Giant Pandas’ Sluggish Metabolism Enables Bamboo-only Diet

Giant pandas are one the laziest species in the animal world, reported a new study. The research, from the Chinese Academy of Sciences, the University of Aberdeen, and the Beijing Zoo, found the bears are equal in sluggishness to slow-moving sloths.

For the study, published in the journal Science, scientists tracked five captive pandas and three wild ones. Pandas, they found, that are much less active than other kinds of bears, and used up only 38 percent as much daily energy as the average for other land animals of their size.

“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,” wrote the study authors.

Pandas are unique in that they are meat eaters, members of the order Carnivora, yet are entirely herbivorous, living almost entirely on bamboo. But unlike most other plant-eating animals, their digestive tract has not evolved the long twists and turns that allow the slower digestion necessary for cellulose-rich plants.

Panda kidneys, livers, and brains were also found to be smaller than the organs seen in other bears. The study identified a gene mutation in the pandas’ DUOX2 gene, matching one seen in humans with underactive thyroids.

Their thyroid hormone levels, the authors note, “are only a fraction of the mammalian norm—comparable to a hibernating black bear’s hormone levels.”

In combination, the bears’ small organs, genetic adaptations and mellow demeanor enable them to survive on bamboo, concluded the study.

China has about 1,600 giant pandas living in their natural habitat, a mountainous southwestern region of China, and another 300 held in captivity.

Exceptionally low daily energy expenditure in the bamboo-eating giant panda
Yonggang Nie, John R. Speakman, Qi Wu, Chenglin Zhang, Yibo Hu, Maohua Xia, Li Yan, Catherine Hambly, Lu Wang, Wei Wei, Jinguo Zhang, and Fuwen Wei
Science 10 July 2015: 349 (6244), 171-174. DOI:10.1126/science.aab2413

Photo: Kyle Taylor/flickr