Human Stress Odour Can Prompt Pessimistic Decisions in Dogs

Dogs can detect human stress, and new research indicates that its scent can produce a comparable emotional response in dogs, leading them towards ‘pessimistic’ choices.

According to the UK research team, this is the first scientific evidence that odours produced by stressed people can affect dogs’ emotions and learning. The findings may also offer important insight into the longstanding relationship between humans and dogs.

Although anyone living with a dog may not be surprised that dogs can pick up on human moods, the research indicates that this sensitivity could be more powerful than many realise.

How dogs respond to human stress odour

"Owners know how attuned their pets are to their emotions, but here we show that even the odor of a stressed, unfamiliar human affects a dog's emotional state, perception of rewards, and ability to learn," says senior author Nicola Rooney, a human-animal interactions researcher at the University of Bristol.

Earlier human studies have found that people can pick up clues about others’ emotions by unconsciously sensing chemosignals in sweat. These concealed signals can, in turn, subtly shape our own feelings and decisions.

Recent research has also shown that dogs can detect these signals from humans. Rooney and her colleagues, however, wanted to establish how the odours associated with our stress affect dogs.

Because dogs are adept at interpreting both human speech and non-verbal behaviour, the researchers chose not to place them directly with stressed people. Instead, the dogs encountered breath and sweat samples taken from three unfamiliar human volunteers while they were either relaxed or undertaking a stressful task.

To create a relaxed state, volunteers watched a nature film. For the stress condition, they received frustrating instructions involving maths and public speaking.

Testing dogs’ optimistic and pessimistic decisions

The researchers enlisted 18 dog-human pairs for trials using the human odour samples. During training, each dog learned that a food bowl in one position always held a treat, whereas a bowl in a second position was always empty.

Once they had learned this, the dogs approached a bowl more rapidly when it appeared at the positive P location, associated with treats, than when it was placed at the negative N location, where no food was provided.

The team describes this enthusiastic rushing towards the bowl as a sign of “optimism”. Previous research has linked positive and negative emotions in people with “optimistic” and “pessimistic” decisions respectively, making this behaviour an indicator of an animal’s emotional state.

After the initial training, bowls were placed in unfamiliar locations between the two established positions, allowing the researchers to assess how willing the dogs were to approach them.

Three additional locations were used, defined by how close they were to the original sites: near-positive NP, middle M, and near-negative NN.

The experiments were repeated while the dogs were exposed either to odour samples from stressed people, samples from relaxed people, or no odour.

Stress odour and dogs’ expectations of food

When they could smell a stressed stranger, dogs were significantly less likely to approach a bowl in the near-negative position than when they detected the scent of a relaxed stranger or a blank cloth.

The stress odour was less discouraging when bowls were placed in the middle or near-positive locations. Yet when the bowl was near the area previously associated with no food, the scent appeared sufficient to reduce the dogs’ expectation of a reward.

Dogs were not apparently put off by the same near-negative bowl position to the same extent when no stress odour was present. This implies that, when judging the chance of finding food, dogs considered the surrounding odours alongside the bowl’s location.

"Working dog handlers often describe stress traveling down the lead, but we've shown it can also travel through the air," Rooney says.

The more subdued reaction of dogs exposed to human stress odour meets the definition of pessimism and points to a negative emotional state. This response could be adaptive, potentially enabling dogs to conserve resources or avoid frustration.

Much remains unclear about this process, and further research will be required to determine precisely how human odours influence dogs’ feelings and learning.

Given dogs’ global importance to people as working partners, companions and friends, it would be sensible to pursue evidence that could strengthen this relationship.

"Understanding how human stress affects dogs' well-being is an important consideration for dogs in kennels," says Rooney, "and when training companion dogs and dogs for working roles such as assistance dogs."

The study was published in Scientific Reports.