Dogs can smell human stress

Dogs can smell human stress — and it alters their own behavior, reveals new research.

Our four-legged friends experience “emotional contagion” from the smell of human stress — leading them to make more ‘pessimistic’ choices, say scientists.

The University of Bristol-led study is the first to test how human stress odors affect dogs’ learning and emotional states.  

Previous evidence in humans suggests that the smell of a stressed person subconsciously affects the emotions and choices made by others near them. 

Bristol Veterinary School researchers wanted to know whether dogs also experience changes in their learning and emotional state in response to human stress or relaxation odours.

The team used a test of ‘optimism’ or ‘pessimism’ in animals, which is based on findings that ‘optimistic’ or ‘pessimistic’ choices by people indicate positive or negative emotions, respectively.

The researchers recruited 18 dog-owner partnerships to take part in a series of trials with different human smells present. 

The participating dogs ranged from eight months to 10-years-old and consisted of two Springer spaniels; two Cocker spaniels; two Labrador Retrievers; two Braque d’Auvergne; a Whippet; a Golden Retriever; a Miniature Poodle and seven mixed breed dogs. 

During the trials, the dogs were trained that when a food bowl was placed in one location, it contained a treat, but when placed in another location, it was empty.  

Once a dog learned the difference between the bowl locations, they were faster to approach the location with a treat than the empty location.  

The researchers then tested how quickly the dog would approach new, ambiguous bowl locations positioned between the original two.

They found that a quick approach reflected ‘optimism’ about food being present in the ambiguous locations – a marker of a positive emotional state – whilst a slow approach indicated ‘pessimism’ and negative emotion. 

Trials were repeated whilst each dog was exposed to either no odour or the odours of sweat and breath samples from humans in either a stressed — arithmetic test — or relaxed — listening to soundscapes – state.

Researchers found that the stress smell made dogs slower to approach the ambiguous bowl location nearest the trained location of the empty bowl, an effect that was not seen with the relaxed smell.  

The findings, published in the journal Scientific Reports, suggest that the stress smell may have increased the dogs’ expectations that the new location contained no food, similar to the nearby empty bowl location.

The researchers suggest that the ‘pessimistic’ response reflects a negative emotional state – and could possibly be a way for the dog to conserve energy and avoid disappointment.

The team also found that dogs continued to improve their learning about the presence or absence of food in the two trained bowl locations and that they improved faster when the stress smell was present.

Study lead author Dr Nicola Rooney said: “Understanding how human stress affects dogs’ well-being is an important consideration for dogs in kennels and when training companion dogs and dogs for working roles such as assistance dogs.

“Dog owners know how attuned their pets are to their emotions, but here we show that even the odour of a stressed, unfamiliar human affects a dog’s emotional state, perception of rewards, and ability to learn.”

Dr Rooney, Senior Lecturer in Wildlife and Conservation at Bristol Veterinary School, added: “Working dog handlers often describe stress traveling down the lead, but we’ve also shown it can also travel through the air.”