Giant pandas may have only recently switched to eating mostly bamboo

When it comes to deciding what’s for breakfast, lunch or dinner, pandas have it easy: Bamboo, bamboo and more bamboo. But that wasn’t always the case.

Although modern giant pandas (Ailuropoda melanoleuca) chow almost exclusively on bamboo in the mountain forests of central China, these bears’ diet was much broader not so long ago, researchers report online January 31 in Current Biology. Analyses using chemical signatures from bones and teeth of both ancient and modern pandas indicate the bears’ hyperdependence on bamboo could have developed as recently as about 5,000 years ago. That’s roughly 2 million years later than previously assumed from molecular and paleontological data.

 “It has been widely accepted that giant pandas have exclusively fed on bamboo for a long time, but our results show the opposite,” says Fuwen Wei, a wildlife ecologist at the Chinese Academy of Sciences’ Institute of Zoology in Beijing. “That made us excited.”

Wei and his colleagues compared the relative abundance of isotopes — atoms of the same element but with a different number of neutrons in the nucleus — in modern and fossil animals, including pandas. Animal diets contain different amounts of naturally occurring “heavy” and “light” isotopes of carbon, oxygen and nitrogen that are then incorporated into bones, hair, nails and teeth. The ratio in which the isotopes occur in the body depends on the animal’s position in the food chain and the climate in which the animal lives. Carnivores, for example, have a higher abundance of the heavy nitrogen-15 isotope because they almost exclusively consume meat, which is made of nitrogen-rich amino acids. And animals living in cold, dry places tend to consume a higher abundance of the heavy oxygen-18 isotope in what they eat and drink because the heavy isotope doesn’t evaporate from that environment as easily as it does in warm, wet conditions.

“Proxies like isotopes are actually able to capture what an animal is doing during its lifetime,” says vertebrate paleontologist Larisa DeSantis at Vanderbilt University in Nashville who was not involved in the study.

Because of their bamboo diet, modern pandas had much lower ratio of heavy-to-light nitrogen isotopes than other herbivores or carnivores living among the bears, Wei’s team found. But when isotopes in panda bone collagen from the mid-Holocene Epoch some 5,000 years ago were compared with those in other animal bones from the same time, pandas were indistinguishable from other herbivores, indicating the bears’ diet wasn’t as specialized as it is today.

The team then examined the oxygen and carbon isotopes of panda teeth dating to the late Pliocene Epoch around 2.6 million years ago. Pandas had a much larger variation in oxygen isotope abundances than they do now, suggesting the bears may have adapted to not only the cool, moist habitats they’re restricted to today, but various other environments, such as hot, dry climates outside of forests, the researchers say. Carbon isotope abundances consistent with a strictly plant diet also indicated that pandas may have already transitioned from the omnivory of their bear ancestors to vegetarianism. 

Given their bear ancestry, it’s not surprising pandas’ diets were once larger, DeSantis says. “But it is very interesting that these researchers were able to document high levels of diet variability in ancient populations as compared to modern ones.”

What factors later coaxed pandas to their strict bamboo diet remain unknown, Wei says. The bears  have specialized teeth that make chewing tough bamboo easier and a modified wrist bone called a pseudothumb that allows them grasp bamboo stalks, though it’s unclear when these traits developed. Determining exactly when pandas switched almost exclusively to bamboo is the next goal, Wei says.