By Ewen Callaway of Nature magazine

Giant pandas don't digest bamboo by themselves. Microorganisms in their guts may help the endangered animals to subsist on plants despite a gut that is better suited to eating meat, finds an analysis published this week in the Proceedings of the National Academy of Sciences.

Pandas (Ailuropoda melanoleuca) are among the pickiest eaters in the animal world. In the wild, they eat more than 12 kilograms of bamboo each day--and little else.

They have to eat so much because, although bamboo contains proteins, sugars and fats among other nutrients, most of its calories are locked in hard-to-digest cellulose fibers that make up plant cell walls. A 1982 study of two pandas, Ling-Ling and Hsing-Hsing in the Smithsonian National Zoological Park in Washington DC, found that 92% of the cellulose and 73% of the hemicellulose (a chemically similar fiber) in the bamboo they ate passed right through their digestive tracts and ended up in their feces.

Most herbivores have developed ways to break down cellulose into sugars; for example, cows and other ruminants have complicated digestive systems--involving multiple stomachs filled with microbes--that process plants many times to extract the maximum nutrition. But pandas are bears, a generally carnivorous family, and neither produce the enzymes necessary to digest cellulose nor harbor the same microbes as ruminants. A broad survey of animal gut microbes found that pandas' microorganisms resembled those of black bears, polar bears and other meat-eaters.

Digestive aid

Fuwen Wei, an ecologist at the Institute of Zoology at the Chinese Academy of Sciences in Beijing, and his colleagues took a closer look at the microbes that live in the guts of giant pandas. The team collected stool samples from seven wild pandas in the Qinling and Xiangling mountains in central and western China, as well as from eight captive pandas. By sequencing stool DNA, the researchers determined the different kinds of bacteria present, as well as the identity of thousands of microbial genes.

Although wild and captive pandas have different diets and lifestyles--the captive pandas eat a more diverse diet that includes fruit and milk--they tended to harbor similar microbe species in their guts. Wei's team found that samples from both groups contained previously unknown genes produced by Clostridium bacteria, which resembled known genes for enzymes that break cellulose into simpler sugars.

The microbial enzymes may help giant pandas to extract extra energy from the small amount of bamboo that they manage to process, says Wei. These microbes are part of a suite of evolutionary adaptations--alongside powerful jaws and teeth, and pseudo-thumbs, bones that allow them to grip plant stalks--that help pandas to live on bamboo, despite having a carnivore's digestive system, he says.

But Ruth Ley, a microbiologist at Cornell University in Ithaca, New York, says that pandas still harbor fewer cellulose-digesting enzymes than even non-exclusively herbivorous species such as humans. "I see a very badly adapted animal," she says. "The main way the panda has adapted to the low-quality diet is not via microbiota, like the vast majority of other animals, but by eating 15 hours per day."

This article is reproduced with permission from the magazine Nature. The article was first published on October 17, 2011.