In recent years, a growing body of research has affirmed that dogs can smell when humans are stressed. A new study shows how it affects them.
As many dog owners can attest, our stress is contagious.
“Dogs can pick up on our stress, and we wondered what effect that had on the dogs,” said Zoe Parr-Cortes, lead author of the study, which was released late last month.
Parr-Cortes - a veterinarian and PhD student at Bristol Veterinary School in Langford, England - ran a series of trials with 18 dogs. She started by teaching the pups that a bowl placed in one location contained food, and when it was placed in another spot, it did not.
“You repeat that over and over until they know that one side is food and one side is never food,” said Parr-Cortes.
Study participant Freddie sitting next to the jar containing the odour sample while waiting for the bowl part of the test to begin. Photo / University of Bristol (Washington Post)
Once they know this, they run faster toward the bowl with the food rather than the one without. Then, Parr-Cortes measured how quickly each dog would approach a bowl placed somewhere between the two spots.
“Those are the locations where there’s no previous association with a reward,” Parr-Cortes explained. “You’re now asking them: How optimistic are you that there’s going to be a food reward in there?”
If a dog ran quickly toward the in-between bowl, it signaled to researchers that the dog was optimistic or in a positive emotional state. If the dog approached the bowl gingerly, it indicated pessimism.
“We first ran that without any [stress] odour, so we had a baseline measure about how optimistic they are about an unknown bowl,” said Parr-Cortes. “Then we did it again introducing the stress odour.”
To collect the stress odour, researchers used sweat and breath samples from humans after a stressful situation, such as a timed math test or public speech. They also collected odour samples after a relaxed situation, like listening to tranquil sounds or watching a peaceful video. Before each situation, participants attached two cotton cloths to their underarms using micropore tape. Participants also exhaled a full breath onto each piece of cloth before sealing them in separate specimen bags.
Researchers used samples from three volunteers - all of whom were strangers to the dogs in the study. They brought the samples to the dogs, allowing them to sniff them before conducting the trial again. Dogs’ sense of smell is at least 1000 times stronger than humans, so they could detect the scent very quickly.
“We were able to see how the odour affected how optimistic or pessimistic they were about receiving the treat in the unknown location,” said Parr-Cortes. “And what we found is that dogs were slow to approach the bowl that was uncertain when the stress smell was present.”
“It suggested more of the glass-half-empty mentality with that odour, and we didn’t see that effect with the relaxed smell,” she said, noting that the majority of the dogs moved slower toward the bowl in an in-between location after being exposed to the stress odour. “Similar tests are used in humans and other animals to measure emotional state, optimism and pessimism.”
Freddie approaching the bowl placed at one of the in-between locations to check if there is a treat inside. Photo / University of Bristol (Washington Post)
The findings were especially interesting, Parr-Cortes said, because the canines involved in the trial did not know the humans who emitted the stress smells.
“This wasn’t the smell of someone they knew that was stressed. It was someone they had never met before,” she said. “It implies there’s a common stress smell that people have.”
It also demonstrates that dogs are able to sense emotions even of people who are not their owners.
“It seems to indicate that they can detect the smell of stress in people generally, and they don’t have to have a prior association with that person being stressed,” said Parr-Cortes.
Other canine experts said they were intrigued by the study.
“I am very impressed by the work; both the originality of it, and the sheer amount of effort that went into it,” said Clive Wynne, the director of the Canine Science Collaboratory at Arizona State University. “I personally find it remarkable how easily dogs are affected by human emotions.”
Wynne said that while the study’s findings are interesting, further research is required to draw a more definitive conclusion.
“The science of understanding dog emotions is very much in its infancy,” he said. “I think it’s enormously important, because hundreds of millions of us live with hundreds of millions of dogs in close proximity…if we can improve and build on how people and dogs understand each other, that will help us.”
Emily Bray, an assistant professor of human-animal interaction in the College of Veterinary Medicine at the University of Arizona, said the results help to decode “all these pieces to the puzzle of communication”. Neither she nor Wynne were involved in the study.
“Canine cognition research in general is so important because the more we understand how they’re perceiving things, the more we can set them up for success.”
While the study focused on smell, “it would be interesting to see how other types of cues play in”, said Bray, pointing to body language and tone of voice.
Parr-Cortes acknowledged that the study used a small sample, and said she hopes to expand on it in future studies.
Still, she said that the study “drives home how important it is to be aware of your emotional state when you’re working with dogs”.
“When you’re stressed, don’t expect your dog to be unaffected by it,” she added. “Doing something relaxing before training your dog might reduce any stress.”
In future research, Parr-Cortes hopes to study how other human emotions - such as happiness - impact dogs’ behaviour, too. She suspects she will see a similar correlation.
“It’s amazing how in tune dogs are with our emotions, and how close we’ve become as species,” Parr-Cortes said.
