Giant pandas eat vegetables even though their bodies are better equipped to eat meat. So how do these black-and-white bears from the remote, misty mountains of central China survive on a diet almost exclusively of a low-nutrient food like bamboo?
Scientists on Thursday provided an answer. They found that the giant panda is downright miserly in the amount of energy it expends on a daily basis - a level similar to that of the famously sluggish three-toed sloth - thanks to low levels of physical activity and low levels of thyroid hormones.
The critically endangered panda is the only one of the world's eight bear species with a vegetarian diet. Pandas, whose ancestors were carnivores, possess a digestive system that evolved to handle a meat diet even though bamboo now makes up about 99 percent of their food. Bamboo is tough to digest and the pandas must devour lots of it to survive.
The researchers studied three wild pandas at Foping Nature Reserve in Shaanxi province and five captive pandas at the Beijing Zoo. They found that the daily energy expenditure of these bears was only about 38 percent of other similar-sized animals.
A key to the panda's remarkably low metabolism is the fact that it boasts extremely low levels of thyroid hormones, most likely due to a mutation in a gene called DUOX2 involved in thyroid hormone synthesis, the researchers said. The thyroid gland controls metabolic processes including energy use.
"Giant pandas achieved this low metabolism through a suite of morphological, behavioral, physiological and genetic adaptations during their long evolutionary history," said biologist Fuwen Wei of the Chinese Academy of Sciences' Institute of Zoology in Beijing, who led the study published in the journal Science.
The researchers said the size of the panda's brain, liver and kidney is relatively small compared to other bears.
"These reduced organ sizes likely contribute to their low energy demands," Wei said.
The researchers found that the wild pandas rested for more than half of any given day and traveled only about 65 feet (20 meters) per hour. With their low metabolism, the researchers said, the panda needs its coat of thick fur to retain body heat in order to keep warm.
There are only about 1,800 giant pandas left in the wild, according to the World Wildlife Fund, citing Chinese government figures.