Dogs have distinct personalities and reactions to their humans and we might be stressing them out. The smell of human stress can be like an emotional contagion that drives them to make more “pessimistic” choices, according to a study published in the journal Scientific Reports on July 22.
For humans, an emotional contagion can be expressed as the tendency to mimic another person’s emotions and then express or experience that same emotion. Some evidence suggests that the smell of a stressed person can affect the emotions and choices of those around us, and we don’t even realize it. Researchers from Bristol Veterinary School in the United Kingdom were curious if dogs also experience changes in their learning and emotional state as a response to the scents given off by humans, both stressed and relaxed.
[Related: Shelter dogs appear ‘less stressed’ when paired with a buddy.]
In this study, the team used a test of ‘optimism’ or ‘pessimism’ that can be used to gauge choices. Optimistic decisions indicate positive emotions, while pessimistic choices indicate negative feelings.
The team recruited 18 dogs and their owners to take part in a series of trials where different human smells were present. The dogs were trained to recognize that when a food bowl was placed in a specific location, it contained a treat. When it was placed in another location, it was treat-less. Once a dog learned the difference between the bowl location, they approached the spot with a treat more quickly than the empty location.
They then tested how quickly the dog would approach new more ambiguous bowl locations that were positioned between the original two food bowls. A quicker approach reflected a dog’s optimism about whether food was present in these new locations–a marker of a more positive emotional state. A slower approach showed pessimism and the presence of more negative emotions. The trials were repeated while each dog was exposed to different odors–no odor, sweat and breath samples from a stressed human taking a math test, and breath and stress samples from a more relaxed person listening to soothing soundscapes.
The team observed that the stress smell made the dogs slower to approach the ambiguous bowl location closest to the trained location of the empty bowl. The more relaxed smells didn’t have any effect on their reactions. The stress smell may have increased the dogs’ expectations that this new location did not have any food, similar to the empty bowl nearby.
[Related: Stressed rattlesnakes just need a little help from their friends.]
According to the team, this pessimistic response reflects a negative emotional state. It may be a way for dogs to conserve energy and avoid disappointment if they do not believe that a treat is present. They also found that dogs continued to improve their learning about whether food was present in either of the bowl locations with repeated experiments. The dogs also appeared to be motivated to learn a bit quicker when the stress smell was present.
“Understanding how human stress affects dogs’ wellbeing is an important consideration for dogs in kennels and when training companion dogs and dogs for working roles such as assistance dogs,” study co-author and University of Bristol animal behaviorist Nicola Rooney said in a statement. “Dog owners know how attuned their pets are to their emotions, but here we show that even the odor of a stressed, unfamiliar human affects a dog’s emotional state, perception of rewards, and ability to learn. Working dog handlers often describe stress traveling down the lead, but we’ve also shown it can also travel through the air.”
