Giant pandas almost exclusively eat bamboo, and they've been doing that for about 2 million years.
So it's quite curious that their gut bacteria isn't really equipped to process all that plant matter. In fact, the animals only digest about 17 percent of the nearly 30 pounds of bamboo they eat throughout the day, according to a study published Tuesday in the American Society for Microbiology's open-access journal, mBio.
Which means that no matter how much giant pandas eat, most of their efforts go into pooping undigested bamboo.
How'd they end up in this mess? Pandas used to eat both meat and plants before they got into the bamboo life. "Unlike other herbivores that have successfully evolved anatomically specialized digestive systems to efficiently deconstruct fibrous plant matter, the giant panda still retains a gastrointestinal tract typical of carnivores," the authors wrote.
Study co-author Xiaoyan Pang of Shanghai Jiao Tong University said in a statement that their findings imply "the giant panda's gut microbiota may not have well adapted to its unique diet, and places pandas at an evolutionary dilemma."
Another article published this week in Scientific Reports compared the gut bacteria in giant pandas to that in red pandas, who also consume bamboo. The researchers found that giant pandas have gut bacteria more closely related to black bears. What's more, a giant panda's gut is completely distinct from that of a red panda.
Taken together, both studies underscore the giant panda's place as a bear in the animal kingdom, while red pandas are more closely linked to the raccoon, said Mike Maslanka, head of the nutrition science department at the Smithsonian's National Zoo in Washington, D.C.
"Regardless of their diet, giant pandas are bears. They're cool and fuzzy, black and white, but they're bears," said Maslanka, who did not work on these particular studies. "It just reinforces that, from the inside out."
For the mBio study, researchers sampled the poop of 45 captive pandas during the spring, summer and fall and looked to older data from nine captive and seven wild pandas. They then used a type of sequencing called 16S rRNA to determine the pandas' microbial profiles.
The findings showed that the guts of these pandas had "extremely low gut microbiota diversity" and lacked the type of bacteria found in other herbivores. Instead, their guts had Escherichia/Shigella and Streptococcus.
While these results aren't all that surprising given what we know about giant panda diets, Maslanka said the studies on the gut bacteria "set a good foundation for future work" on bamboo-eating animals, including exploring possibilities for slightly adjusting diets.
So, okay, how in the world can these creatures continue to live on this planet when their guts are nearly incapable of breaking down the only kind of food they eat? They do so by stuffing their faces all day long and passing a lot of bamboo through their GI tracts.
"Let's say their GI tracts had these microorganisms. Maybe we would only have to feed them a quarter of what they're eating now. But because they don't have that mechanism, to compensate they eat a whole bunch," Maslanka said. "They essentially adapted to a very poorly digestible diet, and they do it well."
Giant pandas at the National Zoo are given somewhere between 50 to 100 pounds of bamboo daily. The leaves have less fiber and are easier to digest; the bamboo stalk may have more fiber and are harder to digest. Giant pandas have to plow through a lot of it so they can get the nutrients they need, and pandas' digestive systems have adapted to pass all that volume.
The result, as you may very well imagine, is basically a lot of bamboo-filled poop.
"If you come to the National Zoo and walk through the exhibit first thing in the morning there will be piles of panda poop, and that's primarily because they're sitting there eating a lot of bamboo, and it looks very much like how it does when they swallow it," Maslanka said.
Hmmm, I think I'll pass on that morning zoo exhibit visit. But nice evolutionary work-around, giant pandas.
[This story has been corrected. An earlier version misidentified the Scientific Reports study authors as the same research group behind the mBio study.]
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