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Horses can smell human fear—and it changes their behavior.

That’s the takeaway of a rather unusual experiment that involved making horses smell material soaked in human sweat and observing what they did next. The findings were published today in PLOS One.

Horses exposed to samples of sweat from people who had had a scary experience appeared more afraid themselves: the animals were easily startled, hesitated to come up to the researchers, and became less likely to interact with unknown objects.

“Our emotions are central when interacting with horses,” says Plotine Jardat, lead author of the study and a horse behavior and welfare researcher at France’s National Research Institute for Agriculture, Food and Environment. “If your horse does not cooperate on an exercise you are proposing, maybe trying it another day when you feel differently can be a game changer.”

Researchers already knew that horses can respond to humans’ emotional cues, including facial expressions and tones of voice. But the new study went further by investigating whether horses could smell different emotions emanating from humans without those visual or oral cues.

In the experiment, a group of people with cotton pads under their armpits watched movie clips geared to produce a sense of joy; these included the dance scene in the film Singin’ in the Rain and the song “We Go Together” from the movie Grease. The researchers then asked the participants—armed with new cotton pads—to watch 20 minutes from the horror film Sinister to stimulate fear.

The sweat samples were then stapled into a custom muzzle for the horses to wear. To limit the stress on each test horse, an “audience horse” served as a witness to the behavior tests.

The researchers first measured how often a test horse would interact with the experimenter, depending on what it was smelling, both while it was being groomed and while the experimenter stood slightly apart from the animal. Horses that smelled the fear samples touched the experimenter less than those in a control group or those that smelled joyful sweat samples.

The team then tested the horses’ reactivity by opening an umbrella near a bucket of food. Once again, horses that smelled the fear sweat showed a different reaction than those that smelled anything else. Their physical reactions to being startled were stronger, and their heart rates were higher.

The last test involved presenting the horses with a novel object—a sculpture of sorts, made of linoleum, plastic and string. The researchers recorded how often a horse gazed at the object and how often the animal touched it. The horses in the fear group touched the novel object less often and stared at it from a distance more than their peers did.

Taken together, the horses’ reactions indicate they can sense fear from odor alone, the researchers conclude. What the study doesn’t answer is why horses can apparently do this: the ability could be a result of domestication, or it could stem from some underlying mammalian characteristic. But regardless, perhaps don’t go up to a horse immediately after watching a horror film.

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Camille Loiseau/500px via Getty Images

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