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Scary stuff. Sweat was collected from the armpits of people doing their first ever skydive. (Source: Olliver Furrer/Getty images)

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Sharing the scent of fear

Tuesday 20th October 2015 12:35 pm

On a recent trip into the Arctic Ocean aboard a sailing boat, we got hit by a weather depression. The air pressure fell to well below 1,000 hPa — in fact, down to 972 hPa.

The seas rose up, and people began to get queasy. The crew advised us to get to our bunks and lie down. The crew knew, from past experience, that as soon as one person started vomiting in the presence of another person, we would all start vomiting. But the passengers were too unwell to make their way down the stairs to their cabins.

Sure enough, very soon after the first person started vomiting, the next one started vomiting. I don’t know why it spread — whether it’s the smell of vomit, or the sight of it — but soon, most of the passengers were vomiting.

But with the concept of fear (that is, being frightened), we now know that it can be spread by the sense of smell.

The story goes back to 1938 and one of the early animal behavioural scientists, Karl von Frisch. He was studying fish — to be more specific, minnows. He noticed that when one minnow became frightened or alarmed, other minnows some distance away would also become alarmed. They would do a mixture of slow swimming, darting away, freezing on the spot, and dropping to the bottom of the tank.

Karl Von Frisch wondered if there was some kind of chemical being transmitted through the water, to trigger the alarm reaction in these other minnows. He gave this hypothetical chemical the name of ‘Schreckstoff’, or in English, ‘scary stuff’.

The next major leap forward was published in 2009 by Professor Lilianne Mujica-Parodi, a cognitive neuroscientist from Stony Brook University in New York.

I have always thought that if you are travelling in a plane, you’re better off staying inside until you land. But some people do like skydiving, that is, jumping out of planes with a parachute.

It sounds reasonable that a person doing their first ever skyjump would be a little fearful in the lead-up to the actual jump – and might sweat a little.

So Professor Mujica-Parodi worked out how to collect their armpit sweat. Even just collecting the sweat was a major task. She had to not interfere with the performance of the novice skyjumper during their first ever jump, nor modify or contaminate in any way the chemicals that were released into the armpit sweat — and still catch all the sweat.

Professor Mujica-Parodi was able to do this. She then worked out how to gently spray essence of armpit from these skydivers onto the noses of volunteers — while they were inside a brain scanner. Amazingly, in the brains of the volunteers, when fearful sweat entered their noses, parts of their brain related to fear (such as the amygdala) would light up.

As a control, she sprayed ‘regular’ exercise sweat onto the volunteers — and in this case, the amygdala did not light up. So it seemed that you could unconsciously detect the smell of fear from another person.

There was a behavioural effect of the fearful sweat. It improved the volunteers’ awareness and vigilance. They became 43 per cent more accurate in judging if another person’s face was neutral or threatening.

In 2012, other scientists from Singapore and Switzerland took the first successful step in working out what the fear chemical might be — at least in fish. Yes, we’re getting back to Karl Von Frisch’s scary stuff or schreckstoff.

It seems to be a mixture of chemicals classified as ‘glycosaminoglycan chondroitin’. These are long molecules, related to sugars — and there are many different types.

The specific chemicals that set off the fear behaviour in fish seem to fall into two categories — heavier and lighter.

For comparison, water is a very small and light molecule — only 18 Daltons. Insulin is bigger and heavier — about 6,000 Daltons. In schreckstoff, the heavy chemicals are about 30,000 Daltons, and seem to evoke slow swimming and dropping to the bottom of the fish tank. The light chemicals in Schreckstoff are around 1000 Daltons, and seem to increase darting behaviour.

Getting back to humans, the effect of the fearful sweat seems to be fairly short-lived — about 15 minutes. But it could be used to make training for sticky situations more realistic — making it easier to know what to do when something smells fishy …



This blog first appeared on Dr Karl's Great Moments in Science

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