Giant pandas are famous for being militant vegetarians. They stick to an almost exclusive bamboo diet, devouring the stalky grass for 12 to 14 hours a day. However, the panda is in the taxonomic clade Carnivora, and its gut is more similar to that of a carnivore than an herbivore, making the animal an evolutionary head-scratcher.
Now, a new study published in journal Current Biology helps make sense of the black and white animal's strange diet. All that bamboo, it turns out, is high in protein and low in carbs and more similar to a meat-based diet than, say, a grass-eating cow’s menu.
To understand the nutritional composition of the panda’s diet, an international team used tracking collars to follow pandas in China’s Foping National Nature Reserve to record the type of bamboo they eat. Susan Milius at Science News reports that for eight months of the year, the bears gnawed on a lowland bamboo species, eating primarily the high-protein new shoots when they were available. Those shoots are 32 percent protein, compared to just 19 percent in bamboo leaves. In the summer months, the pandas migrated to higher altitudes, eating protein-rich shoots of a different species with a similar nutritional makeup.
When the team collected poop from two of the tracked pandas and analyzed it, they found that their guts were extracting more of the protein from the bamboo and leaving carbs and fat behind. According to a press release, despite their vegan lifestyle, the panda diet approximates that of a hypercarnivore, or an animal who gets more than 70 percent of their food from other animals. About 50 percent of a panda’s energy comes from protein, similar to the energy profile of cats or wolves. Other herbivorous mammals typically only get 20 percent of their energy from protein.
The finding was unexpected. “It was a surprise,” co-author Fuwen Wei of the Chinese Academy of Sciences tells Ed Yong at The Atlantic. “[Nutritionally,] bamboo looks like a kind of meat.”
Yong reports that panda critics have often argued that the bears are an evolutionary mistake, seeing as it is an animal with a carnivore's gut that has to spend most of its time eating nutritionally poor bamboo to survive. Some have argued the animal should be allowed to go extinct, a path they believe it was on before humans negatively impacted their habitat.
But the new study suggests the animals are a beautiful example of evolution transforming their carnivorous ancestors to the roly-poly, black-and-white vegans of today with only relatively small modifications to survive in the bamboo forest. According to the press release, pandas have developed a jaw and teeth designed for chewing bamboo, special “pseudo-thumbs” that help them handle the plant and have lost their ability to sense umami, the flavor of meat. However, they kept their carnivore-style gut and the microbes in it. “[T]here’s no need to evolve out of what might still work,” Carrie Vance of Mississippi State University, not involved in the study tells Milius.
Silvia Pineda-Munoz of the Georgia Institute of Technology tells Yong that the adaptation is similar to the way humans adjust to an all-plant diet.
“The giant panda does what human vegetarians do,” she says. “We have high protein requirements, so we wouldn’t be able to survive if we just ate kale salad. Thus, we choose to eat tofu, beans, nuts, and other plant-based foods that compensate for the protein we aren’t getting from animal products. In the end, vegetarians and non-vegetarians don’t have such different diets when it comes to nutrients.”
Yong reports that the study brings into question the concept of herbivore and carnivore. Another way to categorize animals may be by their nutritional requirements, like the amount of protein, fat and carbs they require, something that we don’t yet understand in many species.
The study could also have an impact on panda conservation. Pandas in captivity are fed a pretty steady diet of bamboo, but many still suffer from irritable bowel disease and digestive problems that make them sick and, possibly, less sexually frisky. This study suggests it’s possible that they’re not getting the right, protein-rich bamboo shoots they need to stay healthy and could benefit from dietary supplements or better sources of bamboo.