The mystery of why giant pandas have to spend 14 hours a day eating has been solved - they can digest only about 17 per cent of the 12kg of bamboo leaves and stems they swallow each day.
Researchers have found that pandas' gut bacteria cannot efficiently break down bamboo.
The animals are, in fact, best suited to eating meat, as they have a carnivore-like gut, according to the research published in the journal mBio.
Pandas are one of the most endangered species on Earth, with only 1600 left in the wild.
The study's lead author, Zhihe Zhang, director of the Chengdu Research Base of Giant Panda Breeding in China, said: "Unlike other plant-eating animals that have successfully evolved anatomically specialised 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 of extinction."
The researchers said giant pandas evolved from bears that ate both plants and meat, and started eating bamboo exclusively about two million years ago.