Scientists have determined giant pandas' underactive thyroid glands are the secret of their ability to survive on only bamboo.

These bears do not have stomachs designed to digest the harsh low-nutrient, high-cellulose plants, but still somehow manage to survive on a diet that is almost exclusively made up of bamboo, the American Association for the Advancement of Science reported. The recent study is the first to report measurements of daily energy expenditure (DEE) in bamboo-munching pandas.

A team of researchers studied five captive pandas and three wild ones, and found their DEE was only 38 percent of the average seen in terrestrial mammals of similar body masses. For example, the DEE for pandas is significantly lower than what is seen in koalas, but is closer to measurements taken of sluggish three-toed sloths.

The researchers used GPS loggers to track the movement of bears, and found pandas are much lazier than their close relatives. Further research revealed a panda's brain, liver, and kidneys are relatively smaller than most other bear species'. The panda also has thyroid hormone levels that are only a fraction of the "mammalian norm," and is comparable to hormone levels seen in hibernating black bears.

The researchers compared the giant pandas' genome to those of other mammals, and pinpointed a unique variation on the DUOX2 gene. Loss of this gene is linked to underactive thyroids in humans.

"Taken together, these results suggest that particularly low energy expenditures and thyroid hormone levels enable the carnivorous-looking panda bears to munch on bamboo all day," the researchers concluded.

The findings were published in a recent edition of the journal Science.