Weekly Science Talk | How did giant pandas transform from carnivores to “vegetarians”?

The cute giant pandas are loved by people. They are also very impressive for the way they eat bamboo slowly. Many people think that giant pandas feed on bamboo, but millions of years ago, the ancestors of giant pandas were carnivorous beasts.

How did the giant panda change from a carnivore to a vegetarian animal that only eats bamboo? And how did humans help the giant panda get out of the endangered situation? The research team led by Wei Fuwen, an academician of the Chinese Academy of Sciences and a researcher at the Institute of Zoology of the Chinese Academy of Sciences, gave a scientific answer and provided an effective solution for the protection of giant pandas. Their project "Study on the Adaptive Evolution and Endangered Mechanism of Giant Pandas" won the second prize of the National Natural Science Award in 2019.

In order to study how giant pandas adapt to low-nutrition bamboo, Wei Fuwen's team conducted a systematic study using a variety of technical means and research methods, and discovered a series of adaptations that giant pandas have made to bamboo in terms of morphology, genetics, behavior, physiology, and intestinal microorganisms.

In terms of morphology, giant pandas and red pandas, which also feed on bamboo, have evolved pseudo-thumbs that facilitate grasping bamboo. In terms of genetics, genome analysis shows that both giant pandas and red pandas have undergone similar gene changes to adapt to the development of pseudo-thumbs and the absorption of nutrients from bamboo. One of the receptor genes for the umami taste of meat has become a non-functional pseudogene, while the genes of other carnivores still retain normal functions.

Further research found that in order to adapt to low-nutrient bamboo, giant pandas have adopted strategies such as eating more, digesting more, and exercising less, so the giant pandas we see are round and slow. The first strategy adopted by giant pandas is to forage for food in large quantities, and they are also "picky eaters". They do not eat all kinds of bamboo. They often choose those bamboo species with the best nutrition, and get enough nutrition and energy from bamboo shoots, bamboo leaves and bamboo stems with moderate base diameter to the maximum extent.

Eating more is not enough. Giant pandas also try to improve the efficiency of bamboo utilization. Studies have shown that giant pandas not only use more than 90% of the protein and fat in the bamboo they consume, but also about 8% of the cellulose and 27% of the hemicellulose. However, the digestive tract of giant pandas does not secrete enzymes that break down cellulose and hemicellulose. The secret lies in the presence of a large number of intestinal microorganisms in their gastrointestinal tract that can digest cellulose. These microorganisms can produce a lot of enzymes that break down cellulose and hemicellulose, promote lipid metabolism, and change with the seasons to adapt to the changes in the nutritional content of the bamboo eaten by giant pandas in different seasons. This is the secret of how giant pandas can gain weight by eating bamboo.

It is good to have enough bamboo, but what if there is a shortage of food? Therefore, pandas have another adaptive evolution, which is to reduce energy consumption as much as possible. The most direct way is to reduce activity. Pandas spend more than half of their time resting, and the average moving distance of wild pandas per hour does not exceed 20 meters. Of course, it is not that pandas are born not to like exercise, but they seem to have understood that nutrition is hard to come by and cannot be wasted. In addition to reducing exercise behavior, they have also "thought of many ways" physiologically. For example, the liver, kidneys and other organs of pandas have gradually shrunk during the evolution process to further reduce energy consumption; thick fur can also reduce heat loss, so that their body surface temperature is lower. These adaptive evolutions have led to very low energy metabolism levels in pandas, which can almost compete with slow-moving sloths.

Some people also believe that they do not like to exercise in order to save energy, which even leads to low reproductive efficiency of the population. In this regard, the research of Academician Wei Fuwen's team has denied this speculation. Through the second-generation high-throughput sequencing technology and population genomics methods, the researchers found that the existing giant pandas are divided into three genetic populations, namely the Qinling, Minshan and Qionglai·Xiangling·Liangshan populations. Each subpopulation has high genetic diversity, long-term evolutionary capabilities and high genetic diversity, and there is no risk of endangerment for the time being. Historically, these populations have experienced two expansions, two bottlenecks and two divergences, and the Quaternary climate change is the main cause of their population fluctuations and differentiation. In particular, human activities since the last ice age and the Holocene have caused their populations to decline severely and even endangered. The emergence of the interglacial period has created conditions for the expansion of the giant panda population.

Further research found that the intensification of human activities in modern times has led to the fragmentation of wild giant panda habitats. The population has been divided into smaller groups intentionally or unintentionally by humans, and the genetic diversity of these small groups has been greatly reduced. It can be seen that the current small population of giant pandas is not due to their inability to adapt to environmental changes, but is mainly caused by human activities. If nothing is done, some wild giant panda genetic groups will face the risk of endangerment or even extinction. Therefore, researchers promote the release of captive giant pandas and recommend the construction of ecological corridors between different giant panda habitats to re-establish connections between small populations separated by roads or railways, so as to facilitate genetic exchanges between them and reduce the risk of endangerment.

It is worth mentioning that in the process of studying the adaptive evolution and endangered mechanisms of giant pandas, researchers often have to rely on molecular genetic technology, which requires collecting the panda's DNA. However, since wild giant pandas have a large activity range and erratic whereabouts, their DNA is difficult to collect. Therefore, researchers have invented a non-destructive genetic analysis technology system, which uses the DNA of intestinal cells shed in the feces of giant pandas for genetic analysis, which can accurately identify individual giant pandas. This method has been widely used in surveys and population monitoring of wild giant pandas across the country.

The above research results of Academician Wei Fuwen's team have provided theoretical and practical guidance for the protection of giant pandas in my country, and made an important contribution to the reduction of my country's giant pandas from the "endangered" to the "vulnerable" level.

(Text: Tang Bo, Research Librarian of Zhongguancun Guoke Modern Agricultural Industry Science and Technology Innovation Research Institute. Expert: Zhao Xumao, Researcher of Lanzhou University School of Ecology)

China Association for Science and Technology Department of Science Popularization

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