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There’s now scientific evidence that sheds more light on one of Barkley’s impressive abilities in a long list of endearing traits: the ability to smell when you’re stressed.
According to a study published Wednesday in the journal PLOS ONE, dogs can tell the difference between smells made by humans when they’re stressed and when they’re calm.
Previous research found that dogs may be able to smell when a person is happy or anxious, but this latest study eliminated other competing scents and measured human participants’ stress levels to increase the accuracy of the results.
Researchers first collected breath and sweat samples from study participants to use as a baseline. Then, in front of two researchers, these people performed a mental arithmetic task for three minutes, counting backwards from 9,000 in increments of 17.
“If the participant gave a correct answer, they received no feedback and were expected to continue, and if they gave an incorrect answer, the researcher interrupted them with ‘no’ and told them their last correct answer,” said the lead author of the study, Clara Wilson , PhD student at Queen’s University Belfast in Northern Ireland.
The study team collected another round of breath and sweat samples after completing the task.
In addition, the researchers collected reported stress levels, heart rate, and blood pressure before and after the assigned task. Thirty-six participants who reported feeling stressed and had elevated heart rates and blood pressure were shown their samples to the dogs.
The researchers presented breath and sweat samples after the task from one to 20 people Dogs along with two other blank controls. The canines had to choose the correct sample at least seven out of ten times to move on to the next phase.
In the second and final phase, the study team showed the four dogs that passed phase one the same samples they sniffed in phase one, along with a sample from the same person collected before the task and a blank sample. The dogs presented with these options 20 times had to successfully identify the original “stress” smell after the task at least 80% of the time for the results to be conclusive.
The dogs selected the correct sample 93.8% of the trials, suggesting that the stress odors were significantly different from the baseline samples, Wilson said.
“It was fascinating to see how able the dogs were to discriminate between these smells when the only difference was that a psychological stress response had occurred,” she said.
Dogs have 220 million scent receptors compared to 50 million humans, making dogs “extremely effective at distinguishing and identifying scents,” said Dr. Mark Freeman, clinical assistant professor in the Small Animal Clinical Sciences department at Virginia Tech in Blacksburg. He was not involved in the study.
Olfactory receptors are small nerve endings in your nostrils that allow you to smell, he said.
“Although we cannot know for sure why dogs evolved such a keen sense of smell, it is very likely related to the need to identify prey, potential threats, reproductive status, and family relationships in a pack, among other things,” Freeman said.
Twenty domestic dogs were recruited from around Belfast, Northern Ireland, and four conducted the entire study.
Most dogs didn’t finish because they either showed signs of anxiety when separated from their owner or because they were unable to stay focused all the time.
If the canines in the study were raised from birth with the goal of tracking stress, more dogs most likely would have completed the study, he said.
There was a male Cocker Spaniel, a female Cockapoo, a male Lurcher type, also known as a crossbreed hound, and a female Terrier type. Their ages ranged from 11 to 36 months.
All dogs have a strong sense of smell, but spaniels, terriers, and lurchers would likely have used their scent receptors more regularly than gun dogs, Freeman said. This could have been a factor in their success in the study, or it could be a coincidence because other breeds like retrievers also have excellent smelling abilities.
Assistance dogs that help people with mental illnesses like anxiety and post-traumatic stress disorder could benefit from these findings, Wilson said.
“Knowing that stress contains a detectable olfactory component may stimulate discussion about the value of olfactory-based training with samples from individuals during times of stress versus rest,” she said.
More experiments need to be done outside of a lab to see how applicable the results of this study are in the real world, Wilson said.
These results also open the door for future research to examine whether dogs can distinguish between emotions and how long the smells last, she said.