It’s long been widely believed that dogs can detect extreme emotions by smell. Now scientists at Queen's University Belfast in the U.K. have proven that a dog's nose knows.

Acute stress changes the compounds found in human sweat and breath, research has shown. For the new experiment, four dogs were presented with sweat and breath samples collected from human volunteers — before and after the people engaged in a difficult math exercise.

The canine participants were able to detect with a greater than 90 percent accuracy which samples came from before and which came from after the 36 human volunteers had spent three minutes trying to count backward, aloud, from 9,000 in units of 17, according to the report published Wednesday in the scientific journal PLOS One.

“This study provides further evidence of the extraordinary capabilities of ‘man’s best friend,’” said the study’s first author, animal psychologist Clara Wilson.

The dogs, including Minnie, were presented with samples from 36 people who gave sweat and breath samples before and after doing a fast-paced arithmetic task. (Courtesy Queen’s University Belfast)

The dogs, including Minnie, were presented with samples from 36 people who gave sweat and breath samples before and after doing a fast-paced arithmetic task. (Courtesy Queen’s University Belfast)

“While it is likely that in a real-life context dogs are picking up on our stress from a variety of context cues, we have shown using a laboratory study that there is a confirmed odor component that is likely contributing to dogs’ ability to sense when we are stressed,” Wilson said in an email.

For their study, Wilson and her colleagues first set out to train a variety of 20 pet dogs to point with their noses at samples from a person who was stressed. (By the end of the training period, 16 dogs had been withdrawn for a variety of reasons, including attention issues and boredom.)

The researchers tested the trained dogs with a machine that offered three choices: an unused piece of gauze, a sample from a stressed person and one from the same person when unstressed.

The researchers also collected before and after measurements of heart rate and blood pressure and responses to questionnaires that asked about the volunteers’ stress levels before and after the math task.

The dogs' accuracy at detecting the stress samples — from 90 percent to 96.88 percent — was even better than the researchers anticipated.

Knowing that chemical changes in sweat and breath can result from stress, it was expected the dogs might be able to smell the difference, Wilson said. "However we were still surprised the first time the dogs were shown the pre- and post-math task samples and confidently discriminated between them.”

One thing the research doesn’t reveal is whether dogs feel empathy when a person is stressed.

“Because the dogs were trained with positive reinforcement to find their target, they were visibly excited when they found it in the line-up, rather than showing any kind of stress themselves,” Wilson said.

She compared it to dogs who can smell cancer by picking out breath samples in a line-up. Future studies can investigate whether smell is an important part of a dog’s perception of human emotions, Wilson said.

The findings “make perfect sense,” said Dr. Nicholas Dodman, a professor emeritus at the Cummings School of Veterinary Medicine at Tufts University, and CEO and president of the Center for Canine Behavior Studies.

"Dogs have a formidable sense of smell,” said Dodman, who is also the author of “Pets on the Couch: Neurotic Dogs, Compulsive Cats, Anxious Birds and the New Science of Animal Psychiatry.”

“To put it into numbers, we have 12 million smell receptors," he said. "Dogs have at least 50 times that number.”

Dogs aren’t the only animals with highly sensitive senses of smell.

A therapy cat, Oscar, is thought by some to have sensed death was coming based on a change in how people smelled when they were close to death, Dodman said.

Dr. Katherine Houpt points to a study in horses that showed that based on body odor “the animals could tell the difference between people who watched horror movies and those who watched comedies.”

Houpt commended the researchers on the meticulousness of their methods.

“They really trained their dogs carefully and they carefully cleaned the heads on the machine,” said Houpt, an emeritus professor of animal behavior at the Cornell University College of Veterinary Medicine.

It’s hard to tell whether the dogs equate what they smell with actual feelings of stress, Houpt said.

“Yes, they can tell the difference, but do they care?” she said. “Presumably they do, and that’s why they make good emotional therapy animals.”

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