Pandas Are as Sluggish as Sloths, Study Shows

Panda cub Bao Bao chewing on—what else?—bamboo while playing at the Smithsonian National Zoo in Washington, D.C. Pandas expend very little energy for their size. Gary Cameron / REUTERS

Giant pandas are very bad at digesting bamboo. That's why they spend up to 14 hours eating as much as 27 pounds of the plant every day; they have to, to get enough energy. Their gut microbes more closely resemble those of a carnivore than an exclusive vegetarian, and are inefficient at breaking down fibrous plant matter.

But there's something else that allows them to survive in their own very special panda way: They use very little energy. One might even be tempted to call them lazy.

A study published July 9 in Science finds that the bears expend only a little over one-third of the energy that would be expected for an animal of their size. Their activity levels resemble that of a three-toed sloth, those lords of lethargy, and they use about one-third less energy than koalas and echidnas, which are both known to be rather sluggish as well.

"I have been working on the giant pandas...for 30 years, and it is my longtime dream to understand how low the giant panda metabolism would be," says the study's lead author, Fuwen Wei, a researcher at the Chinese Academy of Sciences. "Finally we did it, and found that the giant pandas have exceptionally low daily energy expenditure so that they could survive on this highly fibrous bamboo diet."

The scientists found that panda bears have relatively low circulating levels of thyroid hormones. In all mammals these chemicals regulate various aspects of bodily functions, such as digestion rate and activity level. In pandas, the concentration of the hormone thyroxine—one of the main thyroid hormones— was found at only 47 percent of what would be expected based on the animal's size, according to the study.

Wei says that despite these apparent physical handicaps, pandas are actually well adapted to their station in life—chewing bamboo all day and being adorable—by the mere fact that they are still around. While they are critically endangered, Wei's research has shown that this is due to climate shifts and human activities, such as habitat destruction. Wei does not think their low activity levels and inefficient digestive abilities "could contribute to their low population size," he says.