A study from the University of Bristol reveals that the scent of human stress acts as an “emotional contagion” in dogs, leading them to make more “pessimistic” decisions.

Published at Scientific Reports, the study is the first “to test how human stress odors affect dogs’ learning and emotional state,” according to the authors.

“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,” Nicola Rooney, PhD, Senior Lecturer in Wildlife and Conservation at Bristol Veterinary School and the paper’s lead author explained.

18 dog-owner pairings were recruited by the research team to take part in a series of trials in which they were exposed to various human odors.

The animals received training using a pair of food bowls by the researchers and their owners during these trials. There was a treat in one bowl and nothing in the other.

The pets moved considerably faster to the bowl containing the treat after they learned which bowl was whose.

A new “ambiguous” bowl was then positioned halfway between the previous two, and the researchers measured the dogs’ speed to determine how quickly they would approach it.

The researchers determined that if the dog came to the new bowl rapidly, it was “optimistic” that it would likewise contain a treat. They suggest that this is indicative of a positive emotional state.

They repeated the trials while exposing the dogs to no odor, breath, and sweat odors from stressed humans (while taking a math test), and breath and sweat odors from relaxed humans (while listening to soundscapes).

Researchers discovered that “the stress smell made dogs slower to approach the ambiguous bowl location nearest the trained location of the empty bowl.” Under the effect of the relaxed scent, the team failed to see that behavior in the animals.

The authors therefore came to the conclusion that the stress odor may have raised the animals’ expectations that there would be no treats in the new location, as evidenced by the empty bowl nearby.

This pessimistic response is a sign of a bad emotional state, the researchers added. Thus, it might be a means for the dog “to conserve energy and avoid disappointment,” according to the authors’ theory.

Lastly, the researchers discovered that the animals “continued to improve their learning about the presence or absence of food in the two trained bowl locations and that they improved faster when the stress smell was present.”

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