In a first, scientists show that dogs can smell when humans are stressed

Winnie, Soot, Fingal, and Treo — pet dogs from Belfast in Northern Ireland — made their mark on science, and they didn’t even pee on anything while doing it. This quartet trained tirelessly to help researchers answer a crucial question: Can dogs smell stress in humans?

Dogs can sniff out a suite of chemical states. They can anticipate when someone is going to have a seizure or smell changes in one’s blood sugar. Not only that, but they also act on these observations, alerting their human with a nudge or nuzzle.

Humans emit chemosignals or chemical signals that the body produces to communicate their emotional state and reproductive status. Stress has its own chemosignal that comes from various physiological changes. The hormones cortisol and epinephrine pour into the bloodstream, resulting in a spike in heart rate, blood pressure, and respiration.

These pups participated in a study that further solidifies dogs’ spots in our hearts as physiologically astounding and emotionally attuned. The Belfast team found that these dogs can, indeed, smell stress. They published their findings in the journal PLOS ONE.

What’s new — On average, these four pioneering dogs correctly identified a person’s stress sample more than 93 percent of the time.

This finding isn’t surprising to Julia Meyers-Manor, a psychology professor studying animal cognition at Ripon College, who wasn’t involved in the research. Since dogs can detect changes in blood sugar by smell, stress hormones are related. Still, to her knowledge, this is the first time researchers have looked specifically at stress hormones, as the study asserts itself to be.

Participating humans provided breath and sweat samples on gauze before and after performing one of the most stress-inducing tasks imaginable: mental math. Researchers directed these poor souls to count backward from 9000 by intervals of 17, sans phone, pen, or paper.

“It is very important that you perform the task as quickly and efficiently as possible,” the researchers warned each participant. “You must keep going until the task is completed.” Just thinking about that will make most people’s cortisol levels spike. If someone gave an incorrect answer, the researcher would interfere with a swift, sharp, “No” and tell them their most recent correct answer. The task went on like this for what were probably three long, grueling minutes.

Samples from 36 people made it into the study. More people participated, but 11 were disqualified because they actually had no stress response to this mental math exercise. In fact, their blood pressure decreased, signaling they experienced the mental math as an invigorating, positive stressor rather than a mildly terrifying negative one.

The four dogs, after dozens of positive-conditioning training rounds to teach them to identify particular samples, were presented with three samples. One was the pre-mental math sample (baseline), one was post-mental math, and one didn’t contain any sample at all.

The researchers trained these dogs with an old-school method known as operant conditioning using positive reinforcement. This type of conditioning rewards an animal for making a correct choice (as opposed to punishing it for making an incorrect choice), encouraging them to repeat that behavior.

Why it matters — For millennia, dogs have been at humankind’s side, domesticating into the cuddly friends we have today. Dogs are loyal companions, yes, but they are crucial to our survival in ways that transcend our ancestors’ need for food or protection dogs afforded them. For some, dogs convey information about surroundings or one’s bodily state that we can’t detect ourselves.

Stress-smelling dogs could be a boon to those with post-traumatic stress disorder (PTSD) like veterans. “Maybe we teach these dogs beforehand not just to comfort but to predict when that stress level’s going up,” Meyers-Manor tells Inverse. “That makes them even more beneficial to the people because now you’re not just reactive, but you’re proactive, trying to prevent that stress response from getting too high.”

Still, this demonstration of a dog’s ability doesn’t mean that your pooch smells when your anxiety spikes. Meyers-Manor stipulates that there’s a difference between ability and actual behavior. “The question here is not whether all dogs do this,” she tells Inverse. “It’s more, ‘Can dogs do this?’”

Digging into the details — While some dog breeds are suited to particular vocations, this study didn’t conclude that any one breed was best at detecting stress. The four dogs that made the cut were a cocker spaniel, a cockapoo, a mixed-breed lurcher, and a mixed-breed terrier.

But that doesn’t mean breed doesn’t matter. Smushy-nosed, or brachiocephalic, dogs, like pugs, probably wouldn’t do too well at this task. They can’t breathe too well, let alone smell. But there may be other unknown factors affecting a dog’s fit for this position.

“Something mattered, because they tested 20 dogs, and only four of them were able to,” Meyers-Manor tells Inverse. At that point, she believes a dog’s viability for the study has more to do with personality than type. It came down to whether dogs could sit still and pay attention long enough, and only a fraction of participating dogs were able to.

What’s next — Meyers-Manor hopes to see two developments from this research.

First, she wants to see this proof-of-concept applied. She wants to know that dogs’ capabilities for detecting stress hold up outside of a lab.

“Can they be used in a real-life setting where there’s a lot more confounds of other people’s stress hormones and other people’s smells?” Detecting three samples in small metal chambers is one thing, but picking up on stress levels on a city street corner is another.

Next, she’s interested in the broader concept of how dogs experience the scent of Eau du Stress.

“Are they actually experiencing the stress when they smell the stress?” she ponders. If dogs don’t merely detect the smell of stress but become stressed out when they catch a whiff, “it has [ethical] implications for using dogs as therapy dogs, and as PTSD dogs, if they’re experiencing that stress themselves.”