Is the giant panda a poorly evolved vegetarian? Or highly specialized, well-designed herbivore, a living link to a time when all animals were vegetarians? Despite having teeth, jaws, and a short digestive tract typical of the more carnivorous members of the bear family, the giant panda depends on a bamboo diet. Though occasionally enjoying a meaty meal,1 the giant panda subsists primarily on a daily diet of 20 to 30 pounds of bamboo leaves, stems, and shoots. The giant panda is an endangered species, and its vulnerability to habitat destruction makes understanding of how it meets its nutritional needs of great importance if it is to be protected rather than allowed to succumb to the evolutionary ideal of survival of the fittest.2
Survival of the Veggie Bear
Scientists have long puzzled how the giant panda survives on its bamboo diet. It digests only about 17% of its cellulose-rich but protein-poor diet, and it must spend most of its time eating in order to consume 12-15% of its body weight in bamboo daily. Most grass-eating mammals have long digestive tracts populated by various cellulose-digesting microbes. The giant panda is notably lacking in the usual cellulose-digesting bacteria, and its digestive tract is too short to take full advantage of any microbial helpers that might be there. Some possible cellulose-digesting bacteria — Clostridia species — were identified in the panda gut as possible helpful agents in a 2011 study, but a new study of the panda's abundant output casts doubt on this. A team of scientists from China's Chengdu Research Base of Giant Panda Breeding confirms in the journal mBio that the giant panda’s gut is inhabited by pretty much the same microbes that live in the gut of carnivores, and those vary seasonally.4 Even the Clostridia species that the giant pandas in their study harbor are not consistently those associated with cellulose digestion.5 Lead author Zhihe Zhang explains the panda's problem:
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. The animals also do not have the genes for plant-digesting enzymes in their own genome. This combined scenario may have increased their risk for extinction.