Many pet parents swear their dog knows exactly how they’re feeling and will even try to provide comfort with a loving cuddle or a gentle paw tap. New research suggests this may be more than just a feeling: Dogs can smell when humans are stressed, according to research published Wednesday in the journal PLOS ONE.
“Our bodies’ psychological stress response changes the smell of our breath and sweat,” says Clara Wilson, a psychologist at Queen’s University Belfast in Northern Ireland and one of the study’s authors, to Gizmodo’s Ed Cara. “Dogs can detect this change.”
Previous research suggests that dogs might be able to smell when humans are happy or fearful. But the researchers behind this latest study were curious about stress, an all-too-common reaction to the hassles of everyday life, especially amid the COVID-19 pandemic.
To probe whether four-legged friends can detect stress, scientists recruited four dogs—Treo, Fingal, Soot and Winnie—and 36 humans to participate in an experiment. They took sweat and breath samples from the human volunteers during two scenarios: a baseline control condition and a stressful condition that required them to do mental math. Separately, the team trained their canine participants to recognize a stressed-out odor sample.
The researchers then exposed these dogs to three different scents: a standard piece of gauze, a sample from an unstressed human and a stressed odor sample. The four dogs were incredibly adept at sniffing out the stressed sample, achieving a combined 93.8 percent accuracy rate. Though the study was small, the findings suggest that humans who are stressed out may emit odors that differ from their normal smell—different enough, at least, that dogs’ highly sensitive noses can tell something’s up.
“This study provides further evidence of the extraordinary capabilities of ‘man’s best friend,’” says Wilson to NBC News’ Linda Carroll.
Compared to humans, dogs are super sniffers—and there are big physiological differences to help explain why. Canines have more than 220 million olfactory receptors, which bind with smell molecules and communicate information to the brain, while humans have just 50 million. That’s why dogs are so effective at sniffing out drugs, bombs, blood sugar changes, corpses, noxious weeds, COVID-19 and nearly anything else with an odor.
“While we can’t know with certainty why dogs developed such keen olfactory senses, it is very probably related with the need to identify prey, potential threats, reproductive status, and familial relationships in a pack setting among others,” says Mark Freeman, a veterinarian and small animal clinical scientist at Virginia Tech who was not involved in the study, to CNN’s Megan Marples.
Though dogs may be able to tell when a human smells stressed, it’s unclear whether they feel or behave any differently because of the odor—the study didn’t attempt to answer that sort of question. But, as Katherine Houpt, a veterinarian and animal behaviorist at Cornell who was not involved in the study, tells NBC News, dogs do seem to care when something is wrong with their humans, and “that’s why they make good emotional therapy animals.”