Dogs can literally sense our stress

Dogs share a long history with our species, which gives them an amazing ability to interpret the signals we send them. They also possess an incredible sense of smell, which allows them to detect, based solely on scent, certain diseases that affect humans, such as COVID-19 or lung cancer. The question of whether these abilities extend to detecting odors associated with psychological states, however, has been much less explored. My colleagues and I wanted to determine whether dogs could use their sense of smell to distinguish between odor samples taken from the same person before and after they had been under stress. It’s important to note that when we’re stressed, hormonal changes and alterations in the nervous system occur, which modify the odors produced by our bodies. To determine whether dogs could indeed detect such differences, we drew inspiration from the protocols used for biomedical detection dogs—sniffer dogs whose talents are utilized in laboratories. We combined these with techniques used to test how our canine companions perceive odors. Our results were published in the journal PLOS One.

The Protocol, Human Side

First, we equipped the human participants in the study with sensors designed to continuously measure their heart rate and blood pressure. We also asked them to rate their perceived stress levels before and after participating in the task we asked them to perform as part of this experiment. The task involved performing a quick mental calculation, which was intended to induce stress. Before the task began, participants wiped a piece of gauze across the back of their neck, placed it in a sterile glass vial, and then exhaled into the vial. After the task, participants provided two additional sweat/breath samples. The time interval between the collection of samples taken “in a relaxed state” (before the task) and “in a stressed state” (after the task) was four minutes. This short interval reduces the likelihood that changes related to events other than exercise-induced stress would affect the participants. We included in the study only samples from participants who reported finding the task stressful and whose heart rate and blood pressure had increased during the exercise. Ultimately, we presented samples from 36 people to the dogs.  

The Dog Training Process

The dogs included in this study were pets whose owners had volunteered them for the study. These animals had been trained by researchers in a laboratory once a week using positive reinforcement (which involves associating an exercise with something that serves as a reward for the animal, ed. ). Before data collection began, the dogs were taught to indicate that they had selected a sample by standing up and remaining motionless over it for several seconds or by sitting in front of it—a behavior we termed “alert behavior.” The animals were then introduced to a matching game, through which they learned to distinguish between samples with different odors. Once it was established that they had successfully completed this game, they were ready to participate in the actual test. During the test, we asked the dogs to distinguish between samples collected from the participants before and after the aforementioned arithmetic task. To teach them which scent to look for during each test session, we first showed them the sweat or breath sample from the stressed person, as well as two “control” samples ”—pieces of clean gauze placed in sterile glass vials, thus containing no sweat or breath. The dogs were allowed to sniff the three samples and were rewarded when they were able to signal the sweat/breath sample to the researchers. After ten trials, a second sweat/breath sample was added to the list: a sample from the same person in a relaxed state. It was at this point that the “discrimination” test began, which took place over the next 20 trials.  

The stress sample was selected in 94% of cases

During this phase, the dogs were required to indicate, through their alert behavior, which sample they perceived as identical to the one shown to them during the previous ten trials—that is, the sample that, to them, smelled like the stress sample. Controls were implemented to verify that the dogs did not rely on information other than that related to the tested sample to aid their choices (such as a lingering odor on the sample presentation devices, or a visual cue unconsciously provided by the experimenter). If the two odors presented seem similar to the sniffer dog, it can be expected to choose one or the other at random. If, on the other hand, the two odors seem distinct to the dog, it should be able to consistently identify the odor that was initially presented to it during training (the “stress” odor). Each set of samples from participants was used only once, so the dogs were presented with samples from a different participant during each session. Result: From their very first exposure to the “stress” samples, the dogs recognized that they had a specific odor. In fact, they correctly identified the stress sample in 94% of the 720 trials. Being subjected to a mental arithmetic exercise that caused them stress thus did indeed alter the odors produced by the participants’ bodies. It should be noted, however, that this study does not determine whether the dogs perceived the “stress” samples as reflecting a negative emotional state. It is likely that, in real life, dogs use various contextual cues—such as our body language, tone of voice, or breathing rate—to help them understand a situation. These results, however, provide strong evidence that the scent of stress is also something dogs can detect. They offer a better understanding of how dogs perceive human psychological states and interact with them. Beyond a better understanding of the relationship that binds us to our canine companions, this knowledge could also be useful for improving the training of assistance dogs for people suffering from anxiety and post-traumatic stress disorder , as these dogs are currently trained to respond primarily to visual cues. Clara Wilson , PhD Candidate, Psychology, Queen’s University Belfast This article is republished from The Conversation under a Creative Commons license. Read the original article.