With their big round eyes and wonderfully diverse fur patterns, the 250 lemurs that prance around the Duke Lemur Center in North Carolina look cute enough to cuddle. But Erin McKenney, a Duke University microbiome researcher, is more interested in their poop. McKenney spends the majority of her time looking for the stiff, upright lemur tails that are “the universal sign of pooping”—and her dirty work might help us better understand the origins of bowel diseases in humans.
McKenney and her fellow researchers recently discovered that the gut microbiomes of two of the lemur species here share surprising similarities with those of humans who suffer from inflammatory bowel diseases, like Crohn’s disease and inflammatory colitis. The reason, they suspect, is diet—which could offer physicians a new perspective on how to treat these diseases. In addition, getting a handle on the flora living in lemur guts could help conservationists better understand—and save—these endangered primates.
In a recent study in the journal Gut Microbes, McKenney studiously compared the poop from three different lemur species. Her goal was to understand how three different factors—the lemurs’ diet, the shape of their guts and the kind of bacteria that lived inside them—shaped their digestion. Her study material came from captive black and white ruffed lemurs and ringtail lemurs—two species that are fed identical diets of fruits, vegetables and supplementary “primate biscuits” at the Lemur Center—and another species, Coquerel’s sifakas, which eat leaves and is perhaps best known as the star of the kids’ show Zoboomafoo.
Thanks to previous studies, the researchers had a sense of what kinds of microscopic life exists inside lemur guts. What they didn’t understand, however, was the relationship between those bacteria and other microorganisms, which occupy different niches in the tiny biome.
During lemur digestion, millions of microscopically minute creatures known as microbes work together to break down food into energy, which is then used for grooming, climbing and exploring. These complex chemical reactions are known as metabolic pathways. “Every time that you eat a piece of food, you or your microbes can use different metabolic pathways to digest the food,” MicKenney explains. “So, one metabolic pathway might be to take a piece of fiber, which is a really complex molecule, and break it down into starch, and then break those starch pieces into sugar.”
In the study, the researchers used several techniques to understand what was happening at each step of the lemurs’ metabolic pathways. First, they analyzed the poop’s chemical makeup, looking for the telltale chemicals that specific reactions leave behind. Then, they sequenced the DNA found in lemur scat to figure out what kinds of microbes were living in it. With a few calculations, they were able to figure out which microbes worked together to digest food, and how.
This led to several unexpected discoveries: First, that the ringtail and the ruffed lemurs, who both eat fruit and vegetables, had the same metabolic pathways. This means that, in spite of their differently shaped intestines, their microbiomes were also very similar. The researchers then determined that five of those metabolic pathways were more common than others. Finally and most surprisingly, they noticed that these five pathways were also common in humans suffering from inflammatory bowel disease (IBD).
In humans, these metabolic pathways are considered to be markers of IBD, and possibly to help cause it by leading to inflammation in the gut. “At first glance, you might think, oh, the lemurs must have been sick,” McKenney says. “But all of the lemurs were healthy.” She and her fellow researchers have a few possible explanations for why these pathways might be more common in lemurs than in healthy humans. Perhaps the microbes that make them up are “adapted to a fast-flowing gut,” she says—or otherwise, they may be more suited to digesting high-sugar food.
To understand a “fast-flowing gut,” just look at the ruffed lemur, which has a very short, straight gut and takes only three hours to digest its food. This lemur likely evolved to have this gut shape because fruit is a main part of its diet, says McKenney. Since fruit can be easily converted to energy by the body, there’s no need for food to hang around and undergo further digestion. By contrast, the ringtail lemur has what she calls an “intermediate” gut: They require around seven hours to digest, and both their digestion time and gut shape resemble a human’s. Meanwhile, the Coquerel’s sifaka, which mainly subsists on plant leaves full of indigestible cellulose, can take up to 24 hours to digest.
Given the vast differences across these guts, the researchers were surprised to see that the two fruit-eating species shared common metabolic pathways. The key, they believe, is diet. In the Lemur Center, both animals were fed exactly the same high-sugar diets from birth. She says it’s possible that other primates with similar diets that contain high sugar might share the same pathways as well, but without more research, there’s no way to know.
The connection between lemurs and sufferers of IBD could have some unlikely implications for human medicine, says Mckenney. To her, it suggests that the bacteria that are part of those pathways may not be harmful in themselves, but could rather be merely bystanders to the disease—and that doctors should look elsewhere for its root cause.
She adds that she would like to see research on how eating a lower-fruit and higher-vegetable diet would affect these pathways. If the result was a decrease in the prominence of these bacteria, that could signal a potential new treatment for IBD-related inflammation. That treatment might be more sustainable than other therapies, such as prescribing steroids to address the inflammation, she says.
On the lemur front, understanding lemur microbiomes could help zoo caretakers give the animals their optimal diet. But it could also help with conservation. “Between habitat disruption and climate change, a lot of the ranges of animals are going to change in the wild,” McKenney says, “and when you change the range, you also change what plants they’re coming into contact with and what food resources they’ll have.” Knowing more about the microbial consequences could help conservationists determine how likely wild lemurs are to survive those changes—and what they can do to help.
Amanda Perofsky, a PhD candidate in biology at the University of Texas at Austin who studies Verraux’s sifaka (another kind of leaf-eating lemur), says the results of this paper are promising and “very thorough.” However, she would like to see a similar study conducted on lemurs in the wild. One big advantage of studying captive lemurs at the Duke Lemur Center is that researchers know lemurs’ exact conditions throughout its entire life. However, “there’s no way we can actually truly replicate an animal’s diet from the wild,” says Perofsky, who was not involved in the recent study. Moreover, “they are limited to the number of animals they have in captivity.”
The good news is that anything we learn about primate guts is likely to eventually benefit human health, says Katherine Ryan Amato, a biological anthropologist who studies gut microbes at Northwestern University. “A lot of microbial research, to some extent, is missing this evolutionary context,” she says, “particularly with respect to comparisons with our closest evolutionary relatives, nonhuman primates.”
And in the case of humans and lemurs, more research may be just what the doctor ordered. Lemurs of all kinds of threatened by deforestation, the illegal pet trade and hunting for food. Moreover, given their millennia of distinct evolution from other primates, any shared traits they may have with primates who evolved outside of Madagascar can help researchers better understand the evolution of both animals—even when those animals walk on two legs and wear clothes.