The domestication of dogs occurred thousands of years ago, and since then, dogs have become extremely sensitive to human emotions and actions. This has been expanded upon in a recent study. Researchers conducted trials to determine whether dogs can use their keen sense of smell to detect the rising tension levels in their human companions. The results showed that they could, even in the case of an unfamiliar person.

In the study, led by researchers at Queen’s University Belfast in Northern Ireland, four domesticated dogs were trained to recognize various odors. Then, thirty-six human participants were given mathematical exercises intended to stress them out. The researchers collected data about their stress levels, including heart rate, blood pressure, and respiration, before and after the exercises. In addition, they wiped the back of their necks to take a sweat sample with a piece of gauze.

This resulted in “stressed” samples that the dogs were trained to identify from “relaxed” ones and completely blank samples for controls. The canines were first rewarded for identifying between stressed and clean samples in a preliminary trial. Then, the relaxed samples were introduced, and the dogs were trained to distinguish between them over 720 trials.

Remarkably, 94% of the time, the dogs accurately identified the stress sample. The researchers view this as “strong proof” that certain smells are associated with stressful moods and that canines can identify such smells from those we generate at ease.

Four dogs trained to smell human stress
Top Left: Soot, Top Right: Fingal, Bottom Left: Winnie, Bottom Right: Treo. (Credit: Wilson et al., doi: 10.1371/journal.pone.0274143)

Clara Wilson, the study author and a Ph.D. student in the school of psychology at Queen’s, concluded:

“The findings show that we, as humans, produce different smells through our sweat and breath when we are stressed, and dogs can tell this apart from our smell when relaxed – even if it is someone they do not know.”

The researchers point out that the study did not examine whether dogs view stress in humans as positive or negative and that in actual situations, they probably make this determination based on other indicators like our breathing or tone of voice. However, they believe the findings could be helpful in training service dogs for veterans with post-traumatic stress disorder, which mainly relies on visual cues.

Wilson added

“This is the first study of its kind and provides evidence that dogs can smell stress from breath and sweat alone, which could be useful when training service dogs and therapy dogs. It also helps to shed more light on the human-dog relationship and adds to our understanding of how dogs may interpret and interact with human psychological states.”

The research was published on September 28, 2022, in the journal PLOS One.

Previous research found that dogs could also be trained to smell can in blood samples with almost 97% accuracy and COVID-19 with almost 100% accuracy. Dogs have also been trained to detect explosives, bacterial infections, malaria, and Parkinson’s disease.

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