Smell of human stress affects dogs’ emotions, study finds

The smell of human stress affects dogs’ emotions and could lead to them making more pessimistic choices, new research has suggested.

University of Bristol-led researchers suggest the findings are important for dogs in kennels and when the animals are being trained for roles such as assistance dogs.

A pessimistic response reflects a negative emotional state and could possibly be a way for the dog to conserve energy and avoid disappointment, the researchers suggest.

The study found that the smell of stress released through human sweat and breath can change the behaviour of man’s best friend.

Dr Nicola Rooney, senior lecturer in wildlife and conservation at Bristol Veterinary School and the paper’s lead author, said: “Understanding how human stress affects dogs’ wellbeing 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.

“Working dog handlers often describe stress travelling down the lead, but we’ve shown it can also travel through the air.”

The researchers 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.

In the study, 18 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, it was quicker to approach the location with the treat.

Researchers then tested how quickly the dog would approach a third bowl, in a new location positioned between the original two.

A quick approach reflected optimism about food being present in these new locations – a positive emotional state – whilst a slow approach indicated pessimism and negative emotion, the researchers say.

These trials were repeated, and each dog was exposed to either no odour or the odours of sweat and breath samples from humans in either a stressed, or relaxed state.

According to the findings, the stress smell made dogs slower to approach the new bowl location nearest the trained location of the empty bowl.

This was not the case after the animals had been exposed to the relaxed smell.

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

This pessimistic response reflects a negative emotional state and could possibly be a way for the dog to conserve energy and avoid disappointment, the researchers suggest.

The scientists also found that dogs continued to improve their learning about the presence or absence of food, and that they improved faster when the stress smell was present.

The 18 dogs comprised ranged from eight months to ten-years-old, and there were two springer spaniels, two cocker spaniels, two labrador retrievers, two Braque d’Auvergne, one whippet, one golden retriever, one miniature poodle and seven mixed breed dogs.