Monkeys in captivity start to resemble humans – in their guts

The mantled howler monkey is one of the species studied by the researchers(Credit: Stock Image/Depositphotos)

Turns out humans aren't the only ones who need more fiber. Researchers have found that monkeys in captivity lose so much of the diversity of their natural gut microbes, that the bacteria in their digestive tracts starts resembling those of modern, Western humans. And no, it's not because of antibiotics – it seems that in captivity, they don't get the diverse, plant-based diet they do in the wild.

The team, led by researchers from the University of Minnesota, studied two different primate species, the endangered red-shanked douc and the mantled howler monkey. They compared the captive monkey gut bacteria to the gut bacteria of their wild counterparts, as well as to modern humans living in developing and Western countries.

Not only did the captive monkeys lose most of their natural gut microbes, but they very consistently acquired the same new and less diverse set of bacteria found in modern humans living in Westernized countries and cities.

The same pattern happened across zoos in three different Western countries.

This could be bad news for primates in captivity. We don't know for certain, but the lack of diversity in modern human gut bacteria is a suspected factor in a range of medical conditions, from type 2 diabetes to cardiovascular disease.

Once the team explored and ruled out factors like genetics, geography and antibiotic use, they came down to a simpler the explanation – the primates just weren't eating enough plants.

The difference in the amount and diversity of plants being eaten by captive vs wild primates is stark. The team performed DNA sequencing on their subjects' stool, and found the captive monkey samples contained almost no plant DNA, while wild primate stool contained up to 40 percent plant DNA.

The researchers also collected samples from a semi-captive population of doucs who live in a sanctuary in Vietnam, where they received much more plant diversity than their western counterparts – about half the normal variety of plants eaten by wild doucs. Sure enough, they found these semi-captive primates had a gut diversity that was closer to their wild counterparts.

"We don't know for certain that these new modern human microbes are bad, but on the other hand, many studies are now showing that we evolved together with our resident microbes," says University of Minnesota professor and research team leader Dan Knights. "If that is the case, then it is likely not beneficial to swap them out for a totally different set."

The full research paper is available on PNAS.org.

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