Living with friends may quietly be altering your gut bacteria - according to a new study from the University of East Anglia.
Research on a colony of tiny island birds reveals they share more of their gut bacteria with the birds they spend the most time with.
And the team say the same principle almost certainly applies to humans too.
Previous human studies have hinted at this phenomenon - with spouses and long‑term cohabitees often having more similar gut microbiomes than strangers, even when their diets differ.
But the new bird research provides unusually clear evidence of how social closeness itself - not just shared environment - drives the exchange of gut bacteria.
How the research happened
Researchers studied the Seychelles warbler, a small songbird found on Cousin Island in the Seychelles.
They collected faecal samples from the warblers, which were then used to analyse the birds’ gut microbiomes - the diverse communities of ‘good’ bacteria living in their digestive systems.
Dr Chuen Zhang Lee, from UEA’s School of Biological Sciences, carried out the work as part of his PhD project.
He said: “To uncover how gut bacteria spreads between social partners, we meticulously collected the birds’ poo over several years. We gathered hundreds of samples from birds with known social roles - breeding pairs, helpers and non‑helpers living in the same group, and in different groups.
“This allowed us to compare the gut bacteria of birds that interacted closely at the nest versus those that did not.
“We studied their anaerobic gut bacteria, which thrive without oxygen.
“And it gave us a rare insight into how social bonds can drive the transmission of gut microbes.”
A natural laboratory in the Indian Ocean
Senior researcher Prof David S Richardson, from UEA’s School of Biological Sciences, said: “Cousin Island is small, isolated, and the warblers never leave it. That means every bird on the island can be individually marked and followed throughout its life.
“This offers scientists an exceptional opportunity to study life-long biological processes in the wild.”
All of the island’s warblers are fitted with coloured leg rings, allowing researchers to track their behaviour, health, and genetics over many years. This creates conditions similar to a laboratory population - but in a completely natural setting.
“It gives us the best of both worlds,” said Prof Richardson. “We can study animals living natural lives, with natural diets and gut bacteria, while still being able to collect detailed data from known individuals.”
Sharing gut microbiota with friends
“We found that the more social you are with another individual, the more you share similar anaerobic gut bacteria,” said Dr Lee.
“Birds who spent a lot of time together at the nest – breeding couples and their devoted helpers – shared a lot of this type of gut bacteria, which can only spread through direct, close contact.
“These anaerobic microbes can’t survive in the open air, so they don’t drift around in the environment. Instead, they move between individuals through intimate interactions and shared nests.”
How home life shapes our microbiomes
The team say that the findings shine a spotlight on what may be happening in human homes.
“Whether you’re living with a partner, housemate, or family, your daily interactions - from hugging, kissing and sharing food prep spaces - may encourage the exchange of gut microbes,” said Dr Lee.
“Anaerobic bacteria are some of the most important for digestion, immunity and overall health.
“Once inside the gut, they thrive in oxygen‑free conditions and often form stable, long‑term colonies.
“That means the people you live with might subtly shape the microscopic ecosystem inside you.
“Translated into human terms, this means that cosy nights in, shared washing‑up duties, and even sitting close on the sofa may bring your microbiomes quietly closer together.
“Sharing beneficial anaerobic bacteria could strengthen immunity and improve digestive health across a household,” he added.
This research was led by UEA in collaboration with Norwich Research Park colleagues at the Centre for Microbial Interactions, the Quadram Institute and the Earlham Institute, as well as the University of Sheffield, the University of Groningen (The Netherlands) and Nature Seychelles.
‘Social structure and interactions differentially shape aerotolerant and anaerobic gut microbiomes in a cooperative breeding species’ is published in the journal Molecular Ecology.
ENDS
Molecular Ecology
Observational study
Animals
‘Social structure and interactions differentially shape aerotolerant and anaerobic gut microbiomes in a cooperative breeding species’
10-Apr-2026