Smithsonian.com

Pandas munch on bamboo for most of the day (courtesy of flickr user clurr)

Giant pandas are weird. They have problems mating, for one. And second, though they are technically carnivores (members of Order Carnivora), with the gastrointestinal tract and gut enzymes to match, their diet consists of 80 percent bamboo. A grown panda consumes around 25 pounds of the fibrous plant each day, but it lacks the multiple stomachs and specialized enzymes that help other plant-eating animals, such as cows and sheep, digest cellulose. And cellulose—the fibrous material that makes plant cell walls strong—contains a lot of energy. “If fully degraded, cellulose can contribute nearly half the calories in bamboo,” Fuwen Wei of the Chinese Academy of Sciences told LiveScience.

Wei and his colleagues, thinking that the pandas might be getting a bit of help from some gut microbes, collected poop from seven wild and eight captive pandas in China and studied the genes of the bacteria in it to determine what kinds of bacteria were likely in the pandas’ guts. (Their findings appear in this week’s issue of PNAS.) They found signs of 85 different species of bacteria, but the ones that interested them the most were species in the genus Clostridium. It’s a genus that includes several bacteria that cause human diseases, but the ones from the pandas of a kind that are good at digesting cellulose. “It is highly possible that it is this kind of bacterium [that] plays an essential role in the degradation of cellulose of the giant panda,” Wei told LiveScience.

The pandas also have a few other adaptations that help them survive on their bamboo diet: They have pseudothumbs that help them grip branches, and strong teeth and jaws to help them chew. But the most helpful of all may be the large quantity of bamboo they eat and the large amount of time (15 hours a day) they spend eating it. Even if most of the cellulose and other fibrous material simply passes through their digestive systems (a 1982 study [PDF] of the National Zoo’s pandas found that 92 percent of the cellulose consumed ended up in the pandas’ poop), they are still able to consume enough calories daily to continue their leisurely lifestyle.