Horses can detect fear in humans by smell, becoming more likely to startle and more wary of people who are scared, a new study has found.

Researchers collected samples of odor compounds from the armpits of human study participants and then observed how the horses behaved when they were exposed to the different odors during standardized tests, according to research published Wednesday in the journal PLOS One.

While previous research has found that horses can pick up on human emotions through speech and facial expressions, the idea that they can smell our fear has remained just a theory due to the difficulties of studying smell, said lead study author Plotine Jardat, a researcher at the French Institute for Horse and Riding (IFCE).

“As humans, we are not really aware of all the smells that are around us compared to what other animals apparently perceive, so that’s not so easy to study,” Jardat told CNN.

To overcome this problem, researchers developed a novel method that involved placing cotton pads in the armpits of the human study participants, where odor compounds are released by the sweat glands.

Samples were taken from people while they watched a scary video and a joyful video, as well as a neutral sample, and these pads were later placed on the nostrils of 43 different female horses, held in place by small nets.

The researchers were careful to prevent contamination by other odor compounds by making sure the pads were only handled by the human providing the sample, and the compounds were preserved by freezing the pads, said Jardat.

The horses were then exposed to a series of tests conducted by experimenters familiar to the animals — for example, whether or not they would freely approach a human in their paddock, or startle at the sudden opening of an umbrella.

Researchers observed the horses’ behavior, as well as collecting data on their heart rate and the level of cortisol in their saliva, a key biomarker for stress.

Analysis showed that both the horses’ behavior and physiology were influenced by the human odor compounds.

Researchers observed that the horses exposed to odor compounds produced by volunteers who had watched scary footage were more likely to startle more easily, and less likely to approach people or investigate unfamiliar objects.

“The fearful odors from humans amplify the reactions of horses,” said Jardat.

“The significance is that horses can smell how we feel, even if they can’t hear or see us,” she said.

Study co-author Léa Lansade, research director at the French National Research Institute for Agriculture, Food and Environment (INRAE), said the study provides evidence of cross-species “emotional contagion.”

And there are practical implications for those who ride or handle horses, including “acknowledging the importance of handlers’ emotional state and its potential transmission through chemosignals during human-horse interactions,” according to the paper.

Although as humans we cannot control the emotional odors that we emit, Lansade said in a statement to CNN that horseback riders should “focus on relaxing, so you can ride calmly and without fear.”

Next, the researchers plan to study whether humans are sensitive to odor compounds produced by horses when they experience certain emotions, and research the chemical compounds involved.

They also plan to investigate whether human-to-horse chemical communication is specific to fear, or whether it also occurs with other emotions.

“We started with fear because this is an emotion that we really expect other animals to also experience, because what fear allows is to detect danger and to avoid it,” said Jardat, who added that researchers plan to investigate sadness and disgust.

“Even if they (horses) don’t feel these emotions precisely, they could smell differences from humans who feel these emotions, and it could mean something for them,” she added.

Gemma Pearson, a Royal College of Veterinary Surgeons (RCVS) specialist in Veterinary Behavioural Medicine (Equine) and a lecturer at the Royal (Dick) School of Veterinary Studies, Scotland, who was not involved in the study, praised the research.

“This is a nice study building on previous work from this group and of a typical standard of evidence for equine behavioural research,” she told CNN. “This group have been careful to control for confounding variables in this study that might result in an incorrect result.”

Pearson also underlined that horses use their sense of smell in conjunction with other sensory inputs.

“Horses are prey animals so it makes perfect sense that they would use any information in the environment to warn them of potential threats,” she said.

“It is important to remember that horses don’t rely on the ‘smell’ of fear from humans but in the real world will be using all their senses to make decisions on the level of threat,” Pearson added.

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