As a rule of thumb, it is easier to grip a stick if you have… well… an opposable thumb. Yet, the adorable fuzzy giant pandas can grip bamboo and chomp on it to their delight, despite never evolving these digits. Instead, they have a radial sesamoid, which is a thumb-like protrusion from the wrist bone, giving them a sixth digit.
This is a unique adaptation and is part of the reason why pandas can eat bamboo – despite other bears being able to eat meat. A paper published in the Scientific Reports looked into a fossil of a bamboo-eating ancestral panda and reports the earliest enlarged radial sesamoid that acted as a “thumb”-like digit.
“Deep in the bamboo forest, giant pandas traded an omnivorous diet of meat and berries to quietly consuming bamboos, a plant plentiful in the subtropical forest but of low nutrient value,” said Dr Xiaoming Wang, study author, in a statement. “Tightly holding bamboo stems in order to crush them into bite sizes is perhaps the most crucial adaptation to consuming a prodigious quantity of bamboo.”
There has been wonder for a while about why the false thumbs of pandas are so underdeveloped. The Ailurarctos is an ancestor to modern pandas, and a 6-7 million-year-old fossil was discovered from the late Miocene site of Shuitangba in Yunnan Province, China. Now that long ago may make you think that they would have less well-developed false “thumbs”, but this is incorrect. In this study, the team looked at the ancient panda fossil and found that this panda had a longer thumber with a straighter end. This is different from the modern panda, which has a more hooked and shorter digit.
So why did these pandas' thumb change shape?
“Panda’s false thumb must walk and ‘chew’,” said Wang. “Such a dual function serves as the limit on how big this ‘thumb’ can become.”
This means that the pandas developed shorter thumbs so they could manipulate the bamboo, scoff it all down, and then allow them to trundle along to find the next bamboo meal.
“Five to six million years should be enough time for the panda to develop longer false thumbs, but it seems that the evolutionary pressure of needing to travel and bear its weight kept the ‘thumb’ short – strong enough to be useful without being big enough to get in the way,” said Denise Su, co-leader of the project that recovered the panda specimens.
“Evolving from a carnivorous ancestor and becoming a pure bamboo-feeder, pandas must overcome many obstacles,” Wang sids. “An opposable ‘thumb’ from a wrist bone may be the most amazing development against these hurdles.”
And… for some fun, here have a video of some cute pandas.