Why do giant pandas love bamboo wholeheartedly? The earliest pseudo-thumbs trace the history of panda bamboo eating to 6 million years ago

The ecological restoration of the first panda in Zhaotong Shuitang Dam, Yunnan (mauricio Antón) shows a pseudo-thumb grip function on the right, and the individual on the left shows a walking posture

Giant pandas clutching and chewing bamboo (Photo by Sharon Fisher)

Anatomy of the pseudo-thumbs of giant pandas (A, C) and pandas (B, D).

The carnivorous giant panda (Ailuropoda melanoleuca) differs from the “stunts” of its brothers, enabling it to adapt to a life that specializes in eating bamboo. And what provides “stunt” support is undoubtedly the giant panda’s extra and mysterious “thumb”. In addition to the normal five fingers on most mammalian forelimbs, giant pandas have a strongly enlarged wrist bone, the radial sesamoid bone, which acts as a sixth finger, forming a “thumb” that can be held oppositely to grasp the bamboo.

Recently, Wang Xiaoming, a visiting researcher at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences and a researcher at the Natural History Museum of Los Angeles, USA, Deng Tao, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and Ji Xueping, a researcher at the Kunming Institute of Zoology of the Chinese Academy of Sciences, cooperated with their peers to publish the sixth finger of the first panda (Ailurarctos), that is, the “pseudo-thumb” evolution paper in the international journal “Science Report”. Deng Tao, one of the co-authors of the paper, told China Science Daily that the results published a new discovery at the site of the Late Miocene Pond Dam in the Zhaotong Basin of Yunnan Province, which, based on the earliest known enlarged radial sesamoid bone of the panda, showed an intermediate morphology, and recorded for the first time the possible time and step of the panda’s evolution to eat bamboo.

The ins and outs of the pseudo-thumb

The key pseudo-thumb and molar fossils of the panda come from the scientific excavation of the Late Miocene site of Shuitangba jointly organized by Deng Tao, Ji Xueping, the Institute of Paleovertebrates, the Yunnan Provincial Institute of Archaeology and the Zhaotong Municipal Government in 2015.

The results of the study showed that the first panda had the earliest enlarged radial sesamoid bone, which had formed a “thumb” function of the opposite hand.

However, since the late Miocene, the panda’s “thumb” has not been further enlarged, because it must also take into account the load-bearing function of walking in a walking style while grasping bamboo, and cannot be abandoned by either side. The restriction of sharing weight may be the main reason why the panda’s pseudo-thumb never evolved into a full finger, but this simple “thumb” suggests that the giant panda’s habit of eating bamboo specifically originated at least 7 million to 6 million years ago.

The panda’s pseudo-thumb has long attracted the attention of naturalists. In recent decades, thanks to the enthusiastic introduction of renowned evolutionary biologist Steven J. Gould, the thumb-like structure of giant pandas gaining grips has become a well-known example of evolutionary adaptation, emphasizing the unique link between this unique anatomy and the equally unique habit of eating bamboo. The giant panda is a highly specialized member of the bear family, which has evolved into a focused plant-eating animal.

Although the panda’s pseudo-thumb is very famous, it is only a rather rudimentary small, flat structure that once puzzled early anatomists. Fossils are the most important evidence to understand the ins and outs of the pseudo-thumb, but the earliest previous records were found in the Late Pleistocene sediments of about 102,000 to 49,000 years ago in Shuanghe Cave, Guizhou.

If grasping bamboo is a major function of this feature, since the sesamoid bone of mammals is easily elongated during development, why didn’t giant pandas evolve a more elongated radial sesamoid bone, a sesamoid bone that more resembles a true counter-grip thumb in order to grip bamboo more efficiently? This question was not previously answered because of the lack of relevant fossil evidence in the Echizoite-Giant Panda lineage, which preceded the Late Pleistocene.

The result is the first to document the possible time and steps for pandas to evolve to feed on bamboo. Among carnivores, the molars of the bear family are the most complex because they also chew large amounts of non-meat foods. Among bears, the panda has the most delicate tooth pattern, with many very pronounced tip of the folds, which are conducive to chewing hard bamboo with the help of strong mandibles and temporomandibular joints. The tooth morphology of the first panda has reached the level of complexity of modern giant pandas, or even more complex.

Has the panda started eating bamboo? The pseudo-thumb that can help grasp a bamboo pole is another key piece of evidence.

The evolution of the pseudo-thumb

One of the most important characteristics of humans and their primate relatives is the evolution of a thumb that can be held against other fingers for precise grip, which requires not only joint flexibility, but also complex interactions of flexors and extensors.

The grip of the giant panda is fundamentally different from that of humans, and its radial sesamoid bone forms a functional complex with the first metacarpal bone and the moon bone by fixing the joints, which is linked to the other metacarpal bones, and cannot be independent of other finger movements like the human thumb. This passive grip system is far less effective than the active grip system of humans, but it is enough to provide giant pandas with the gripping ability needed to eat bamboo. In addition, from an evolutionary point of view, even if the radial sesamoid bone is initially only slightly enlarged, it can form a simple but useful grip function, moderately preventing the bamboo from slipping off the finger.

The radial sesamoid bone of the living giant panda has a sharp inward curved hook near the end, and its function is to rely on a single bone to form a claw. In contrast to the human thumb, which has two knuckles, the end section of the human thumb can be bent for easy grip. The radial sesamoid bone lacks a curved hook at the end, indicating that the evolutionary process is divided into two steps: the pseudo-thumb is initially simply elongated, and later a finer end curved hook appears, accompanied by a slight shortening of the end.

Whether in absolute or relative length, the pseudo-thumb of the panda exceeds its present offspring, but lacks the end curved hook of the giant panda. This raises the question that longer fingers will certainly enhance the ability to grasp bamboo poles, so why didn’t the panda’s pseudo-thumb stretch further?

The study believes that the radial sesamoid bone did not extend further, and it was the pseudo-thumb that took into account the functional reconciliation between grasping the bamboo pole and bearing the weight. Unlike the toe line of running carnivores such as canines, cats, and hyenas, obese bears are in a toe manner, where the front and back paws touch the ground when walking. The strongly elongated radial sesamoid bone is more conducive to grasping bamboo, but inevitably conflicts with long walks, thus impairing the dual function of the pseudo-thumb – the inner surface is used for grasping and the outer surface is used for load bearing.

Any further enlargement of the radial sesamoid bone will result in a more pronounced prominence on the palmal surface, which interferes with walking, in layman’s terms, the foot.

Therefore, the living giant panda balances the conflict between functions by sharply bending the end of the pseudo-thumb inward into a hook and flattening on the outside. Of the extant bears, only giant pandas have a large fleshy cushion to cushion the radial sesamoid bone, indicating that the weight-bearing function of this bone is important.

Why pandas love bamboo wholeheartedly

Because the bamboo in the habitat is very abundant, the giant panda does not need to travel long distances every day to find food, and it is more important to eat more and eat fast. When a panda tears a bamboo pole with its teeth, clinging to the bamboo pole may be the most useful function of the pseudo-thumb.

Bamboo poles are very hard, especially when completely wooden in winter, requiring the panda’s hands to have considerable grip to twist and pull in order to cooperate with the chin for powerful nibbling and tearing. Therefore, grasping the bamboo pole is more important for the panda than grasping more bamboo at once, so there is no strong requirement for the length of the pseudo-thumb.

The panda’s humble pseudo-thumb is difficult to use to collect the usual omnivorous raw materials of the bear family, such as seeds, nuts, berries, and even dwarf grasses, suggesting that the only feeding target for enlarged radial sesamoid bones is bamboo, and the pseudo-thumb is a key adaptation for effective bamboo collection in the panda lineage. Although its digestive system appears to be inefficient, the giant panda’s strategy of eating sea has allowed it to successfully expand to large parts of southern China and Southeast Asia and become an important member of the Pleistocene giant panda-saber-toothed elephant fauna.

Deep in China’s bamboo forests, giant pandas eat its bamboo quietly and live in isolation and solitude, away from the dominance of their carnivorous relatives in the food chain.

Pandas’ transition from widespread omnivorous to single-mindedly eating bamboo has logically required multiple changes based on anatomy, physiology, and genetics. However, even after at least 6 million years of focusing on eating bamboo, these shifts are still limited and focused on feeding skills, while the digestive system of giant pandas is still a carnivorous configuration. After the late Miocene, the fact that the panda family’s pseudo-thumbs were not further extended shows that they have gained enough grip to deal with bamboo. That is to say, it is enough to grasp a bamboo pole or a handful of bamboo, while also supporting the fat body to walk, the pseudo-thumb is not long or short.

The research was funded by the Natural Science Foundation of China, the National Science Foundation of the United States, the Chinese Academy of Sciences and the Zhaotong Municipal Government. (Source: China Science Daily Cui Xueqin)

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