Human emotional chemical cues collected in situations of disgust or sadness modulate horses’ behaviour and physiology in a multimodal context
This study demonstrates that horses can discriminate between human chemical cues of sadness and disgust, exhibiting distinct physiological and behavioral responses—such as calmness to sadness and heightened arousal to disgust—that align with the specific arousal levels of these emotions when presented alongside matching visual and auditory cues.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Animals have long been known to read human faces and voices, but a quieter form of communication has recently come into focus: the invisible chemical signals we release when we feel strong emotions. When a person is afraid, their body produces a specific scent, a chemical signature that dogs and horses have been shown to detect, often triggering a state of alert or avoidance in the animal. For years, scientists assumed this ability was limited to fear, or perhaps a simple contrast between fear and happiness. However, emotions are complex experiences defined by two main qualities: whether they feel good or bad, and how much energy they require from the body. Fear is a high-energy, negative feeling, but sadness is a low-energy, negative feeling, while disgust is another high-energy, negative feeling. It remained a mystery whether animals could distinguish between these different negative states just by smell, or if they simply reacted to any sign of human distress.
A team of researchers set out to solve this puzzle by testing whether horses could tell the difference between the scent of a human feeling sad and the scent of a human feeling disgusted. They worked with twenty-eight adult mares, bringing them one by one into a quiet stall where they could watch a screen. The setup was designed to be a complete sensory experience. While the horses watched a video of a human actor displaying a specific emotion—either a sad face or a disgusted face—they were also exposed to a scent. This scent came from cotton pads that had been worn by volunteers who had just watched films designed to make them feel either deep sadness or intense disgust. To ensure the horses were reacting to the smell and not just the video, the researchers also ran a control test where the horses saw the same sad or disgusted faces but smelled only a neutral, odorless pad.
The results revealed that the horses were not just reacting to the negative nature of the emotions; they were responding to the specific type of emotion and its energy level. When the horses watched a video of a sad human while smelling the scent of sadness, they became noticeably calmer. They spent more time with their heads lowered, a posture that indicates relaxation, and they moved around less. Their heart rates also dropped more significantly compared to when they smelled a neutral scent. This suggests that the scent of sadness acted as a soothing signal, reinforcing the calming effect of the sad face they were watching.
In stark contrast, the reaction to the scent of disgust was one of heightened alert and agitation. When the horses watched a video of a disgusted human while smelling the scent of disgust, they moved around significantly more and their heart rates rose. They appeared more tense and less relaxed than when they smelled a neutral odor. This reaction aligns with the nature of disgust, which is a high-energy emotion that signals something is wrong or potentially harmful. The horses seemed to sense this danger and responded by becoming more active and vigilant.
These findings suggest that horses possess a sophisticated ability to read human emotional states through smell, distinguishing not just between good and bad feelings, but between different kinds of negative feelings based on their intensity. The study indicates that horses do not simply react to human distress as a single, uniform event. Instead, they appear to extract specific information from our chemical cues, adjusting their own behavior to match the emotional energy they detect. Whether a human is feeling the heavy, low-energy weight of sadness or the sharp, high-energy spike of disgust, the horse seems to know the difference, responding with a calm stillness to one and an alert restlessness to the other. This adds a new layer to our understanding of the bond between humans and horses, showing that their connection extends beyond what they can see and hear, reaching into the invisible world of scent.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.