BEIJING -- Despite two million years of munching almost exclusively on bamboo, the giant panda's gut has not adapted to eating the plant -- putting the creatures in an "evolutionary dilemma", scientists said Tuesday.

The surprising study, reported by online U.S. journal mBio, examined 45 giant pandas over the course of a year and found that the animals appeared to have a digestive system "entirely differentiated from other herbivores".

Instead, the pandas still retained the gut bacteria of the omnivorous bears they evolved from, the report's summary said.

"Unlike other herbivores 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".

This, the report said, "may adversely influence the coevolutionary fitness of this herbivore".

Giant pandas spend up to 14 hours a day eating 12.5 kilogrammes (27.5 pounds) of bamboo, but can digest only about 17 per cent of what they consume.

"This result is unexpected and quite interesting, because it implies the giant panda's gut microbiota may not have well adapted to its unique diet, and places pandas at an evolutionary dilemma," said study co-author Xiaoyan Pang from Shanghai Jiao Tong University, according to China's state-run Xinhua news agency.

Pandas, whose natural habitat lies in mountainous southwestern China, have a notoriously low reproductive rate and are under pressure from factors such as habitat loss.

China has about 1,600 pandas living in the wild and another 300 held in captivity.