There has been a long-standing debate about whether animals can sense fear in humans through their sense of smell. However, answering this question is not as simple as just asking a pet and expecting an answer.
Researchers have attempted to answer this question by conducting studies where they remove human presence from the equation. This is because animals, such as dogs, tend to respond to human expressions and body language. Instead, researchers have focused on how animals, such as horses and dogs, react to different smells given off by humans watching happy vs. fear-inducing videos.
In a study with horses published in the journal Scientific Reports in 2023, participants were asked to watch clips from a comedy movie one day and a horror movie the next. After each clip, the researchers collected sweat samples from the viewers’ armpits using cotton pads and asked the participants to report their level of joy or fear while watching each clip. The researchers then presented the two swab samples, one from each clip, to a horse to see if it could differentiate between the odors produced by humans during times of happiness and distress.
“At first we weren’t sure if the horses could differentiate between the odors,” lead study author Plotine Jardat, a doctoral student at the University of Tours in France, told Live Science.
The horses exhibited different reactions depending on the cotton pad that was presented to them.
“When the horses smelled the joy samples, they used only their left nostrils,” Jardat said. “That indicates which part of the brain they are using to analyze the odor. In all mammals, the two brain hemispheres have different functions, and in an emotional context, it seems like the odor from the joy samples were perceived as positive by the horses.”
But when the horses were given the samples swiped during the horror film, the animals reacted much differently and not only sniffed the sample longer but also “used both nostrils” to catch a whiff, Jardat said.
Jardat was quick to point out that horses may not know what fear is when they smell another animal. It’s not like the word “fear” crosses their minds. However, it has been discovered that horses are capable of distinguishing odors from different emotional states in humans.
This discovery raises a question: What specific compounds in human sweat cause a shift in the behavior of horses? Researchers suggest that chemosignals, which are chemicals produced by animals that can affect the behavior of another animal, may be responsible for the horses’ reactions. In humans, sweat contains several compounds, such as adrenaline or androstadienone (a pheromone-like protein), that may cause a change in odor during moments of fear. According to the study, these compounds could also be carrying “emotional information” from one species to another.
In a future study, scientists plan to examine whether smelling fear could cause a fearful reaction in horses, and what kind of emotional impact it could have on the animals. They will have the horses perform a range of tests after smelling the samples to determine the extent of the impact.
“We want to see if fear will modify their reactions to the tests,” Jardat said.
In a study published in the journal Animal Cognition in 2018, scientists conducted an experiment with Labrador retrievers. They were asked to sniff samples taken from the underarms of male participants after watching either a scary or happy video clip. The samples were placed in a box with an opening, and the box was then placed inside a closed room with two people: a stranger and the dog’s owner.
The study found that, similar to the horse study, the dogs reacted differently based on whether they smelled the scent of a fearful or a happy human.
“When the dogs smelled the odor of a happy person, they increased their interactions with the stranger in the room,” study lead author Biagio D’Aniello, a professor of zoology at the University of Naples Federico II in Italy, told Live Science.
When the dogs smelled a sample from a fearful person, their reaction was completely different.
“When they smelled fear, they would either go to their owner or they would go to the door and try to leave the room,” study co-author Anna Scandurra, a postdoctoral researcher at the University of Naples Federico II, told Live Science.
The researchers came to a similar conclusion as the scientists in the horse study: The dogs’ reactions were likely due to chemosignals, suggesting that “interspecies emotional communication” was at play, the authors wrote in their study.