The smell of fear influences horse behaviour

Olfaction is the sense most widely used by animals to communicate. Although this 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. Ethologists from INRAE and IFCE are exploring this hypothesis through studies of human–horse interactions.

Exposing horses to human emotional odours

The research team collected fear- and joy-related odours 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^[1].

The researchers then presented 43 horses with a pad impregnated with the human odour of fear, the human odour of joy, or no odour at all—used as a control. They measured behaviour, heart rate, and cortisol (stress hormone) levels in saliva during various tests:

• Two interaction tests: one in which the experimenter groomed the horse, and an approach test in which the experimenter stood in a defined area to measure whether the horse would approach.
• Two ‘fear’ tests: a suddenness test in which an umbrella was suddenly opened next to the horse, and a test in which an unknown object was placed near the horse.

The human smell of fear causes fearful behaviour in horses

Horses exposed to the smell of human fear exhibited higher levels of fearful behaviour 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. The results point to emotional contagion of fear through chemical cues between two different species, humans and horses. 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.

[1] 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.

“There is a chemical communication of emotions, and it crosses the species barrier. This new study conducted by INRAE and IFCE and published in PLOS One shows that, when exposed to the odours of humans experiencing fear, horses in turn exhibit fear responses through a mechanism of emotional contagion.” Léa Lansade, INRAE research director and co-author of the study