Giant pandas have the wrong type of gut bacteria for efficiently digesting bamboo, scientists suggest.
Despite spending up to 14 hours per day munching about 12.5 kg (27.5 lbs) of the plants' stems and leaves, the animals can only digest about 17 per cent of what they consume.
Scientists in China have shown giant pandas' gut microbiota - the microscopic plant and animal life naturally found in the intestine - have not seemingly adapted to deal with bamboo.
Giant pandas have eaten an almost exclusive bamboo diet for about 2 million years, but they evolved from bears that ate both plants and meat.
Previous research has suggested the bears' digestive systems are better suited to a carnivorous diet, leaving researchers intrigued by how the animals actually digest and gain nourishment from bamboo.
It was thought that pandas' gut microbiota, which aid in the digestion and absorption of nutrients, may have shown a specialisation towards a herbivorous diet.
However, the results of the new study found the bears' gut microbiota is not made up of plant-degrading bacteria such as Ruminococcaceae and Bacteroides, but other bacteria such as Escherichia and Streptococcus.
"Unlike other plant-eating animals that have successfully evolved anatomically specialised digestive systems to effectively deconstruct fibrous plant matter, the giant panda still retains a gastrointestinal tract typical of carnivores," said research team member Dr Zhihe Zhang from the Chengdu Research Base of Giant Panda Breeding, China.
Furthermore, pandas do not have the genes for producing plant-digesting enzymes.
"This combined scenario may have increased their risk of extinction," said Dr Zhang.
Details of their findings are published in mBio, an online journal published by the American Society for Microbiology.
To carry out the research, the team examined 121 droppings from 45 captive giant pandas, using a laboratory sequencing method to evaluate gut microbiota.
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