Horse behavior influenced by smell of fear

By Stephen Beech

Horses' behavior is influenced by the smell of fear, reveals new research.

Mares and stallions exposed to the smell of human fear exhibited higher levels of fearful behavior and were less inclined to interact with people, say scientists.

They explained that smell - also known as olfaction - is the sense most widely used by animals to communicate.

Although the sense has been studied mainly in the context of reproduction within a species, its broader role remains largely unknown.

Recent data suggest that olfactory signals may also be involved in interactions between individuals of different species, facilitating emotional communication.

French scientists are exploring the hypothesis through studies of human–horse interactions.

The research team collected fear- and joy-related odors from 30 volunteers.

To do so, they placed cotton pads under the armpits of participants while they watched 20-minute video clips depicting situations of fear or joy.

To evoke fear, an excerpt from the horror film "Sinister" was shown.

To elicit joy, the participants saw a montage of various excerpts from comedy sketches, musicals and comedies.

The researchers then presented 43 horses with a pad impregnated with the human odor of fear, the human odor of joy, or no odor at all - used as a control.

The team measured behavior, heart rate, and levels of the stress hormone cortisol in saliva during a series of tests.

In the first "interaction" test, the experimenter groomed the horse, and in an "approach" test the experimenter stood in a defined area to measure whether the horse would approach.

In a "suddenness" test an umbrella was suddenly opened next to the horse, and in another test an unknown object was placed near the horse.

Study co-author Dr. Léa Lansade said: "Horses exposed to the smell of human fear exhibited higher levels of fearful behavior and were less inclined to interact with humans.

"In the suddenness test, they showed stronger reactions to the sudden opening of an umbrella and reported more gazes at the unfamiliar object. Their heart rates were also higher.

"In the interaction tests, they had less physical contact with humans, both in the approach test and during grooming.

"More than a simple reaction to a smell, these findings suggest that horses adopt the emotional state of fear."

She says the results, published in the journal PLOS One, point to emotional contagion of fear through chemical cues between two different species, humans and horses.

Dr. Lansade, research director at the National Research Institute for Agriculture, Food and Environment (INRAE), said: "While this phenomenon has already been demonstrated in dogs, this is the first time such emotional communication has been documented in another animal species.

"These findings highlight the importance of olfactory signals in interactions between different species and provide new insights into the impact of domestication on emotional communication between humans and animals."

She added: “There is a chemical communication of emotions, and it crosses the species barrier.

"This new study shows that, when exposed to the odors of humans experiencing fear, horses in turn exhibit fear responses through a mechanism of emotional contagion.”