Giant pandas give bamboo two thumbs up

Who loves bamboo and has two thumbs? Giant pandas!

The giant panda (Ailuropoda melanoleuca), a member of the order Carnivora, has adapted to the extreme lifestyle of eating almost exclusively bamboo.

While most mammals have five digits on each hand, the giant panda has evolved an enlarged wrist bone, called the radial sesamoid, that acts like a sixth digit and second opposable thumb. This was thought to enable the giant panda to better manipulate bamboo.

A new fossil specimen of the sesamoid “thumb” of the ancient panda relative Ailurarctos has been discovered in the late Miocene deposit (between six and seven million years old) of Shuitangba in Yunnan Province, China. Along with isolated teeth and a partial humerus, this represents the oldest evidence in the world of this adaption.

This indicates that giant pandas are likely to have been specialised bamboo eaters for millions of years.

Compared to the other members of the bear family (Ursidae), the panda is quite special in becoming a dedicated vegetarian, and even has a greatly altered gut microbiome. Because of this trade-off for high-protein diet to low nutritional high-fibre bamboo, the panda consumes up to 45kg per day, and spends about 15 hours per day just eating. Because carnivores have shorter digestive tracts, the panda also rapidly but poorly digests its food, absorbing less than 20% of the nutrients, and defecates up to 100 times per day.

While an older sesamoid fossil of the extinct ailuropodine bear Indarctos arctoides that lived 9 million years ago has been described, the new Ailurarctos specimen is much more similar tothe modern giant panda. Both the giant panda and Ailurarctos specimens are larger, wider and more hooked, while the Indarctos specimen lacked these characteristics that indicate the thumb-like sixth digit adaptation.

Since the Miocene, the sesamoid “thumb” has not enlarged further, possibly because it is constrained by its function in balance and weight bearing when the giant panda walks on its plantigrade (palm down) posture. This weight distribution function may also be the reason why the panda’s crude sesamoid “thumb” never evolved into a full digit.

This research comes out of the Natural History Museum of Los Angeles County (US). A description and analysis of these new fossil findings have been published in Scientific Reports.