It's old news that having a dog provides a lot of benefits. Playing with a pooch can help our brains concentrate and relax, a family dog can help prevent food allergies in children, and even fulfill our primal need to nurture. They also may have some sway over some of the tiniest organisms around -- the microbes that live in our bodies. A study published December 3 in the journal iScience found that the family dog prompts changes in our gut microbiome that result in better mental health.
"Raising dogs has beneficial effects, especially for adolescents, and these effects may be mediated through symbiosis with microorganisms," Dr. Takefumi Kikusui, a study co-author and animal behaviorist at Azabu University in Japan, said in a statement.
Living inside of all of our intestines is a bevy of bustling natural bacteria and other microorganisms working together called the gut microbiome. Most of these microorganisms have a symbiotic relationship with us, meaning we both benefit from the relationship. Our bodies provide food and shelter, while they help out bodies in return. The gut microbiome helps our bodies digest important nutrients, train our immune system, and can even stimulate the production of important neurotransmitters in our brains. Importantly, these organisms also help to keep potentially harmful microorganisms in check.
Some studies have found that dogs also have important differences in their gut microbiomes, including greater microbial diversity. Kikusui's team previously found that young people have a dog from a young age and then continue to have dogs later in life have better levels of social support and companionship.
For this new study, the team explored if some of the beneficial effects that dogs have on adolescent mental health might also be tied to these differences in the microbiome.
"Adolescent children who keep dogs exhibit higher mental well-being, and we also found that dog ownership alters the gut microbiota," said Kikusui. "Since the gut microbiota influences behavior through the gut-brain axis, we conducted this experiment."
Initially, they found that a person's dog-owning status at age 13 could predict their future mental health and behavioral scores. Adolescents with a dog at home reported fewer social problems than those without a dog.
The team then collected saliva microbiome samples from 247 non-dog owning and 96 dog-owning adolescents. After sequencing the microbes, they saw similar species diversity and richness between the both groups of teens. However, the composition of the microbiome composition was different in dog owners. According to the team, this means that having a dog in the house might shift the abundances of some mouth bacteria -- potentially bacteria that might correlate with the adolescents' psychological scores.
To test that psychological hypothesis, the team treated laboratory mice with the microbiota found in dog-owning teens to see whether and how it affected their social behavior. The mice with this new microbiome spent more time sniffing the other mice in their cages. These animals also showed a more social approach toward a trapped cage-mate -- a behavior test that is often used to test prosocial behavior in mice.
"The most interesting finding from this study is that bacteria promoting prosociality, or empathy, were discovered in the microbiomes of adolescent children who keep dogs," Kikusui said. "The implication is that the benefits of dog ownership include providing a sense of security through interaction, but I believe it also holds value in its potential to alter the symbiotic microbial community."
According to the team, more research is needed to weed out how other variables affect the adolescent microbiome. For example, poverty can negatively affect the gut microbiome diversity, as it can lead to malnutrition and make accessing regular healthcare difficult.
Still, these results suggest that a family dog can change the microbiome in ways that support prosocial behavior, empathy, and mental health in general. We can thank the tens of thousands of years of human-canine coexistence for this very special relationship inside and out.