Dogs can smell their humans' stress, and it makes them sad

Humans and dogs have been close companions for perhaps 30,000 years, according to anthropological and DNA evidence. So it would make sense that dogs would be uniquely qualified to interpret human emotion. They have evolved to read verbal and visual cues from their owners, and previous research has shown that with their acute sense of smell, they can even detect the odor of stress in human sweat. Now researchers have found that not only can dogs smell stress—in this case represented by higher levels of the hormone cortisol—they also react to it emotionally.

For the new study, published Monday in Scientific Reports, scientists at the University of Bristol in England recruited 18 dogs of varying breeds, along with their owners. Eleven volunteers who were unfamiliar to the dogs were put through a stress test involving public speaking and arithmetic while samples of their underarm sweat were gathered on pieces of cloth. Next, the human participants underwent a relaxation exercise that included watching a nature video on a beanbag chair under dim lighting, after which new sweat samples were taken. Sweat samples from three of these volunteers were used in the study.

Participating canines were put into three groups and smelled sweat samples from one of the three volunteers. Prior to doing so, the dogs were trained to know that a food bowl at one location contained a treat and that a bowl at another location did not. During testing, bowls that did not contain a treat were sometimes placed in one of three "ambiguous" locations. In one testing session, when the dogs smelled the sample from a stressed volunteer, compared with the scent of a cloth without a sample, they were less likely to approach the bowl in one of the ambiguous locations, suggesting that they thought this bowl did not contain a treat. Previous research has shown that an expectation of a negative outcome reflects a down mood in dogs.

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The results imply that when dogs are around stressed individuals, they're more pessimistic about uncertain situations, whereas proximity to people with the relaxed odor does not have this effect, says Zoe Parr-Cortes, lead study author and a Ph.D. student at Bristol Veterinary School at the University of Bristol. "For thousands of years, dogs have learned to live with us, and a lot of their evolution has been alongside us. Both humans and dogs are social animals, and there's an emotional contagion between us," she says. "Being able to sense stress from another member of the pack was likely beneficial because it alerted them of a threat that another member of the group had already detected."

The fact that the odor came from an individual who was unfamiliar to the dogs speaks to the importance of smell for the animals and to the way it affects emotions in such practical situations, says Katherine A. Houpt, a professor emeritus of behavioral medicine at Cornell University's College of Veterinary Medicine. Houpt, who was not involved in the new study, suggests that the smell of stress may have reduced the dogs' hunger because it's known to impact appetite. "It might not be that it's changing their decision-making but more that it's changing their motivation for food," she says. "It makes sense because when you're super stressed, you're not quite as interested in that candy bar."

This research, Houpt adds, shows that dogs have empathy based on smell in addition to visual and verbal cues. And when you're stressed, that could translate into behaviors that your dog doesn't normally display, she says. What's more, it leaves us to wonder how stress impacts the animals under the more intense weight of an anxious owner. "If the dogs are responding to more mild stress like this, I'd be interested to see how they responded to something more serious like an impending tornado, losing your job or failing a test," Houpt says. "One would expect the dog to be even more attuned to an actual threat."