Giant pandas are a favorite of zoo-goers and seekers of general cuteness on the Internet, but for biologists looking at dietary habits, they represent a great mystery.
Pandas' diet is almost exclusively bamboo. But in theory, they should not be able to digest cellulose and hemicellulose, the main components of bamboo. These organic compounds are used in the metabolism of carbohydrates, which give pandas their basic energy, said Fuwen Wei, deputy director of the Institute of Zoology at the Chinese Academy of Sciences.
Pandas don't have the enzymes required to digest these compounds, since their gastrointestinal tracts are more like those of carnivores than herbivores. Researchers reasoned that there must be organisms living in the panda’s gut to help out.
For the first time, Wei and colleagues have identified gut microbes that allow pandas to digest cellulose and hemicellulose. Their results are published in the journal Proceedings of the National Academy of Sciences.
Using gene-sequencing techniques, scientists scoured more than 5,000 ribosomal RNA sequences corresponding to microbes that turn up in wild and captive giant pandas’ fecal samples. They then looked at how those sequences compare to what’s found in herbivores.
It appears that giant pandas have gut microbes similar to clostridium bacteria, which digest cellulose.
"This would help to further understand the nutritional ecology of this species," Wei said.
These microorganisms that help the panda eat bamboo are just part of the story of how pandas have adapted to this diet. They also have large molar teeth, strong chewing muscles, and a lot of mucus in the digestive tract. They have “pseudothumbs,” or bones that allow for grasping bamboo stems – an extraordinary manipulation system among mammals.
And they eat a lot – 20 to 40 pounds of bamboo a day – to make up for inefficient digestion, with a relatively brief digestion time. (Pandas spend 10 to 16 hours a day foraging and eating.) All of these traits have allowed the giant panda to eat a bamboo diet while maintaining a digestive system that’s closer to a carnivore’s than a herbivore’s.
Pandas are omnivorous, meaning they can eat meat too - and sometimes they will eat small rodents or musk deer fawns. But about 99% of the panda's food is bamboo, Wei said in an e-mail. "In the wild, we rarely find bones and hairs of other animals in their fresh feces," he said.
The panda's earliest known ancestor, Ailuropoda microta, which lived about 2 million years ago, appears to have evolved to handle a vegetarian bamboo diet, too.