The giant panda — the rarest member of the bear family, a charismatic global symbol of conservation, and one of China’s most successful global brands — eats bamboo all day long, but has the digestive system of a carnivore. Giant pandas are, indeed, taxonomically classified as carnivores. Their ancestors were omnivorous and, about 7 million years ago, started to develop an interest in bamboo. The estimate is that they switched to an almost exclusive bamboo diet 2.4 million to 2 million years ago while, at the same time, developing strong jaws, and a pseudothumb that makes them not only expert grinders and kneaders with their mouths, but also skillful bamboo handlers.
Giant pandas eat different bamboo types and different plant parts (leaves, stems and shoots) at different times of the year — leaves and stems all year long, and fresh, soft bamboo shoots in spring and summer. However, they consume large quantities of bamboo every day, up to fifty pounds — or even more. Because pandas lack the long intestinal tract characteristic of other herbivores, researchers wanted to understand how these bears digest bamboo cellulose — the main constituent of the plant cell walls and vegetable fibers — and how they extract nutrients from it. Now, results from a new study show that, in giant pandas, the gut microbiome — the trillions of microorganisms that help with digestion (among other functions) — is optimized for meat consumption.
To study the panda’s gut microbiome, researchers in China sequenced ribosomal RNA in 121 poop samples collected from 45 captive individuals of different ages, at different times — spring, summer, and late autumn — over the course of a year. Then, they compared the microbes found in pandas with those found in other mammals — herbivores, omnivores, and phylogenetically related carnivores.
The researchers found that the giant panda gut microbiota is low in diversity, and highly variable across seasons. None of the cellulose-degrading bacteria typically seen in other plant-eaters were present in panda guts. Instead, the microbiome consisted mostly of Escherichia, Shigella and Streptococcus bacteria, which are normally found in carnivores.
Zhihe Zhang, director of the Chengdu Research Base of Giant Panda Breeding and lead study author, said in a press release: “Unlike other plant-eating animals 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. The animals also do not have the genes for plant-digesting enzymes in their own genome. This combined scenario may have increased their risk for extinction.”
According to the Conservation Biological Institute of the Smithsonian National Zoological Park, giant pandas live in a few mountain ranges in central China, in Sichuan, Shaanxi, and Gansu provinces. They once lived in lowland areas, but farming, forest clearing, and other development now restrict giant pandas to the mountains. There are only about 1,600 giant pandas left in the wild, while more than 300 live in zoos and breeding centers around the world, mostly in China.
Jonathan Eisen, a microbial ecologist at the University of California, Davis, told Nature that some of the microbes in the panda gut might still be highly efficient at breaking down cellulose. The study’s authors examined only microbial composition, not function. He said that microbes can change function rapidly, making it hard to predict how the microbiome may perform.
Indeed, results from a study published in 2011 showed presence, in panda guts, of microbes able to digest cellulose and break it down into simple sugars. The researchers that carried out the 2011 study thought that, in combination with adaptations related to feeding, physiology, and morphology, giant pandas could have evolved a number of traits to overcome the anatomical and physiological challenge of digesting a diet high in fibrous matter.
Zhihe Zhang and his collaborators are planning a follow-up study combining different scientific techniques to more fully understand the function of the panda’s gut microbiota on the animals’ nutrition and health.