Hair From Dogs and Humans Show Biological Signs of a "Unique Interspecies Bond" 

When we’re stressed, we sometimes seek solace in the fur of our oldest, most loyal companions. But do we ever stop to wonder if the daily stresses of human life are also taking a toll on our pets? An analysis of biological stress markers from humans and their dogs published Thursday in Scientific Reports suggests that yes, when we enter the stress spiral, our best friends aren’t far behind.

High-stress humans also tend to have high stress-dogs, at least according to an analysis of their levels of cortisol (a stress-related hormone), explains Lina Roth, Ph.D., an assistant professor of zoology and ethology at Sweden’s Linköping University. Her analysis of hair and fur samples from 58 humans and dogs revealed a correlation between the cortisol levels seen in owners, and the one noted in their dogs. It revealed what she called “long-term stress synchronization” between species.

Roth tells Inverse that her results are just a correlation, and she says we should interpret them with caution, but she’s comfortable saying that the results represent a “unique interspecies bond” that goes beyond stress. It may be more about empathy.

“Dogs and humans have adapted to each other during the past 15,000 years, and we have developed a unique interspecies bond,” says Roth. “We are two social animals that share everyday life together and one could speculate that that this long-term stress synchronization is related to emotional empathy.”

Roth’s evidence for this long-term stress synchronization comes from a comparison between 58 people who owned either a Shetland sheepdog or a border collie. Roth included in her study a mix of male and female dogs, those who competed in agility or obedience competitions, and typical (but good) family dogs. She included the obedience competition, she adds, because dogs who actively compete do tend to have higher cortisol levels — which she found in an earlier study on German shepherds.

When Roth analyzed cortisol levels in hair samples from humans and from their animals, she found her correlation, noting that it seemed to be slightly stronger in female dogs compared to male dogs. But she also took the research a step further and conducted a personality analysis of dogs and their owners.

For people, Roth evaluated personality using the “Big Five Inventory,” which measures five aspects of human personalities: agreeableness, openness, extraversion, conscientiousness and neuroticism. Dogs were evaluated according to a dog personality questionnaire, which was filled out by their owners.

There was no correlation between the dog personality questionnaire and dog cortisol levels, but the personality test did find significant correlations between human personality traits and their animals’ cortisol levels. Not all of them held for both dog breed and seasons. One finding Roth does note is that people with high neuroticism scores, strangely, tend to have dogs with lower cortisol levels. But this comparison largely serves to highlight the idea that it’s really humans who are driving this shared cortisol relationship, and not the other way around.

Dogs May Empathize With Owners

On one hand, her results provide evidence that dogs may internalize their humans’ stress (at least hormonally speaking), but her speculative interpretation is that they may also empathize with their owners to an extent. The stress synchronization may may be an example of a a concept called “emotional contagion,” Roth says.

In her study, Roth and her co-authors explain emotional contagion as a “the mirroring of emotional or arousal states between individuals.” In other words it’s the idea that a feeling — like stress — is catchy amongst animals that spend a lot of time together. In humans, stress-based contagion has been demonstrated before in elementary school students who had burned-out teachers. But Roth’s early-stage work builds on the idea that this happens between species too, if the bond is tight enough:

“Dogs and humans are both social species that share everyday life together, and function as social support for each other which would fit the idea of emotional contagion,” Roth says.

We don’t need peer-reviewed evidence to show that the human-dog connection is strong, and now we have some hormonal evidence to speak to its strength. That, adds Roth, speaks to our truly unique relationship with dogs.

Abstract:

This study reveals, for the first time, an interspecifc synchronization in long-term stress levels. Previously, acute stress, has been shown to be highly contagious both among humans and between individuals of other species. Here, long-term stress synchronization in dogs and their owners was investigated.We studied 58 dog-human dyads and analyzed their hair cortisol concentrations (HCC) at two separate occasions, reflecting levels during previous summer and winter months. The personality traits of both dogs and their owners were determined through owner-completed Dog Personality Questionnaire (DPQ) and human Big Five Inventory (BFI) surveys. In addition, the dogs’ activity levels were continuously monitored with a remote cloud-based activity collar for one week. Shetland sheepdogs (N=33) and border collies (N=25), balanced for sex, participated, and both pet dogs and actively competing dogs (agility and obedience) were included to represent different lifestyles. The results showed significant interspecies correlations in long-term stress where human HCC from both summer and winter samplings correlated strongly with dog HCC (summer: N=57, χ2=23.697, P <0.001, β=0.235; winter: N=55, χ2=13.796, P<0.001, β=0.027). Interestingly, the dogs’ activity levels did not affect HCC, nor did the amount of training sessions per week, showing that the HCC levels were not related to general physical activity. Additionally, there was a seasonal effect in HCC. However, although dogs’ personalities had little effects on their HCC, the human personality traits neuroticism, conscientiousness, and openness significantly affected dog HCC. Hence, we suggest that dogs, to a great extent, mirror the stress level of their owners.