For a bear with a stomach of carnivorous, modern pandas have set a bizarre niche in the world of eating plants.
Today's gigantic pandas (Ailuropoda melanoleuca) live in the cool mountains of southwestern China, where the days are almost exclusively bamboo-eating.
But their ancient ancestors, living at least 5,000 years ago, had a much more diverse diet of bamboo and other plants, according to a new study published today in Current Biology.
"We do not know which are the exact plants, but their diet was similar to herbivores as deer at the time," said lead author of the study, Fouwe Wei of the Chinese Academy of Science.
Modern pandas have distinctive teeth, skull and muscles and a specialized toe to bend and break between 12-38 kilograms of bamboo every day.
But they have a short stomach and a microbial which is not well suited for digestion of plant material, indicating that they originally developed from a disappeared carnivorous ancestor.
Previous fossil evidence suggests that the panda family began to flourish for plants about 2 million years ago.
At that time, the ancestors of today's Pandas lived through the much wider spectrum of China and Southeast Asia; from Beijing to the north, to Myanmar, northern Vietnam, Laos and Thailand to the south.
To understand the diet of extinct species, Dr. Wei and his colleagues analyzed chemical signatures in the bones of 12 ancient pandas that lived for up to 5,000 years.
They compared this with carbon and nitrogen rates from collagen and teeth from modern pandas and other mammals from the same area gathered between the 1970s and 2000s.
The analysis showed that all pandas species lived on a pure C3 plant nutrition – the most common group of plants typical of forests – over the past 2 million years.
But the nitrogen isotopes in the bones and teeth of modern and ancient groups were quite different.
"The modern panda that feeds on bamboo has had very low nitrogen isotopes, but ancient pandas have had very high levels like herbivores," says Dr. Wei.
This suggested that ancient pandas had a much more complicated diet than today.
They also said that they may have lived in many different dwellings, such as subtropical zones and forest fires, supporting archaeological records from southern to northern China where fossils were found.
Switch to 99 percent bamboo
It's unclear when and why the giant pandas became almost exclusively bamboo eaters – though with a strange little grass or meat on the menu in the wild.
The first description of their bamboo diet is only a few hundred years, but researchers suspect that the change took place about 5,000 years ago.
To find out, they hope to learn more panda fossils.
"We need to get more samples from different years 5000 years ago, but it's hard to do this," said Dr. Wei.
Dr Wei said that it is possible for the switch to come as an adaptation to the reduction in bandwidth, "but we do not know the exact reasons."
"It may be complicated [mix of] climate change, human endangerment, and the competitiveness of resource types, "said Dr. Wei.
Bamboo provides pandas with a constant source of food that can be used by very few other animals, says David Raubenheyer of the University of Sydney, who was not involved in this study, but previously explored the animal's diet habits.
However, not all bamboos are equal.
"It's not a matter of them just eating anything in front of them," said Professor Raubenheimer.
"We now have evidence that they carefully choose among the various options available to them from the sea [of bamboo] in which they live ".
This evidence shows that pandas must migrate to pick the bamboo that satisfies their seasonal nutritional needs.
"They migrate in the summer to get high protein doses, and then migrate to low-protein habitats in winter, but high calcium is needed for reproduction," said Professor Raubenheimer.
"Probably ancestrally that is not necessary, because in every dwelling they had a wider diet because they had protein in some food and calcium than others."
Today's giant pandas, which are listed as vulnerable, live on various types of bamboo scattered through 20 isolated patches in six mountain ranges in the provinces of Sichuan, Shaanxi and Gansu.
In a world of shrinking habitats, being a cautious nuisance is also a blessing and a curse.