On the outside, giant pandas look like herbivores. They spend nearly all of their waking hours eating bamboo.
But on the inside, they’re built like carnivores. About half of the calories they eat come from protein, according to a new study.
That puts the giant panda diet on a par with wolves, feral cats and other animals that depend on meat to survive, the study authors said. A typical herbivore, on the other hand, gets less than a quarter of its calories from protein.
“We were absolutely amazed,” said David Raubenheimer, a nutritional ecologist at the University of Sydney and senior author of the study published Thursday in the journal Current Biology. “It looks like no other herbivore that we know of — none whatsoever. But it looks astonishingly similar to a carnivore.”
Long ago, the ancestors of giant pandas were omnivorous. They ate both animals and plants, and had the digestive system and gut bacteria to metabolize them. They had umami taste receptors, to appreciate the savory flavors of meat.
But sometime between 2.4 million and 2 million years ago, things began to change. The gene for their umami taste receptor became inactive. Their jaw and teeth evolved to help them crush bamboo, and their wrist bone became something of an extra digit — a “pseudo-thumb” — to help them grasp the stalks of their favorite plants.
Scientists think the iconic black and white bears switched to eating bamboo in part because it’s extremely abundant and they don’t have to fight with other animals to get it. Bamboo is high in fiber but has a low concentration of nutrients, so pandas have to eat 20 to 40 pounds of the stuff every day just to get by.
Raubenheimer and his colleagues wanted to learn more about this extreme herbivore way of life. So they put GPS trackers on two giant pandas and followed their movements throughout the year.
They discovered that the pandas followed the protein. Between August and April, they foraged in low elevations on China’s Qinling Mountains. At the start of the cycle, they ate Bashania fargesii leaves until they got the chance to feast on young shoots, which contained more protein. The more the shoots grew, the more their protein was diluted by fiber.
That prompted the pandas to move to higher ground, where Fargesia qinlingensis grew. First they ate the shoots, but these, too, went from being protein-rich to fiber-rich as they grew. The pandas responded by switching to the leaves. These tided them over until they went back down the mountain and resumed eating B. fargesii leaves.
The researchers found that about half of the calories the pandas ate were in the form of protein (48%). That was about the same as the calories from fat and carbohydrates combined (52%).
Although the pandas chewed through so much protein, the researchers didn’t assume the animals actually digested it all. So they collected and analyzed the pandas’ manure .
For the most part, the ratio of protein to fat and carbohydrates in the feces was similar to, or lower than, the ratio in the bamboo. That meant the bears were absorbing and using the protein they worked so hard to find.
”They weren’t defecating out the protein,” he said.
Pandas’ penchant for protein begins early in life, while cubs are still nursing. The researchers analyzed panda milk and found that about 20% of its calories came in the form of protein. That’s about the same as for carnivores that live on land.
Silvia Pineda-Munoz, a diet ecology researcher at Georgia Tech, said the findings show that pandas are resourceful.
”They can know exactly where to go, [and] when to go, so they can get the most of the nutrients that their ecosystem can provide,” said Pineda-Munoz, who was not involved in the study.
The work also shows that classifying an animal as herbivore or carnivore is more complex than one might assume.
“It’s not whether you’re eating plants,” she said, “but what part of the plant you’re eating.”