Come on, Le Le. That's going to bother your stomach. Photo via Candace Williams

Nature is a brutal teacher. To survive harsh environments, predators, and disease, Earth’s plants and animals have evolved some spectacular abilities and traits. And then there are pandas, who seem to lack any sense of self-preservation whatsoever. Their disinterest in mating is well-known. Less known is the folly of their insistence on eating bamboo—a food they literally cannot digest. Now researchers have found that that same diet may be a trigger for frequent and serious gastrointestinal illness. The report was published in the journal Frontiers in Microbiology.

"Gastrointestinal diseases are a major cause of mortality in wild and captive pandas but scientists understand very little about their digestive process," said Mississippi State University chemist and co-author Ashli Brown-Johnson in a press statement.

The illness affects not only pandas’ eating habits but also their already tenuous reproduction. The cycle looks something like this: During the heat of mating season, pandas switch from eating bamboo stalks to eating the leaves. Around this same time, many pandas get sick and stop eating. They poop out gooey globs called mucoids, and expectant female pandas may lose their pregnancies.

To understand how the cycle began, Brown-Johnson and her colleagues examined the feeding habits, poop, and mucoids of two pandas at the Memphis Zoo. Both Le Le (male) and Ya Ya (female) have been under mealtime supervision since 2003, so the zoo had already amassed a lot of information about their health and behavior.

The data showed that, like other pandas both captive and wild, Le Le and Ya Ya changed their diets drastically in the summer. In winter and spring, leaves accounted for less than 1 percent of their food. In July and August, nearly 60 percent of mealtime was spent eating leaves, and both bears began passing more mucoids, expressing pain, and regularly refusing food.

That's a serious problem, since they’re not exactly loading up on nutrients to begin with. Pandas are related to carnivores and built like carnivores and have teeth and guts like carnivores, but what do they eat? Bamboo. The bears consume up to one-third of their body weight in the woody plant every day, but it pretty much passes straight through them, since their stomachs can do almost nothing to break it down. Healthy panda poop (shown here) looks a lot like chunks of fresh bamboo:

The mucoids are a lot less pleasing to the eye, and, according to first author Candace Williams, they smell “ghastly.”

Zoo staff wrapped up samples of both types of bear waste in tinfoil and shipped them to the lab. The research team ran DNA tests on the bears’ fresh-looking fecal samples (five from Ya Ya, 13 from Le Le) and smelly mucoids (one from Ya Ya, 5 from Le Le), looking for differences in their gut bacteria. They found quite a few. First, the pandas’ healthy poop had much lower bacterial diversity than fecal samples from other plant-eaters. In general, the more diversity an animal has in its gut flora, the healthier it will be, so this alone was concerning.

Oddly, the poop’s bacterial diversity increased as summer and mucoid season approached, and peaked inside the mucoids themselves. The species of bacteria in the mucoids were perhaps the most telling: some, like Actinobacteria, are known for causing GI disease in people, while others are typically found in gut lining. 

"What we think might be happening is that their diet is causing a strong internal reaction, leading to an inflammatory response," said Suen. "Pandas are basically shedding their gastrointestinal lining to allow for the replacement of those microbes. It's kind of like resetting the microbiome." Unfortunately, that reset has a cost.

With just two bears and 24 samples, this was a small study, but it’s a start. "Until recently, the gut microbiome hasn't really played a role in the management of animals," said Williams. "Having a balanced gut is important, and it's also important that we know these things, especially about such unique animals."

Unique is right.