What makes up almost the entire diet of a giant panda?

Bamboo accounts for almost everything a giant panda eats, which is a strange position for an animal whose body is still built like a bear's. Its digestive tract is short and simple, the gut of a carnivore, with none of the long fermenting chambers that let a cow or a deer break down tough plant fibre efficiently. The panda extracts only a small fraction of the energy in what it swallows, and compensates by eating for many hours a day and passing food through quickly. Two adaptations help. An extended wrist bone works as a crude opposable thumb for gripping stems, and the skull carries huge jaw muscles and broad flat molars for crushing them. Bamboo is a risky staple because a species flowers and dies off together across a whole region at long intervals, which is one reason pandas historically needed access to several bamboo types and large connected areas of forest.

Panda gut bacteria have been found to include species that break down cellulose, partly offsetting the unsuitable digestive tract.