The Smell of Human Stress Leads Dogs to Make More Pessimistic Decisions, Study Suggests

Dogs are renowned for their incredible sense of smell. With noses up to 10,000 times more sensitive than our own, our canine companions can sniff out bombs, track down missing people and sense illness or stress in humans. Now, in a first-of-its-kind study, researchers have discovered that smelling stressed-out humans may influence how a dog behaves.

“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,” Zoe Parr-Cortes, a veterinarian at the University of Bristol, tells Scientific American’s Sara Novak. “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.”

In the new study, Parr-Cortes and her colleagues in the United Kingdom found that dogs were more likely to approach a new situation with pessimism after sniffing a sweaty cloth from an anxious human. They published their findings this week in the journal Scientific Reports.

To collect their data, the researchers stressed out 11 human volunteers by asking them to perform math and public speaking tasks on the spot. During another session, the participants relaxed by watching 20-minute video compilations of forest and seaside scenes in a calming environment. In both scenarios, researchers affixed cloths under the armpits of participants to collect sweat and took saliva and breath samples. The team also measured participants’ heart rates and cortisol levels and administered pre- and post-test anxiety questionnaires.

Separately, the researchers trained 18 dogs to recognize that a bowl in one location of a room always contained food, while a bowl in another part of the room did not. Then, they introduced new bowls in intermediate spots to see how the dogs would respond.

“Do they approach quickly, optimistic that there’s going to be a food reward in there, or do they approach more slowly, with a more pessimistic outlook that there might not be food in that bowl?” Parr-Cortes, the lead author of the paper, says to NPR’s Rachel Treisman.

The dogs were then exposed to the sweat cloths of stressed humans for 20 seconds before being released into the room. Researchers recorded how likely the dogs were to approach the new, ambiguous bowls. Next, the dogs repeated the test after sniffing a sweat cloth from the same, relaxed human. The experiment was also performed without exposing the dog to any specific smells, as a control. Researchers conducted the tests several times, and all dogs were unfamiliar with the humans they were smelling.

After sniffing cloths from anxious humans, the canines were slower to approach new bowls placed close to the known position of the empty bowl, suggesting the canines were less likely to believe they contained a treat. The relaxed human scent didn’t seem to have any effect on the animals’ behavior.

Yet, stress has also been shown to reduce hunger, which may be another explanation for the dogs’ behavioral changes, Katherine A. Houpt, a professor emeritus of behavioral medicine at the Cornell University College of Veterinary Medicine who was not involved with the study, tells Scientific American.

“It might not be that it’s changing their decision-making but more that it’s changing their motivation for food,” she tells the publication. “It makes sense, because when you’re super stressed, you’re not quite as interested in that candy bar.”

Still, the research shows dogs are empathetic and highly sensitive to even strangers’ emotions, scientists say.

“Importantly, it highlights how in-tune dogs are at picking up on mood,” Parr-Cortes tells NPR. “So, keeping your relationship with your dog … based on positive reinforcement and happy, fun engagement is the best way to have a good relationship and a happy dog.”

Margaret Osborne | | READ MORE

Margaret Osborne is a freelance journalist based in the southwestern U.S. Her work has appeared in the Sag Harbor Express and has aired on WSHU Public Radio.