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.