Human Fear Smell Changes Horse Behaviour, INRAE Study Finds

Have you ever considered whether animals can detect human feelings without hearing a word or seeing a gesture? Many pet owners believe their animals recognise emotions almost immediately.

New research by INRAE indicates that scent alone can transfer emotions from people to animals. The study shows that human fear can be conveyed through odour and influence animal behaviour in striking ways.

Smell drives animal communication

Animals depend greatly on their sense of smell to interpret their surroundings. Of all the senses, odour has the most important role in animal communication.

Scientists have largely examined this sense in relation to mating or communication between members of the same species. Far less research has explored how scent functions across different species.

Ethologists from several organisations set out to investigate this overlooked area. Their research concentrated on relationships between humans and horses.

Horses have close connections with people and frequently react to human emotions during routine handling and training.

The researchers posed a central question: can horses detect human emotions through smell alone? To investigate, they devised a controlled experiment centred on human fear and joy.

Scientists collected fear sweat

The researchers recruited 30 human volunteers. Every participant watched a 20-minute video intended to provoke a strong emotional response. Some clips contained frightening scenes, whereas others featured joyful moments.

While the videos were playing, cotton pads were positioned beneath the volunteers’ armpits. Human sweat contains chemical signals associated with emotional states.

The pads collected odours linked with either fear or joy. Researchers also used pads carrying no human odour, which served as a control.

The team then exposed 43 horses to the pads. Each animal took part in one of three conditions: one group encountered human fear odour, another encountered human joy odour, and the final group encountered no odour.

Scientists subsequently monitored the horses’ responses in a range of settings. This design enabled the team to make clear comparisons between their emotional reactions.

Testing horse reactions

Horse responses were assessed in four scenarios, two of which examined interactions with people.

For one assessment, an experimenter groomed the horse. In another, the experimenter stood within a marked zone to determine whether the horse would come closer.

The remaining two assessments examined reactions associated with fear. In one, the experimenter abruptly opened an umbrella near the horse. In the other, the team positioned an unfamiliar object close by.

Researchers carefully documented behaviour throughout every test. They also recorded heart rate and measured cortisol in saliva. Cortisol is a stress hormone, and elevated levels commonly indicate fear or tension.

Combining behavioural observations with physical measurements allowed the researchers to assess both outward responses and underlying stress.

Human fear affects horse behaviour

Horses that encountered the odour of human fear responded noticeably. Fear-related behaviours rose across several of the tests.

In the unexpected umbrella test, these horses displayed more pronounced startle reactions. Many also spent longer looking at the unfamiliar object.

Their heart rates rose, indicating stress or fear. The horses were also less willing to remain in close contact, engaged in less physical interaction while being groomed, and were less likely to approach a person.

Horses exposed to human joy odour did not react in the same way, and the control group was also calmer. These contrasts suggest a specific response to fear odour, rather than a reaction to smell generally.

Emotional contagion across species

The results suggest more than a straightforward response to an unpleasant odour. Instead, they indicate emotional contagion, whereby one individual takes on another’s emotional state.

Here, the horses seemed to acquire human fear. Chemical signals in sweat transmitted emotional information between species. Although the effect had previously been identified in dogs, this study provides the first clear evidence in horses.

Domestication could be relevant. The long history shared by humans and horses may have increased their emotional sensitivity. Scent appears to provide a subtle yet powerful connection between species.

“There is a chemical communication of emotions, and it crosses the species barrier,” said Léa Lansade, INRAE research director and co-author of the study.

“This new study conducted by INRAE and IFCE and published in PLOS One shows that, when exposed to the odors of humans experiencing fear, horses in turn exhibit fear responses through a mechanism of emotional contagion.”

The study creates new opportunities for understanding animal emotions. It also underlines that human emotional states may affect animal behaviour more strongly than was previously thought.