According to a new study, dogs not only can smell human stress but also make pessimistic decisions based on their owners' negative feelings.
The study, conducted by researchers from the United Kingdom and published in the scientific journal Scientific Reports, provides the first evidence that human stress odors influence emotion and learning in dogs.
"We know how attuned pets are to their owners’ emotions, but here we show that even the scent of an unknown stressed human affects a dog’s emotional state, reward perception, and ability to learn," explained Nicola Rooney, senior author and researcher of human-animal interactions at the University of Bristol.
Previous studies in humans have shown that we can sniff out clues about other people's emotions, subconsciously detecting chemical cues in sweat, and these hidden signals can subtly affect our own emotions and decisions.
Now, researchers have found that dogs also detect these signals coming from us. Since dogs are skilled at reading verbal and non-verbal cues from humans, the researchers decided not to directly expose them to stressed humans.
Instead, they collected sweat from three unknown human volunteers while they were either relaxing or experiencing something stressful. The relaxing activity involved watching a nature video, while the stressful activity included frustrating math instructions and public speaking.
The researchers recruited 18 pairs of dogs and humans to participate in tests with human odor samples. During training sessions, the dogs learned that a food bowl in one location always contained a treat, while a bowl in another location was always empty.
Dogs that learned this began to approach more quickly if a bowl was placed in the positive location (associated with treats) compared to the negative location (associated with no treats).
According to the researchers, the dogs' anxious behavior indicates optimism or a behavioral signal that suggests an animal's emotional state, based on previous research linking people's positive and negative emotions with optimistic or pessimistic decisions, respectively.
After the initial training, they repeated these experiments while exposing dogs to odors of stressed or relaxed humans, or to no odor.
They found that dogs were significantly less likely to approach a bowl in the almost negative position when they smelled a stressed stranger, compared to when exposed to the scent of a relaxed stranger or a blank fabric.
The moderate response of dogs exposed to human stress odor is characterized as pessimism and suggests a negative emotional state. This may be adaptive, possibly helping dogs conserve resources or avoid frustrations.
Understanding how human stress affects the well-being of dogs is an important consideration for shelter dogs and when training companion dogs and dogs for working roles like assistance dogs, added Rooney.