Taken once in summer and once in winter to account for seasonal variability, cortisol concentrations tended to be high in the sheepdogs and collies when their human guardians' levels were also high. And this key measure of chronic stress was lower in dogs whose owners' hair samples indicated lower levels of chronic stress.
The big surprise came when the researchers looked for a link between dogs' cortisol levels and their personalities (as reported by their owners -- yes, these exist).
If a pet parent described her pooch as timid, fearful or anxious, the researchers expected they might find higher cortisol levels in the dog's hair samples, and to find lower concentrations in samples from confident, easygoing dogs, according to Lina S.V. Roth, the paper's senior author.
But they didn't. Canine cortisol levels did not seem to rise and fall with their position on the temperamental spectrum from fearful to calm. The cortisol levels of their humans were actually a much better predictor of a dog's stress level.
That, the researchers wrote, suggests that "it is the dogs that mirror the stress levels of their owners rather than the owners responding to the stress in their dogs."
Brian Hare, a Duke University professor of evolutionary biology and expert in animal cognition, cautioned that the findings show only an association at this point. While provocative and original, the new research will need to take some further steps to show that an owner's stress levels are what cause stress in their dogs, he said.
But the finding delighted University of Nebraska at Omaha developmental psychologist Jonathan Santo, who co-authored a 2015 study finding clear evidence of short-term emotional contagion between dogs and their owners.
Not only did the researchers find evidence of lasting emotional synchrony between dog and human, that association was apparent in a sample of just 58 dogs and their owners, Santo said. Most human personality traits can only be clearly seen in much larger samples, he said.
"What this paper seems to hint at is some of the underlying mechanisms behind why humans and dogs or wolves have been able to domesticate each other over thousands of years," Santo said. "We're both social species, and once we became integrated into each other's lives, it was to everyone's advantage that dogs and humans would keep tabs on each other emotionally."
Santo, who co-parents a dog with strong St. Bernard features, said he suspects that dogs' anxieties affect their guardians' stress levels as well. But he added: "I'm inclined to believe it's more one way than the other. With dogs as they stand now, they are so much more dependent for their survival on humans than the other way around. It makes sense as an evolutionary strategy that they have become very sensitive to humans, and adopted properties that endear them to us."