The ability of dogs to sniff out stress levels in humans has been known for some time. But the relationship between woman and man’s four-legged best friend is two-way, and scientists now believe they have discovered that when a dog senses a person in distress, it has a knock-on effect in the behaviour of the animal – most notably, it makes them pessimistic. New research is believed to be the first to test how dog owners’ emotional states can affect their pets. Dr Nicola Rooney, senior lecturer at the University of Bristol’s veterinary school and lead author of a paper on the subject, said: “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 also travel through the air.” She added that understanding how human stress affects dogs’ wellbeing was important for dogs in kennels and training companion, working and assistance dogs. The team used a test of “optimism” or “pessimism” in animals to determine whether they were feeling positive or negative emotions after exposure to people’s stressed and relaxed odours. The researchers recruited 18 dog-owner partnerships to take part in trials in which 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 dogs learned the difference between these bowl locations, they were faster to approach the location with a treat than the empty location. Researchers then tested how quickly the dog would approach new, ambiguous bowl locations positioned between the original two. A quick approach reflected optimism about food being present in these ambiguous locations – a marker of a positive emotional state – while a slow approach indicated pessimism and negative emotion. These trials were repeated while each dog was exposed to either no odour or the odours of sweat and breath samples from humans who were feeling stressed from a maths test or relaxed after listening to soundscapes. Researchers discovered that the stress smell made dogs slower to approach the ambiguous bowl location nearest where they had been trained to expect empty bowls. This effect that was not replicated after exposure to the relaxed smell. These findings suggest that the stress smell may have increased the dogs’ expectations that this new location contained no food, similar to the nearby empty bowl location. The researchers suggested this could 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 learned the difference faster when the stress smell was present.