Can People Really Smell Fear?

koosen/iStock via Getty Images
koosen/iStock via Getty Images

Fictional characters, and even real-life folks, often talk about animals and people—particularly snarling dogs and knife-wielding lunatics—being able to “smell fear” on people. No one ever seems to be able to describe just what fear smells like, though.

The lack of detail—Is it musty? Does it have a hint of vanilla?—leads one to think that the smell of fear is more metaphorical than literal. But scientific evidence suggests that fear might really have a chemical component that we sense through our noses without even realizing it. The matter is far from settled, though.

Right Under Your Nose

In 2009, a team of German researchers collected sweat from two groups of students, one where the sweat came from exercising on bikes and one where it came from the stress of waiting to give a graded oral presentation.

A third group of students, lying in fMRI scanners and wearing modified oxygen masks, then smelled air odorized by the two groups’ sweat. Asked about what they were smelling, the students didn’t even notice an odor in half of the trials. When they did notice they were smelling something, they were unable to tell the difference between the two sources and rated both as low in intensity, weakly pleasant, unfamiliar, and having no effect on their own emotions.

The brain scans told a very different story, though. After smelling the sweat of the students nervously waiting for their exams, the smellers’ brains showed increased activity in areas that are involved with empathy and processing social signals and the emotional states of other people. The sweat from exercise didn’t cause these same activations, suggesting that the nervous students’ sweat contained some sort of chemical signal of their anxiety that triggered a response in the smellers’ brains without registering as the sensory experience of a smell.

That same year, a pair of psychologists at Rice University collected sweat from different volunteers while they watched horror movies or slapstick comedies, and then asked other volunteers to smell the sweat while they looked at images of faces that switched expressions from happy to ambiguous to fearful. As the faces morphed, the volunteers were asked to indicate whether they thought the expressions were happy or fearful.

The smellers were more likely to judge the ambiguous faces as fearful after being exposed to the the horror watchers’ sweat than when they smelled the comedy watchers’ sweat or a control sweat. That behavioral change suggests that not only did the sweat contain some chemical signal that communicated emotion, but also affected people’s visual perception of emotions and biased them towards the one being communicated by the sweat (that second part is consistent with other findings that emotional cues from faces and voices can regulate each other).

Just last month, Dutch psychologists found evidence that fear-induced sweat not only biases someone who smells it toward seeing fear, but might also push them to feel it themselves. Volunteers either watched scenes from a scary movie or from Jackass, and their sweat was collected. People in another group were then exposed to the smell of one of the sweats while they took a visual test that asked them to find a target object on screens full of different items. While this was going on, the researchers recorded their facial expressions and tracked their eye movements.

The people who got the horror sweat made facial expressions suggesting fear or anxiety shortly after they were exposed to the sweat. The Jackass-sweat smellers, meanwhile, made disgusted faces. (This was determined by comparing their faces to “distinctive facial-muscle signatures” associated with emotions. For more on face reading, check out Paul Ekman and his facial action coding system.) The two sweats also appeared to have affected the smellers’ behavior, with the horror sweat smellers attempting to acquire more sensory information while exposed to the sweat by taking bigger sniffs and scanning more and fixing their gaze less during the visual exercise.

What the Nose Knows

What all this, and a bunch of other studies looking at the same sort of thing, suggests is that humans might not communicate by just sight, sound, and touch. Like other animals, we might also use chemical signals embedded in our sweat, and maybe elsewhere, to let each other know about our emotional states.

That has been a contentious idea for a very long time, with some people claiming “definitive evidence of human pheromones,” and others saying “no, not so fast.” While there’s a good amount of evidence for behavioral and physiological changes in people in response to “chemosignals,” no one has been able to nail down just what the chemicals are that trigger these responses, and how people detect them. The vomeronasal organ, which many animals use to detect pheromones, is present in some humans, but doesn’t appear functional. When scientists can find them in people, the genes that code for their receptors don’t seem to have done their job, and their sensory neurons have little or no connection with the central nervous system.

Those are two very important dots that need to be connected before the line between someone’s sweaty armpit and someone else’s schnoz can be drawn clearly.

Why Are Sloths So Slow?

Sloths have little problem holding still for nature photographers.
Sloths have little problem holding still for nature photographers.
Geoview/iStock via Getty Images

When it comes to physical activity, few animals have as maligned a reputation as the sloth. The six sloth species, which call Brazil and Panama home, move with no urgency, having seemingly adapted to an existence that allows for a life lived in slow motion. But what makes sloths so sedate? And what horrible, poop-related price must they pay in order to maintain life in the slow lane?

According to HowStuffWorks, the sloth’s limited movements are primarily the result of their diet. Residing mainly in the canopy vines of Central and South American forests, sloths dine out on leaves, fruits, and buds. With virtually no fat or protein, sloths conserve energy by taking a leisurely approach to life. On average, a sloth will climb or travel roughly 125 feet per day. On land, it takes them roughly one minute to move just one foot.

A sloth’s digestive system matches their locomotion. After munching leaves using their lips—they have no incisors—it can take up to a month for their meals to be fully digested. And a sloth's metabolic rate is 40 to 45 percent slower than most mammals' to help compensate for their low caloric intake. With so little fuel to burn, a sloth makes the most of it.

Deliberate movement shouldn’t be confused for weakness, however. Sloths can hang from branches for hours, showing off some impressive stamina. And because they spend most of their time high up in trees, they have no need for rapid movement to evade predators.

There is, however, one major downside to the sloth's leisurely lifestyle. Owing to their meager diet, they typically only have to poop once per week. Like going in a public bathroom, this can be a stressful event, as it means going to the ground and risking detection by predators—which puts their lives on the line. Worse, that slow bowel motility means they’re trying to push out nearly one-third of their body weight in feces at a time. It's something to consider the next time you feel envious of their chill lifestyle.

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Are Any of the Scientific Instruments Left on the Moon By the Apollo Astronauts Still Functional?

Apollo 11 astronaut Neil Armstrong left the first footprint on the Moon on July 20, 1969.
Apollo 11 astronaut Neil Armstrong left the first footprint on the Moon on July 20, 1969.
Heritage Space/Heritage Images/Getty Images

C Stuart Hardwick:

The retroreflectors left as part of the Apollo Lunar Ranging Experiment are still fully functional, though their reflective efficiency has diminished over the years.

This deterioration is actually now delivering valuable data. The deterioration has multiple causes including micrometeorite impacts and dust deposition on the reflector surface, and chemical degradation of the mirror surface on the underside—among other things.

As technology has advanced, ground station sensitivity has been repeatedly upgraded faster than the reflectors have deteriorated. As a result, measurements have gotten better, not worse, and measurements of the degradation itself have, among other things, lent support to the idea that static electric charge gives the moon an ephemeral periodic near-surface pseudo-atmosphere of electrically levitating dust.

No other Apollo experiments on the moon remain functional. All the missions except the first included experiment packages powered by radiothermoelectric generators (RTGs), which operated until they were ordered to shut down on September 30, 1977. This was done to save money, but also because by then the RTGs could no longer power the transmitters or any instruments, and the control room used to maintain contact was needed for other purposes.

Because of fears that some problem might force Apollo 11 to abort back to orbit soon after landing, Apollo 11 deployed a simplified experiment package including a solar-powered seismometer which failed after 21 days.

This post originally appeared on Quora. Click here to view.

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