Who someone hangs out with may influence the bacteria found in their gut.

Researchers from UT Austin and collaborating institutions recently studied 32 black howler monkeys in the jungles of Mexico to examine the effects of kinship and social contact on the monkeys’ gut microbiota, or the population of small bacteria occupying their intestines.

The researchers found that monkeys that spent time in close proximity to one another had more similarities in their gut bacteria. The researchers published their findings in Microbial Ecology on Jan. 26. 

The microorganisms living in our guts have been known to have impact on our health, affecting things such as immune function and digestion, according to the study. Differences in gut microbiota, which are caused by variance in diet and social contact, can help scientists study the evolution of different species and how populations interact with their environments.

Katherine Amato, assistant professor of anthropology at Northwestern University and first author of the study, said the environment and food availability of black howler monkeys likely mirrors how human ancestors lived. 

“It’s especially important to look at nonhuman primates because they’re looking at a type of environment that is probably more similar to the environment human ancestors lived in when these relationships with microbes were evolving,” Amato said. “Understanding what patterns we can pick up in nonhuman primates in terms of microbial interactions can tell us something about the past.”

The researchers collected genetic data on how the howler monkeys were related to one another, as well as the microbe populations in their guts. They found that increased proximity and social contact was associated with more similar gut microbes, supporting previous studies. The study also found that kinship was negatively linked to gut microbiota, which contrasts with previous studies on humans — however, the association was weak and may be attributed to a small sample size in the study, Amato said.

Black howler monkeys live in small groups consisting of a few males, females and their offspring, according to the study. Unlike humans, they don’t often interact with individuals outside of this group, said Sarie Van Belle, UT anthropology postdoctoral researcher and co-author of the study. 

Female-female pairs of monkeys within each group were the strongest driver for the link between social contact and more similar microbial composition. According to the study, this may be due to increased proximity and how microbial transmission confers health benefits that female monkeys gain more from, given the cost of reproduction. 

“It seems that females that spent more time close to one another had more similar gut microbiomes,” Van Belle said. “In males, although they had three adult males living in the same group, those individuals don’t climb close to one another so we didn’t find any similarity in terms of gut microbiomes.”

Amato said that studies on primates and gut microbiota may serve to enhance scientists’ understanding of ecology and behavior.

“We think about social behavior in terms of certain aspects of our biology. People think of it being a good thing to protect people from predation and the downside is that it’s easier to spread disease,” Amato said. “I think this aspect — microbial communities that aren’t negatively affecting host health, but may be beneficially helping host health — is an interesting perspective for looking at why animals form groups.”