Pandas seek out bamboo that's high in protein.Mike Blake, Reuters

The giant panda, a consummate vegetarian, belongs to a group of mammals called Carnivora, so-called because almost all of them—dogs, cats, hyenas, weasels, mongooses, raccoons, and more—eat meat. But the giant panda’s diet of bamboo, and little else, makes it a vegetarian.

At least, outwardly.

Yonggang Nie and Fuwen Wei of the Chinese Academy of Sciences have spent years tracking wild pandas, analyzing exactly what kinds of bamboo they eat, and measuring the chemicals within those mouthfuls. And they found that the nutrient profile of a panda’s all-bamboo diet—very high in protein, and low in carbohydrates—is much closer to that of a typical carnivore than to that of other plant-eating mammals. “It was a surprise,” Wei says. Nutritionally, “bamboo looks like a kind of meat.”

In other words, “the giant panda does what human vegetarians do,” says Silvia Pineda-Munoz of the Georgia Institute of Technology. “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 nonvegetarians don’t have such different diets when it comes to nutrients.” And so it is with China’s black-and-white bear.  

This discovery explains some puzzling parts of panda biology. The panda’s ancestors switched to a vegetarian diet more than 2 million years ago. In that time, the panda has evolved stronger jaws for chewing tough, fibrous mouthfuls, and one of its wristbones has become a false thumb, for gripping bamboo stems. But despite these superficial hardware changes, it still has a meat eater’s digestive system.

Plant-eating mammals almost always have enlarged, elongated guts to slow the passage of food, and to give their inner bacteria more time to digest their meals. The panda, however, has the short, vanilla gut of a carnivore. Even its gut microbes are closer to a bear’s than, say, a cow’s or deer’s. Nie and Wei’s study makes sense of this paradoxical combination of traits. The giant panda has the plumbing of a half-committed herbivore because it has the diet of a closet carnivore.

The team used tracking collars to follow pandas in China’s Foping National Nature Reserve, which harbors the highest density of these bears in the world. The pandas, it turned out, migrate over long distances to exploit the shoots and leaves of two bamboo species, which grow at different altitudes. Every year, the bears cycle from low-growing leaves, to low-growing shoots, to high-growing shoots, to high-growing leaves, and back again. The team analyzed these varied mouthfuls and determined that the pandas’ decisions seem largely motivated by protein. They’re always selecting the species and tissues that offer the most protein and the least fiber.

Their selective tastes mean that at least 50 percent of their energy comes from protein, while just 39 percent comes from carbohydrates, and 13 percent from fat. That’s comparable to feral cats and wolves, which also get half their energy from protein. And it’s starkly different from other plant-eating mammals, which typically get 20 percent of their energy from protein.

Panda poop, which the team also collected and analyzed, told the same story. So did panda milk. Nutritionally, it stands apart from most herbivore milk, and falls in with typical carnivore milk.

This suggests that the move from meat to plants might have been easier for ancestral pandas than commonly assumed. By simply choosing parts of plants that are richer in protein, they could switch to vegetarianism without needing to radically overhaul their bodies. “If you’re going to switch to a specific plant, bamboo isn’t too bad, as it does have respectable plant protein levels, as well as a swath of different vitamins,” says Garret Suen of the University of Wisconsin at Madison.

These results should help to counter the tiresome myth that pandas are evolutionary dead ends: lazy, poorly adapted creatures that eat deficient diets, are inept at sex, and should be allowed to go extinct. Nonsense. Pandas have beautifully adapted to eat an extremely plentiful food source—bamboo—and they go to great, careful lengths to get exactly the right balance of nutrients.

Perhaps by felling large expanses of China’s bamboo forests, humans have disrupted the panda’s ability to find the specific protein-rich morsels that it needs. And perhaps captive pandas are so famously prone to digestive problems, and loath to breed, because they’re not being fed the right kinds of bamboo.

Pandas aside, Nie and Wei’s study should also make us rethink how we classify animals. Terms such as herbivore and carnivore can be misleading if they only account for what a species is eating, and not the nutrients that it’s actually using for energy. The same goes for labels such as generalist or specialist. The former implies versatility, and the latter, inflexibility. But a generalist animal might eat a wide range of foods precisely because it needs to keep its nutritional levels within narrow boundaries. Its versatility in choices might reflect a hidden, underlying inflexibility.  

Samantha Price of Clemson University wants to know what kind of nutrient levels other bear species shoot for, especially because their diets are so varied. “Sloth bears predominantly eat insects; spectacled bears predominantly eat plants, especially bromeliads; sun bears eat fruit and insects; polar bears rely on marine mammals; while grizzly bears [and] American and Asiatic black bears are omnivorous and will eat fruits, seeds, leaves, insects, and mammals,” she says. Do they all resemble the panda, or do they differ?

Even in these species, appearances can be deceiving. Black and brown bears in the U.S. “have a diet that is about 80 percent vegetation,” Pineda-Munoz says. “During the summer, they load [up] on animal protein for a few weeks, but in general they are herbivores. Diet is more complex than we think.”

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