Why are China's giant pandas so lazy?

They eat and lie around. Then they eat some more, and maybe lie around some more. 

But giant pandas aren't just lazy – their low-key lifestyle serves a purpose.

A study published in Science on Friday reveals that giant pandas get very little energy from their diet, so their metabolic rates are low.

After monitoring the hourly behavior of three wild and five captive pandas, the China-based research team found that pandas use only a third to a half as much energy as other large mammals. The captive pandas’ daily energy expenditure was 38 percent of what would be expected for large mammals, while wild pandas used 45 percent.

The problem is their diet: bamboo. Like celery, bamboo is low in calories and high in fiber, with the result that pandas have little energy to burn.

To meet their energy needs, pandas must eat up to 83 pounds of bamboo every day – and that still gives them barely enough energy to move to the next clump of bamboo.

"Giant pandas have exceptionally low [daily energy expenditure], which may facilitate survival on their diet of bamboo," the authors write.

That explains pandas' lazy lifestyle.

In addition, pandas have a smaller brain, liver, and kidney than other mammals their size, as well as under-active thyroid glands. As Sci-News notes, "its thyroid hormone levels are only a fraction of the mammalian norm – comparable to a hibernating black bear’s hormone levels."

The pandas' low metabolic rate does not mean that they are always cold; their thick coat of fur helps them maintain high body temperature.

Ancestors of today’s pandas ate both meat and plants, but they gradually switched to their almost bamboo-exclusive diet between 7 million and 2 million years ago.

While modern pandas’ daily diet consists almost entirely of various bamboo species, about one percent of their diet includes other plants and even meat, such as pikas and small rodents, according to the World Wildlife Fund (WWF).

Pandas love bamboos even though they can not digest it very efficiently. A study published in May found that pandas digest about 17 percent of the bamboos they eat throughout the day.

According to WWF's latest census, in 2014 there were 1,864 giant pandas in the wild, an increase from around 1,000 in the late 1970s. In the past decade, giant panda numbers have risen by 17 percent, but they are still considered an endangered species, threatened by habitat loss and fragmentation, as well as hunting.

The giant panda’s habitat once stretched throughout southern and eastern China, as well as Myanmar and northern Vietnam, but they now live only in zoos and in six mountain ranges in China, WWF reports.