Why pandas don’t eat meat: Bamboo molecules shed light on origins of plant-based diet

The giant panda is a true bear, just like brown bears and polar bears, powerful predators. The beloved animal, endemic to China, also has a carnivorous bacterial flora, but it follows an essentially vegetarian diet, which is reflected in its teeth and also in the musculature of its jaw. Bamboo is the panda’s signature dish and it can spend up to 14 hours a day chewing it, despite the fact that it lacks sufficient enzymes to digest cellulose. It has been a dissident of its genus for about seven million years and the reason could be due to the microRNA of this plant from the Asian continent. This is suggested by a new study published in the journal Frontiers in Veterinary Science, since these small molecules — which intervene in the processes of development and adaptation of plants in the environment — could be capable of entering the bloodstream of these animals and adapting the response of pandas to bamboo.

Through comparative analysis, researchers from China West Normal University identified 57 bamboo-derived microRNAs in blood samples from seven giant pandas — three adult females, three adult males, and one juvenile female — collected between May and June 2022. The study found that the molecules can adjust dopamine levels in giant pandas, thereby influencing their food preferences, which could play a vital role in their dietary adaptation process.

“MicroRNAs in bamboo are also involved in regulating smell and taste, which are related to their eating habits,” Feng Li, lead researcher on the discovery, said in a statement. During infancy, microRNAs are acquired from breast milk, where they regulate growth and development. As the panda matures and bamboo is incorporated into its diet, microRNAs continue to modulate its genetic expression. Scientists say that individuals of this emblematic species for biodiversity conservation accumulate the plant’s molecules over the years. And these can regulate different physiological processes, including growth and development, biological rhythms, behavior, and immune responses.

Silvia Díaz, a technician from the WWF Spain Species Program, explains that this animal is “very specialized in its diet. It only eats bamboo 99% of the time and the research offers a novel finding that could explain its behavior,” she says. However, of the 12 kilos of bamboo leaves and stems that the giant panda eats on average daily, they only manage to digest approximately 17%.

The giant panda’s adaptation to a herbivorous diet has long puzzled researchers. In 2009, scientists discovered that it does not possess the genes needed to digest plant food after sequencing the entire genome of the bear for the first time. Another analysis of the gut microbiome, published in the journal Nature, determined that the panda’s gut has microbes typical of carnivorous animals.

Alejandro Cabrera, from the conservation department of the Madrid Zoo Aquarium, says that bamboo is able to penetrate their blood cells, which favors the metabolism of dopamine. “This makes the giant panda feel the need to keep eating bamboo,” he says. The study also found that giant pandas of different ages and sexes had different compositions of microRNA in their blood. For example, they regulate reproductive processes; for this reason, they can only be found in the blood of pandas of a certain sex or age. The scientists discovered that adult males had higher concentrations of microRNA.

On the other hand, the ability of microRNAs to send signals from plants to animals may allow for the study of disease treatment and prevention. “Plant microRNAs also have the ability to regulate the immune system of animals, boosting their resistance to disease,” Li said. The next step for the scientists behind the new study will be to collect blood samples from young pandas that have not yet eaten bamboo, to understand a little more about how this process takes place.

Panda conservation efforts

The International Union for Conservation of Nature has now classified this species as vulnerable. WWF has been involved in campaigns and collaborations since the 1980s. The NGO became the first international organization invited by China to work on the conservation of this species. To date, more than 70% of the giant panda’s natural habitats across the country have been protected.

“We helped the Chinese government by creating a network of nature reserves for giant pandas, as well as ecological corridors to connect populations that were isolated. These actions have helped to protect not only the pandas, but also the other species that live there,” says Díaz.

Efforts have also been made in Spain to conserve this species. The Madrid Zoo Aquarium has successfully carried out a conservation and reproduction program by breeding six specimens. “It has been historic for a European zoo. We had twins in 2010, and again in 2021, and two individual specimens in 2013 and 2016,” says Cabrera, who worked as a caretaker of these bamboo-eating bears.

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