Dogs can smell stress in people and it changes their own behavior, a new study has found.
Our four-legged friends experience “emotional contagion” from the smell of human stress, leading them to make more “pessimistic” choices, scientists say.
The study, led by the University of Bristol, is the first to examine how human stress odours affect dogs’ learning and emotional state.
Previous human studies have suggested that the smell of a stressed person can subconsciously influence the emotions and choices of those nearby.
Researchers from Bristol Veterinary School wanted to find out whether dogs also experience changes in their learning or emotional state in response to the smells of human stress or relaxation.
The researchers used tests of “optimism” or “pessimism” in animals, based on research showing that “optimistic” or “pessimistic” choices in humans indicate positive or negative emotions, respectively.
The researchers recruited 18 dog-owner pairs to take part in a series of experiments in which they exposed the dogs to a variety of human scents.
The dogs involved ranged in age from 8 months to 10 years old and included two springer spaniels, two cocker spaniels, two Labrador retrievers, two black d’Auvergne, one whippet, one golden retriever, one miniature poodle and seven mixed-breed dogs.
During the experiment, the dogs were trained so that a food bowl would contain treats when placed in one location, but would be empty when placed in another location.
Once the dogs learned the different bowl locations, they were quicker to approach the area with the treats than the empty area.
The researchers then tested how quickly the dogs approached a new, ambiguous location halfway between the original two bowls.
The researchers found that a quick approach reflected “optimism” about the presence of food in the ambiguous location — an indicator of a positive emotional state — while a slow approach indicated “pessimism” and negative emotions.
Each dog was repeatedly exposed to either no odor or human sweat and breath samples while in a stressed (arithmetic test) or relaxed (soundscape) state.
The researchers found that smelling the stressful scent slowed dogs down in approaching the location of the ambiguous bowl closest to the trained location of the empty bowl, an effect that was not seen when smelling the relaxing scent.
The study, published in the journal Scientific Reports, suggests that the smell of stress may have heightened the dogs’ expectations that the new location was free of food, as was the location of a nearby empty bowl.
The researchers suggest that the “pessimistic” response reflects a negative emotional state and may be a way for dogs to conserve energy and avoid disappointment.
The researchers also found that the dogs continually improved their ability to learn about the presence or absence of food at the two trained bowl locations, and improved more quickly when the stress odor was present.
Dr Nicola Rooney, lead author of the study, said: “Understanding how human stress affects canine health is an important consideration when training dogs in kennels or for working roles such as companion dogs or service dogs.”
“Dog owners know how sensitive their pets are to their emotions, but here we show that even the scent of a stressed stranger affects a dog’s emotional state, reward perception and ability to learn.”
Dr Rooney, senior lecturer in wildlife and conservation at Bristol Veterinary School, added: “Working dog owners often say that stress can be transmitted through the lead, but we have demonstrated that stress can also be transmitted through the air.”
