China News Service, Beijing, July 1 (Reporter Sun Zifa) Why do giant pandas belonging to the order Carnivora only eat bamboo?

How did its skillful grasping bamboo "stunt" originate and evolve?

These issues are not only of concern to the academic community, but also of the public.

Deng Tao, a researcher from the Institute of Paleo-Spine of the Chinese Academy of Sciences, was interviewed in Beijing to introduce the latest research results on the origin and evolution of the giant panda's diet.

Photo by China News Agency reporter Sun Zifa

The history of giant panda eating bamboo can be traced back to 6 million years ago

  Wang Xiaoming, a visiting researcher at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences (Institute of Paleoanthropology, Chinese Academy of Sciences), a researcher at the Natural History Museum of Los Angeles, USA, Deng Tao, a researcher at the Institute of Paleo-Spine, Chinese Academy of Sciences, and Ji Xueping, a researcher at the Kunming Institute of Zoology, Chinese Academy of Sciences, cooperated with scientific colleagues. Research on the key fossils of the Pseudo-thumb (also known as the sixth finger) and molar teeth collected from the Late Miocene (about 7 million to 6 million years ago) site in Zhaotong Shuitangba, Yunnan found that the Panda has a The earliest enlarged radial sesamoid has formed the "thumb" function of the opposite grip.

  However, since the late Miocene, the "thumb" of pandas has not been further enlarged, because it must also take into account the weight-bearing function of plantar walking (front and rear feet touching the ground) while grasping bamboo, and no one can be favored.

This weight-sharing limitation may be the main reason why pandas' pseudo-thumbs never evolved into full fingers, but this rudimentary "thumb" suggests that giant pandas' habit of eating bamboo exclusively originated at least 6 million years ago .

  This important achievement paper of paleontology research was published online in the international academic journal "Scientific Reports" on the night of June 30th, Beijing time. It provides important fossil evidence for the evolution of the Elephant Panda-Giant Panda lineage earlier than the Late Pleistocene, and also provides answers for the answer to Elephant Panda. Whether it has started to eat bamboo and whether the pseudo-thumb can help pandas grasp the bamboo poles provides key evidence.

Schematic diagram of the anatomy and grasping of the pseudo-thumb of the giant panda (A, C) and the primordial panda (B, D).

Photo courtesy of the Institute of Ancient Spine, Chinese Academy of Sciences

Giant panda's pseudo-thumb "stunt" is the most famous and mysterious

  Researcher Deng Tao, the co-author of the paper, said in an exclusive interview with a reporter from China News Agency on the day the paper was published that the giant panda belonging to the carnivorous order is a highly specialized member of the bear family, and it has evolved into a dedicated herbivore.

The giant panda is different from its "brothers" because it has many "stunts" that allow it to adapt to the life of eating bamboo exclusively.

  Among these "stunts", the panda's extra "thumb" is arguably the most famous but also the most mysterious.

In addition to the normal five fingers on the forelimbs of most mammals, giant pandas have a strongly enlarged wrist bone, the radial sesamoid, which acts as a sixth finger and forms a "thumb" that can be gripped to grasp bamboo .

  The giant panda's pseudothumb has long attracted the attention of naturalists, and in recent decades, thanks to the introduction of the famous evolutionary biologist Steven J. Gould, giant pandas have acquired the thumb-like structure of the opposite grip. Having become a well-known example of evolutionary adaptation, the unique link between this unique anatomy and an equally unique bamboo-eating habit has drawn attention.

  Deng Tao pointed out that although the pseudo-thumb of giant pandas is very famous, fossils are the most important evidence to understand the ins and outs of the pseudo-thumb. Pleistocene sediments.

If grasping bamboo is the main function of this trait, since mammalian sesamoid bones are easily elongated during development, why didn't giant pandas evolve a more elongated radial sesamoid, a more The sesamoids of the grip thumb for a more effective grip on bamboo?

This question has not been answered before because of the lack of relevant fossil evidence in the proto-Panda lineage earlier than the Late Pleistocene.

Comparison of the hands of giant pandas (B, D, E) with primitive bears (A) and humans (C).

Photo courtesy of the Institute of Ancient Spine, Chinese Academy of Sciences

Panda grasping is fundamentally different from human grasping

  Deng Tao said that this research is based on the new discovery of the pseudothumb and molar fossils of the Elephant Panda at the Shutangba site of the Late Miocene in the Zhaotong Basin, Yunnan Province in 2015. According to the earliest known enlarged radial sesamoid bone of Panda, its Pseudo-thumbs show an intermediate form, recording for the first time the possible timing and steps of the evolution of pandas towards eating bamboo.

  Among carnivores, bears have the most complex molars because they also chew a lot of non-meat food.

Among the bears, pandas have the most delicate tooth pattern, with many very distinct cusps, which are helpful for chewing hard bamboo with the assistance of strong mandibles and temporomandibular joints.

The tooth morphology of the primitive panda has reached the level of complexity of the modern giant panda, or even more complex.

"Has the beginning panda already started eating bamboo? The pseudo-thumb that helps grasp the bamboo pole is another key piece of evidence."

  Deng Tao pointed out that one of the most important characteristics of humans and their primate relatives is the evolution of a thumb that can hold with other fingers for precise grasping, which requires not only joint flexibility, but also flexor and extensor muscles. complex interactions.

  The grasping of giant pandas is fundamentally different from that of humans. The radial sesamoid bone forms a functional complex with the first metacarpal and scapholunate through fixed joints, and is linked with other metacarpals, and cannot be independent of the human thumb like the human thumb. other finger movements.

This passive grasping system is nowhere near as effective as a human's active grasping system, but it is sufficient to provide pandas with the grasping ability they need to eat bamboo.

Furthermore, from an evolutionary point of view, even if the radial sesamoid was initially only slightly enlarged, it formed a simple but useful grasping function that moderately prevented the bamboo from slipping off the fingers.

  The radial sesamoid bone of living giant pandas has a sharply inwardly curved hook near the end, which functions as a grasping forceps formed by a single bone.

In contrast to the human thumb, which has two knuckles, the distal segment of the human thumb can be bent to facilitate grasping.

Detail of a giant panda grasping and chewing on bamboo.

Photo by Sharon Fisher

Pseudo-thumb evolution takes into account both grasping bamboo poles and bearing weight

  Deng Tao said that the lack of terminal hooks in the radial sesamoid bone of the ancestral panda indicates that its evolutionary process was divided into two steps: the pseudothumb initially simply elongated, and later a finer terminal hook appeared, accompanied by a slight shortening of the terminal.

Both absolute and relative lengths, the pseudo-thumb of the primordial panda exceeds that of its living offspring, but lacks the terminal hook of the giant panda.

  "This raises the question, longer fingers definitely enhance the ability to grasp bamboo poles, so why didn't the pseudo-thumbs of pandas elongate further?"

  The research team believes that the radial sesamoid does not extend further, and it is the function of the pseudo-thumb between grasping the bamboo pole and bearing weight.

Unlike canines, cats, and hyenas, which are toe-walking (where their toes are on the ground), obese bears are plantar-walking.

The strongly elongated radial sesamoid is more conducive to grasping bamboo, but inevitably conflicts with long-distance walking, thereby impairing the dual functions of the inner surface of the pseudothumb for grasping and the outer surface for weight bearing.

Any further enlargement of the radial sesamoid would result in a more pronounced projection of the sesamoid on the palm, which would interfere with walking, "in layman's terms, stump the foot."

  Therefore, the living giant panda balances the conflict between functions by sharply bending the pseudothumb end into a hook shape inward and flattening outward.

Among living bears, only giant pandas have a large fleshy pad to cushion the radial sesamoid, suggesting that the weight-bearing function of this bone is important.

  Because the habitat is very rich in bamboo, giant pandas do not need to travel long distances every day to find food, so it is more important to eat more and eat faster.

When pandas tear the bamboo pole with its teeth, holding on to the pole is probably the most useful function of the pseudothumb.

Bamboo poles are very hard, especially when they become completely woody in winter, requiring considerable grip of the panda's hands to twist and pull in order to cooperate with the jaws for powerful gnawing and tearing.

Because it is more important for pandas to grasp a bamboo pole than to grasp more bamboo in one hand, there is no strong requirement for the length of the pseudo-thumb in evolution.

Evolutionary relationship between pseudo-thumbs and molars and bamboo-eating habits in the panda family.

Photo courtesy of the Institute of Ancient Spine, Chinese Academy of Sciences

Pseudo-thumb is a key adaptation for effective bamboo harvesting

  Deng Tao believes that the simple pseudo-thumb of pandas is difficult to collect the common omnivorous raw materials of bears, such as seeds, nuts, berries, and even dwarf grass, which indicates that the only feeding target of the enlarged radial sesamoid is bamboo. The pseudothumb is a key adaptation for efficient bamboo collection in the panda lineage.

Although its digestive system appears to be inefficient, the giant panda's strategy of "eating the sea to eat" has enabled it to successfully expand into much of South China and Southeast Asia and become an important member of the Pleistocene giant panda-sabertooth fauna.

  Deep in China's bamboo forests, the giant panda eats its bamboo quietly, living a secluded, solitary life, far from the dominance of its carnivorous "relatives" in the food chain.

  The panda's transition from an extensive omnivorous diet to a single-minded eating of bamboo would arguably require multiple changes based on anatomy, physiology, and genetics.

However, even after at least 6 million years of focusing on eating bamboo, these transitions are still limited and largely focused on feeding skills, while the giant panda's digestive system is still a carnivore configuration.

  The fact that the pseudothumb of the panda family did not elongate further after the late Miocene suggests that they had acquired enough grip to handle bamboo.

"That is to say, enough to grasp a bamboo pole or a handful of bamboo, and at the same time support a fat body to walk, the pseudo-thumb evolved by the giant panda is suitable for neither long nor short." Deng Tao concluded.

(Finish)