Dogs can sense human stress through smell, leading them to make more pessimistic choices, according to groundbreaking research from the University of Bristol. Published in Scientific Reports, the study is the first to explore how human stress odours influence dogs’ learning and emotional state.
Previous research suggests that the scent of a stressed person subconsciously affects the emotions and decisions of those around them. To investigate whether dogs also respond to human stress odours, researchers at Bristol Veterinary School conducted a series of trials with 18 dog-owner pairs.
The study deployed a test of ‘optimism’ or ‘pessimism’ in animals. In this context, ‘optimistic’ or ‘pessimistic’ choices reflect positive or negative emotions, respectively.
During the trials, dogs were trained to recognise that a food bowl placed in one location contained a treat, while another location had an empty bowl. Once the dogs learned the difference, they were quicker to approach the treat location. Researchers then introduced new, ambiguous bowl locations positioned between the original two to observe the dogs’ reactions.
A fast approach to these new locations indicated ‘optimism’ – a sign of a positive emotional state – while a slow approach suggested ‘pessimism’ and negative emotions. These trials were repeated with dogs exposed to different human smells: no odour, the odour of sweat and breath samples from stressed individuals (after an arithmetic test), and relaxed individuals (after listening to soundscapes).
The results revealed that the smell of human stress made dogs slower to approach the ambiguous bowl nearest to the empty bowl’s location. This effect was not observed with the relaxed smell, suggesting that the stress smell increased dogs’ expectations of finding no food, similar to the empty bowl.
This ‘pessimistic’ response likely reflects a negative emotional state, potentially serving as a strategy for dogs to conserve energy and avoid disappointment. Interestingly, dogs improved their learning about the presence or absence of food in the two original bowl locations more quickly when exposed to the stress smell.
Dr Nicola Rooney, Senior Lecturer in Wildlife and Conservation at Bristol Veterinary School and lead author of the paper, highlighted the significance of these findings: “Understanding how human stress affects dogs’ wellbeing is crucial for dogs in kennels, training companion dogs, and dogs in working roles like 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 also shown it can travel through the air,” she added.
Dr Zoe Parr-Cortes, PhD student at Bristol Veterinary School and primary researcher on the project, expressed her gratitude to everyone involved in the study, particularly the participants and dog owners.
The paper titled ‘The odour of an unfamiliar stressed or relaxed person affects dogs’ responses to a cognitive bias test’ by Z. Parr-Cortes, N. J. Rooney et al., is available in Scientific Reports: https://www.nature.com/articles/s41598-024-66147-1.
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