from 20 distinct patches of their skin, all areas that are often afflicted by skin disorders. These areas represent a broad range of habitats from the oily lakes of the eyebrows, nose, inner ear and upper chest, to the moist jungles of the groin, nose, armpit and navel, to dry deserts of the forearms and palm.
By sequencing the distinctive 16S rRNA gene of over 112,000 bacterial samples, Grice catalogued a much broader menagerie of microbes than expected, with representatives from 19 different phyla and 205 different genera. Previous attempts at doing this have been biased towards species that grow easily in laboratory conditions such as Staphylococcus, but Grice’s more thorough approach revealed a surprising degree of diversity. It also showed that bacteria from a specific body part have more in common than those from a specific person. Your butt microbes have more in common with mine than they do with your elbow microbes.