What's in and not in giant panda stomachs may be key to survival

It's little wonder that the giant panda is the darling of China and considered a national treasure.

Its rolly-polly, cuddly size, its gentle comical ways, the fashionable charm of its distinctive coat, its low birthrate combined with its endangered status, its lonely solitary life in the wild and its vanishing habitat, all add up to a open-and-shut case for compassion and sympathy. How could anyone not love this rarest member of the bear family?

Efforts to save the iconic creature from extinction have been massive and worldwide and on multitude fronts, from mapping the species' population back 3 million years to exhaustive analysis of its reproductive behavior in captivity. All aimed at figuring out ways to pump some vitality into the bears and make their numbers more robust.

As of last December, 49 giant pandas were living in captivity outside of China in 18 zoos in 13 different countries. About 250 live in captivity in China and estimates put the population in the wild there at about 1,600 individuals.

Now, a group of scientists in China have taken a new look at one of the other quirky things about the giant panda - its diet and, by association, its stomach.

A giant panda's diet consists of 99.9 percent bamboo. But as the panda's exclusive appetite for the jaw-breaking grass evolved over the millennia, its digestive tract did not.

"Unlike other plant-eating animals 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," according to the study's lead author, Professor Zhang Zhihe, director of the Chengdu Research Base of Giant Panda Breeding in China.

"The animals also do not have the genes for plant-digesting enzymes in their own genome," Zhang said. "This combined scenario may have increased their risk for extinction."

It should come as no surprise that giant pandas are continually chewing on bamboo, they said, since they can only digest 17 percent of what they consume. They spend 14 hours a day eating "a remarkable quantity of bamboo, which can reach 12.5 kilograms each day," the study said.

It's been estimated that the ancient ancestral giant panda was, like other members of the Ursidae (bear) family, ate both plants and meat, and started to eat bamboo about 7 million years ago, becoming an exclusively bamboo-eating mammal species about 2 million years ago.

The scientists speculate that the switch was probably brought about by mutations in the panda's genome, including changes to its taste receptor and appetite-reward system genes. And to keep up with the highly specialized food source it evolved powerful jaws and teeth. Other studies have suggested that geographical isolation by two ice ages may have also forced the issue of what was available to eat.

But, unlike other herbivores who along with such adaptations also evolved elongated digestive tracts to get the maximum nutrition out of the high-fiber diet, the panda kept its relatively simple, faster-acting stomach.

"Despite the dietary switch, the giant panda did not evolve any enzymes for bamboo digestion" while retaining those for digesting meat, the study said.

The study started with the scientists' simple curiosity over how the giant panda digested bamboo fiber and extracted nutrients from it and their results are "unexpected and quite interesting, because it implies the giant panda's gut microbiota may not have well adapted to its unique diet and places panda at an evolutionary dilemma," study co-author Associate Professor Pang Xiaoyan of the School of Life Sciences and Biotechnology at Shanghai Jiao Tong University, told Asian Scientist.

Contact the writer at chrisdavis@chinadailyusa.com.