Research shows that dogs and horses respond differently to odor samples collected from fearful and happy people.
In the study, horses' behavior changed depending on the scent humans released in times of fear or joy. Image: iStock
Can animals smell fear? To find the root answer, the researchers removed human presence from the experiment, because animals like dogs respond to human expressions and body postures. Instead, they focused on how animals, including horses and dogs, responded to the different odors that the human body released when watching funny and scary videos.
In a study with horses, published in the journal Scientific Reports In 2023, a team of experts let volunteers watch comedy videos one day and horror movies the next. After each session, the research team used cotton pads to collect sweat samples from the volunteers' armpits and asked them to indicate how happy or scared they felt while watching each video.
Next, the team of experts gave two cotton samples from the same volunteer to the horses to see if they could distinguish between the odors released during times of happiness and fear. As a result, they react differently depending on the cotton pad provided.
“When sniffing the happy sample, the horses only used their left nostril. This reveals which part of the brain they are using to analyze smells. In all mammals, the two hemispheres of the brain have different functions, and in terms of emotions, it seems that odors from happy patterns are perceived by horses as positive,” said Plotine Jardat, PhD student at the University Tours, lead author of the study, said.
In samples collected after volunteers watched horror movies, horses responded very differently. Not only did they smell the sample longer, but they also used both nostrils. However, Jardat argues that this does not necessarily mean horses understand fear. “When horses sniff another animal, the word 'fear' doesn't come to mind. But we know that horses can distinguish smells from different human emotional states,” she explains.
So what substance in human sweat causes horses to change their behavior? The team proposes that chemical signals – chemicals that animals secrete and influence the behavior of other animals – may be responsible. In humans, certain compounds in sweat such as adrenaline or androstadienone (a pheromone-like protein) can cause a change in odor when afraid. These compounds can also transmit “emotional information” from one species to another.
Dogs have different reactions to the scent of fearful and happy people. Image: momtastic
In a study in the journal Animal cognition In 2018, scientists had Labrador dogs sniff samples taken from the armpits of male volunteers after they watched horror or happy videos. The team of experts put the sample in an open box and placed the box in a closed room with two people: the dog owner and a stranger.
Similar to the horse study, they found that dogs reacted differently depending on whether they smelled a scared or happy person. "When they smell a happy person, they will increase their interactions with strangers in the room," said Biagio D'Aniello, the study's lead author and professor of zoology at the University of Naples Federico II.
But when sniffing samples from frightened people, the dogs reacted completely differently. "When they smell fear, they will go to their owner or go towards the door and try to escape the room," said Anna Scandurra, a postdoctoral fellow at the University of Naples Federico II and co-author of the study.
In the end, the scientists concluded similarly to the horse research team: The dogs' reactions were likely due to chemical signals, suggesting that "interspecies emotional communication" was taking place.
Thu Thao (Follow Live Science)
