A giant panda may look like a vegetarian on the outside, but it definitely looks like a carnivore on the inside.
A genetic analysis of 121 samples of panda poop finds that the community of microbes living inside these animals' guts is optimized to digest meat. This is despite the fact that giant pandas have been eating bamboo for at least 7 million years, and that the plant has been the bears' sole food source for at least 2 million years.
The findings, published Tuesday in the journal mBio, may not bode well for the endangered species. Only about 1,600 giant pandas remain in the forests of northern and central China.
"The peculiar characteristics of its gut microbiota may put it at high risk of extinction," the study authors wrote.
Scientists have come to appreciate that the microbes in our guts play an essential role in keeping us healthy. These bacteria help their hosts digest the foods they eat and extract -- or synthesize -- the needed nutrients.
In theory, the microbes in the panda gut should be pretty busy. Pandas eat as much as 28 pounds of bamboo each day, a diet that requires as much as 14 hours of munching. Once the plant material reaches the panda intestine, digestive enzymes should get busy digesting all that fiber.
In some ways, giant pandas have adapted to their unusual vegetarian eating habits. They have "powerful jaws and teeth" to break down copious amounts of cellulose, the study said.
But then, it appears, evolution abandoned giant pandas. They did not develop a longer gut to give themselves more time to break down stubborn plant parts, as other herbivores did. Nor did they adjust their DNA to make different kinds of enzymes that would have helped them digest bamboo.
So how do pandas cope? Only one possibility remained, the researchers wrote: "The giant panda appears to have no alternative but to rely on symbiotic gut microbes to adapt to its highly fibrous diet."
So the researchers, from various universities and research centers in China, went looking for panda scat. They collected scores of fecal samples from 45 wild pandas -- 24 adults, 16 juveniles and five cubs that had not yet been weaned. Among these pandas, 33 provided samples during spring, summer and fall.
After sequencing the DNA in the samples they collected, the researchers made some surprising discoveries. For starters, the components of the gut microbiome varied more in a single panda over the course of a year than they did between two different pandas.
Also, the diversity of these microbial communities was "extremely low" compared with that of other mammals. Scientists believe that high diversity makes animals more resilient to changes in their environment. If the reverse is true as well, it could spell trouble for pandas and their "highly fragile lifestyle," the researchers wrote.
But the biggest surprise was that the collection of microbes that did make their home in the panda gut were dominated by Escherichia/Shigella and Streptococcus bacteria. That meant that they looked a lot like the communities found in meat eaters, especially black bears, polar bears and spectacled bears.
What was missing were Ruminococcaceae and Bacteroidetes bacteria, two groups that are good at degrading fiber. These microbes typically help herbivores like cattle, sheep, horses and rabbits digest their food.
Pandas are notoriously poor breeders, but the new study provides another reason giant pandas are endangered in the wild. Without the anatomy, biology or microbiome to efficiently digest their only source of food, perhaps it's a wonder they've survived this long.