Dogs act sad when they sense stress
Dogs can smell when people are stressed and will appear depressed.
Haley Seelig/Getty Images
Anthropological and DNA evidence suggests that humans and dogs have been close companions for perhaps 30,000 years. So it’s no surprise that dogs have a unique ability to interpret human emotions. Dogs evolved to pick up on verbal and visual cues from their owners, and previous studies have shown that their keen sense of smell can even detect the stress odor of human sweat. Now, researchers have discovered that dogs can not only smell stress (in this case, as manifested by elevated levels of the hormone cortisol), but they can also respond emotionally to it.
In a new study published Monday, Scientific Reports, Scientists at the University of Bristol in the UK recruited 18 dogs of various breeds and their owners. Eleven volunteers who were unfamiliar with dogs underwent stress tests including public speaking and arithmetic, during which armpit sweat samples were collected on a piece of cloth. The human participants then performed relaxation exercises, such as sitting in beanbag chairs under dim lighting and watching a nature video, after which they collected a new sweat sample. Sweat samples from three of the volunteers were used in the study.
The dogs were divided into three groups and were asked to sniff a sweat sample from one of the three volunteers. Before that, the dogs were trained to recognize that a food bowl in one location contained a treat and a bowl in another location did not. During the experiment, a bowl without a treat was sometimes placed in one of three “ambiguous” locations. In one experiment, dogs that had sniffed a sample from a stressed volunteer were less likely to approach the bowl in the ambiguous location than dogs that had sniffed a cloth without a sample, suggesting that the dog thought that this bowl did not contain a treat. Previous studies have shown that the expectation of a negative outcome reflects a depressed mood in dogs.
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Study participant Freddie sits next to a jar containing an odor sample while waiting for the test to begin.
The findings suggest that dogs become more pessimistic about uncertain situations when they are near stressed people, but this effect is absent when they are near people who smell relaxed, says Zoe Parr-Cortes, a PhD student at the Bristol School of Veterinary Medicine at the University of Bristol and lead author of the study. “For thousands of years, dogs have learned to coexist with humans and have been alongside them for much of their evolution. Both humans and dogs are social animals and there is emotional contagion between us,” she says. “Being able to sense stress in other members of the pack would probably have been beneficial, as it would have alerted them to a threat that other members of the group had already sensed.”
The fact that the dog picked up the smell from a stranger speaks to how important smell is to animals and how it affects emotions in such real-life situations, says Katherine A. Haupt, professor emeritus of behavioral medicine at Cornell University College of Veterinary Medicine. Haupt, who was not involved in the new study, suggests that the smell of stress may have reduced the dogs’ hunger, because it is known to affect appetite. “It may not have been that the smell of stress changed the dog’s decision-making, but rather that it changed their motivation for food,” she says. “That would make sense, because when you’re extremely stressed, you’re not going to be as interested in a candy bar.”
Haupt added that the study shows that dogs have an olfactory sense of empathy, in addition to visual and verbal cues. And stressed owners may behave in ways that dogs don’t normally display, he said. And it raises questions about how stress affects animals when combined with the pressures of anxious owners. “If dogs are responding to such mild stress, I’d be interested to see how they would react to something more serious, like an impending tornado, losing their job, or failing an exam,” Haupt said. “Dogs should be even more sensitive to real threats.”
