Giant pandas only eat bamboo, a low-nutrient food they can barely digest. So how can these massive creatures exist on such a terrible diet?

By being big ol' sacks of lazy, basically. That's at least the prism from which I view the latest findings on giant panda metabolism researchers published this week in the journal Science. (Disclosure: I have feelings on giant pandas.)

[Why giant pandas have to eat and poop all day]

The researchers tracked the daily energy expenditure (DEE) of five captive and three wild giant pandas, and found the creatures don't move around much. The giant pandas' energy measurements "are among the lowest, relative to body mass, ever made" for mammals not in torpor, which is sort of like suspended animation, the authors write.

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

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The captive pandas studied spent a third of their time being physically active, while the wild pandas spent about half of their time being active.

[Just asking: The National Zoo’s Bill McShea on pandas]

Giant pandas, from an energy perspective, are close to sloths.

Aside from not moving around as much as mammals of their stature, other features contribute to the giant panda's low metabolic rates, including thyroid hormone levels comparable to a hibernating black bear. Relatively small brains, livers and kidneys "probably contribute to their low energy demands," the authors write.

While some animals turn to daily stretches of torpor or hibernation to reach low metabolism rates, pandas don't. Nope. They can sustain high body temperatures because of their thick coat of fur, the researchers suspected.

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Pandas actually used to eat both meat and plants until they began their bamboo-exclusive diet about 2 million years ago, and their guts haven't adapted to the new menu yet. A giant panda's gastrointestinal tract is similar to that of a carnivore.

Another study published this year in the journal mBio zeroed in on just how poor of a job pandas do in digesting bamboo. Those researchers found the animals digest about 17 percent of the nearly 30 pounds of bamboo they eat throughout the day -- which means a lot of sitting around, eating and pooping.

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