Whether animals can sense fear in humans or not has long intrigued scientists. Since directly asking them is not feasible, scientists have been conducting experiments to find out the answer. So, what do these scientific studies say?
Can Animals Truly Smell Human Fear? Insights from Scent-Based Research on Horses and Dogs
Olfactory Systems: The Dual Mechanisms of Smell
Equine scientist Nancy Diehl from Penn State said that there are two distinct olfactory systems. The primary olfactory system is responsible for consciously recognizing smells, such as "grandma's apple pie," and relies on the nose's ability to detect volatile molecules in the air.
For example, when you smell brewing coffee, you're inhaling coffee molecules that bind to olfactory receptors in your nasal passages. Emotional states like fear are typically not associated with the production of scented molecules.
The second olfactory process is known as the accessory olfactory system, which begins its communication in the vomeronasal organ located above the soft palate in the nasal cavity. This organ detects highly specific smell molecules, transmitting them to the accessory olfactory bulb for collection and processing.
Nerves from both the accessory and main olfactory bulbs project to the limbic system, the brain region responsible for emotional perception and response.
Unlike the main olfactory system, the accessory olfactory system doesn't detect airborne molecules. Instead, it "reads" messages from non-volatile pheromones-communicative chemicals emitted by all animals. Research indicates that pheromone molecules convey information about territory, aggression, and prominently, reproduction. Does this mean animals can detect fear in humans?
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Horses and Dogs Demonstrate Emotional Sensitivity to Human Odors
Researchers aimed to investigate if animals, particularly horses, could discern human emotions by observing their reactions to scents associated with joy and fear.
In a 2023 study published in Scientific Reports, participants watched comedy and horror videos and sweat samples were collected for horses to assess their responses. Horses displayed distinct behaviors, engaging only their left nostrils for joy-related odors and both nostrils for fear-related scents, indicating an ability to differentiate emotional states in humans.
Lead author Plotine Jardat noted the initial uncertainty but highlighted horses' perceptiveness, particularly in positively perceiving joy-inducing scents through left-nostril engagement.
The study didn't suggest horses comprehend fear as humans do but revealed their capability to differentiate odors linked to various human emotions. This prompted further exploration into specific compounds in human sweat influencing horse behavior. Chemosignals, chemicals affecting other species' behavior, were proposed as potential factors.
Compounds like adrenaline or androstadienone in human sweat were hypothesized to trigger odor shifts during fear-inducing moments, potentially conveying "emotional information" across species. Future studies aim to investigate if the scent of fear induces fearful reactions in horses and assess its emotional impact through tests.
Meanwhile, the 2018 study on Labrador retrievers in the journal Animal Cognition supports the findings. Dogs sniffed samples from participants after watching scary or happy videos, reacting differently based on the scent. Dogs increased interactions with strangers after smelling a happy person's odor but showed avoidance behaviors, such as going to their owner or attempting to leave the room, when exposed to the scent of fear.
This study, like the horse research, pointed to chemosignals, suggesting "interspecies emotional communication" at play between humans and dogs.
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