According to a new study led by Dr Yonggang Nie from the Chinese Academy of Sciences’ Institute of Zoology, bamboo-eating giant pandas (Ailuropoda melanoleuca) have a very low daily energy expenditure (DEE).

In the wild, the diet of giant panda consists almost entirely of bamboo, but because its gut remains designed for digesting meat, this causes the bear huge problems. Bamboo contains very little nutritional value so the bears must eat 10 – 40 kg every day to meet their energy needs.

Until now, how giant pandas survive on this poor diet has been a mystery, but it has been widely speculated that an essential component of their ability to do so is having a low energy demand.

In a new study, Dr Nie and his colleagues from China and the United Kingdom have measured the metabolic rates of pandas living both in captivity and in the wild.

The team studied five captive pandas and three wild ones, discovering that the animal’s DEE was just about 38 percent of the average for a terrestrial mammal with the same body mass. For example, a 90-kg panda expends less than half the energy of a 90-kg human.

“Measurements of DEE across five captive and three wild pandas averaged 5.2 megajoules/day, only 37.7% of the predicted value (13.8 megajoules/day). For the wild pandas, the mean was 6.2 megajoules/day, or 45% of the mammalian expectation,” the scientists said.

“The DEE values for giant pandas are substantially lower than those for koalas and more akin to those of three-toed sloths.”

“Pandas save a lot of energy by being frugal with the energy they spend on physical activity. Using GPS loggers attached to pandas we discovered that they rest for more than half of the day and on average, only traveled at 20 meters an hour,” explained Prof John Speakman from the University of Aberdeen, a co-author on the study.

A big problem with having such a low metabolic rate is keeping warm. Giant pandas have thick fur which traps what body heat they have inside, however this means that the surface temperature of pandas, which the scientists measured with a thermal camera, is much lower than other black and white animals, such as zebras and Dalmatian dogs.

Further research revealed that the giant panda’s brain, liver, and kidney are relatively small compared to other bears, and that its thyroid hormone levels are only a fraction of the mammalian norm – comparable to a hibernating black bear’s hormone levels.

Finally, the team compared the giant panda genome to those of other mammals, identifying a panda-specific variation on the DUOX2 gene, loss of which is associated with 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.

“It has been a real pleasure to be involved in this international collaboration which has revealed some amazing insights into how the panda manages to survive on its bamboo diet. The combination of behavioral and physiological responses that enable this low metabolic rate is fascinating,” Prof Speakman said.

“It is my long-time dream to accomplish this fascinating study to understand how low the giant panda metabolism would be, and why the giant pandas can survive on their specialized and low quality bamboo diet. Finally we have done it,” added study senior author Prof Fuwen Wei from the Chinese Academy of Sciences’ Institute of Zoology.

The results appear online today in the journal Science.

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Yonggang Nie et al. 2015. Exceptionally low daily energy expenditure in the bamboo-eating giant panda. Science, vol. 349, no. 6244, pp. 171-174; doi: 10.1126/science.aab2413