Giant pandas are the new couch potatoes of the animal world and just as sluggish as slow-moving sloths, according to a study published on Thursday in the journal Science.
Researchers in China tracked five captive pandas at Beijing Zoo and three wild ones at Foping Nature Reserve in Shaanxi province.
The study found that pandas are far less active than other bears, expending just 38 per cent of the average daily energy of other bears.
"The daily energy expenditure values for giant pandas are substantially lower than those for koalas, for example, and more akin to those of three-toed sloths," the study said.
The 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 bamboo, which accounts for 99 per cent of their food. Bamboo is tough to digest and pandas must devour lots of it to survive.
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, which is involved in thyroid hormone synthesis, the study said.
The thyroid gland controls metabolic processes, and thyroid hormones are important for regulating body weight and energy. Low levels can lead to sluggishness.
"Giant pandas achieved this low metabolism through a suite of morphological, behavioural, physiological and genetic adaptations during their long evolutionary history," said biologist and lead researcher Fuwen Wei of the Institute of Zoology at the Chinese Academy of Sciences in Beijing.
The researchers said the size of the brain, liver and kidneys in pandas is relatively small compared with 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. When they do move it is at a rate of 20 meters per hour. With their low metabolism, pandas need their coat of thick fur to retain body heat, the researchers said.
The natural habitat of the panda is mountainous southwestern China. The wild population is about 1,600, with another 300 pandas living in captivity.