Sorry Toto! Dogs Can Sniff Out Human Stress, Study Finds

In humans, stress is associated with different physiological changes, including increased heart rate, blood pressure, respiration, and the release of epinephrine and cortisol into the bloodstream. For the purposes of the study, “stress” is defined as a physiological and psychological response to a challenging situation.

The US has been training dogs to identify human health conditions for quite some time. Now, a new study confirms that dogs can also smell subtle changes in human biology with startling accuracy. The research could prove useful in the training of therapy dogs, scientists say.

Scientists have known that long-term cortisol levels in dogs mirror the cortisol levels of their owners through a process called “emotional contagion,” which is a phenomenon that describes a process where the emotional states between individuals is mirrored regardless of exercise (which can cause cortisol spikes). The researchers wanted to know how, exactly, dogs were detecting their owner’s stress levels, and if the indicators went beyond the visual.

Scientists at Queen’s University Belfast conducted a first-of-its-kind study that compared baseline biological samples with biological samples of 36 people under experimentally-induced stress.

Researchers collected samples of sweat and breath from participants before and after being given difficult math problems to solve. Then, they selected samples from individuals whose heart rate and blood pressure increased while solving the problems, indicating stress. “Stress” samples and “relaxed” samples were subsequently arranged in a line up where dogs were taught how to identify the correct sample.

“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," said Clara Wilson, a PhD student in Queen’s University Belfast School of Psychology, in a release.

The research also indicates that dogs, unlike humans, don’t need to rely on visual or audio cues to notice when someone is stressed. The researchers said that the findings will “[help] to shed more light on the human-dog relationship and [add] to our understanding of how dogs may interpret and interact with human psychological states.”

The study also called attention to exactly how much more reliant dogs are on their noses to “see” the world around them. Helen Parker, who owns Treo, one of the study’s canine participants, said that she noticed Treo being more attuned to the emotional barometer in her home after participating in the study.

“The study made us more aware of a dog’s ability to use their nose to ‘see’' the world. We believe this study really developed Treo’s ability to sense a change in emotion at home. The study reinforced for us that dogs are highly sensitive and intuitive animals and there is immense value in using what they do best – sniffing!”

The research team is hopeful that the findings will contribute to the training of service and emotional support dogs tasked with responding to acute stress in their owners. Further study is required, they said, to detect the time frame for which the odor can be detected.

The research was published in the journal PLOS One.