Human Stress Odours Can Make Dogs More Pessimistic, Study Finds

Dogs can detect human stress by smell, and new research suggests that this scent can produce a comparable emotional response in dogs, leading them to make ‘pessimistic’ choices.

Human stress odours and dogs’ emotions

According to the UK research team, this is the first scientific evidence that odours produced by human stress can affect dogs’ emotions and learning, potentially offering important insight into the long-standing relationship between humans and dogs.

For anyone who shares their home with a dog, their ability to pick up on human moods may not seem remarkable. However, the findings indicate that this sensitivity may be more powerful than many realise.

"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 detect signs of others’ emotions through smell, unconsciously picking up chemosignals in sweat. These concealed signals can, in turn, subtly influence our own feelings and decisions.

Other recent research has shown that dogs can also recognise these signals in humans, but Rooney and her colleagues wanted to investigate what effect stress odours have on dogs.

Testing dogs’ responses to scent samples

Because dogs are highly capable of interpreting human verbal and non-verbal signals, the researchers chose not to place them directly with stressed people.

Instead, the dogs encountered sweat and breath samples taken from three unfamiliar human volunteers while they were either relaxed or undertaking a stressful task.

To induce relaxation, volunteers watched a nature video. The stress task, meanwhile, involved frustrating directions connected with maths and public speaking.

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

Dogs that had learned this pattern approached a bowl more rapidly when it appeared in the positive P location, linked with treats, than when it was placed in the negative N location, where no treats were available.

The researchers explain that this keen rushing towards the bowl represents “optimism”: a behavioural indication of an animal’s emotional state. Previous research has associated positive and negative human emotions with “optimistic” and “pessimistic” decisions, respectively.

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

Three additional positions were used, defined by their distance from the original sites: near-positive NP, middle M, and near-negative NN.

The experiments were repeated while the dogs were exposed to samples from either stressed or relaxed humans, as well as with no odour present.

Dogs made more pessimistic decisions around stress odours

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

The stress odour was less off-putting when the bowl was positioned in the middle or near-positive location. Yet when it accompanied a placement close to the area where food was absent, the scent appeared sufficient to reduce the dogs’ expectation of a reward.

Dogs also seemed less discouraged by the same near-negative bowl position when they were not exposed to the stress odour. This indicates that, when judging the chance of finding food, they were weighing up surrounding scents 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 restrained behaviour of dogs exposed to human stress odour meets the definition of pessimism and suggests a negative emotional state. Such a response could be adaptive, perhaps enabling dogs to preserve resources or prevent frustration.

This process remains largely unclear, however, and further studies will be required to establish precisely how human odours influence dogs’ feelings and learning.

Given dogs’ widespread importance to people around the world-as co-workers, partners 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.