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Scientists already know that dogs can smell human stress. But a team from the National Research Institute for Agriculture, Food and the Environment (INRAE) in France wanted to see if the same is true for horses, another domesticated animal that has lived alongside us for thousands of years.
The scent of fear
Researchers used cotton pads to collect sweat samples from 30 human volunteers while they were watching a horror movie called Sinister. They also collected samples from the same group later, while they were watching uplifting or funny clips and in a more joyful, relaxed emotional state. This allowed the team to compare how the horses reacted to two distinct emotions.
The team placed the pads into a special Lycra muzzle fitted to 43 female Welsh horses. These horses were randomly assigned to one of three groups. The fear group was only exposed to sweat collected from the horror movie viewers, while the joy group only smelled sweat from people watching the uplifting or funny clips. The control group wore muzzles with clean pads that had no human scent.
The horses then underwent four behavioral tests while the study authors monitored their reactions. These were being groomed, reacting to an umbrella suddenly opening, being approached by a human and exploring a new object.
When smelling the fear sweat, the horses were jumpier, stared at strange objects longer, had higher heart rate spikes and were less likely to touch or approach a human. These horses could not see that participants were fearful, so there were no facial expressions or body language clues to pick up on. They were most likely only reacting to chemical triggers in the sweat, although the scientists did not analyze the chemical composition of the samples.
"These findings highlight the significance of chemosignals in interspecific interactions and provide insights into questions about the impact of domestication on emotional communication," commented the research team in their paper.
Improved animal welfare
This discovery could have major implications for animal welfare, as it suggests that a handler's emotional state can be communicated to a horse through scent, even when the handler tries to hide their feelings. This could affect how we approach horse training and their overall welfare.
"Practical implications include acknowledging the importance of handlers' emotional state and its potential transmission through chemosignals during human-horse interactions," added the researchers.
Publication details
Plotine Jardat et al, Human emotional odours influence horses' behaviour and physiology, PLOS One (2026). DOI: 10.1371/journal.pone.0337948
Anyone who is around mammals for some time realizes they can detect the mental state of their counterpart. However they do it.