Pandas’ Gut Bacteria May Help Them Fatten Up While Eating Bamboo

The lengths to which giant pandas go to maintain their virtually all-bamboo diet make the sacrifices many humans endure during Veganuary look very bearable. To ensure they get enough energy from the low-fat leaves and shoots that make up 99% of their food intake, pandas must eat up to 84 pounds of bamboo a day. The effort involved in eating these tough, reedy plants (see the strangely ASMR video below) means that pandas often spend upwards of 10 hours a day eating.

These time constraints leave little time for other activities, like making new pandas, and even when a panda embryo gets fertilized, researchers believe their limited diet also explains their odd pregnancies, which feature a “delayed implantation” phase, an extremely short gestation and very scrawny newborns. In short, a panda’s lifestyle is sculpted by its bizarre dietary choices.

One thing that hasn’t been changed by the panda’s insatiable hunger for starch is its digestive system, which, with its short digestive tract, remains that of a carnivore. Despite this, the panda manages to stay healthy and to put on a huge amount of weight during the shoot season from April to August. To try and work out what adaptations this strange bear’s gut has taken in response to its almost meat-free diet, Chinese researchers examined the microbiome of pandas in the Foping National Nature Reserve, situated in Shaanxi Province.

Their study has been published in Cell Reports. Their central finding: pandas’ gut bacteria play a key role in helping them tack on mass and survive the nutrient-scarce period from late August to April, where only bamboo leaves are available to eat.

What’s different in chubby pandas’ guts?

“This is the first time we established a causal relationship between a panda’s gut microbiota and its phenotype,” said first author Guangping Huang of the Institute of Zoology at the Chinese Academy of Sciences in a press release. “We’ve known these pandas have a different set of gut microbiota during the shoot-eating season for a long time, and it’s very obvious that they are chubbier during this time of the year.”

The researchers followed a cohort of 8 wild pandas across the Reserve, collecting 14 feces samples from the leaf-eating season and 15 from the shoot-eating season. The researchers identified that there was a higher abundance of the bacterium Clostridium butyricum in the panda poo during the shoot season. The team used a powerful metaproteomics toolkit, which can suggest the function that bacteria are likely to play in the gut, to show that the overall panda microbiome’s metabolic functions shifted between seasons.

Panda-to-mouse transplants

To investigate whether these changes in the microbiome might help the panda pack on the pounds, they transplanted gut microbiota from leaf- and shoot-eating bears into germ-free mice, which contain no microbiota of their own. This transplant technique is common in the field, although such transplants are normally done between different mice or from humans to mice.

Huang explained the innovative use of mice seeded with panda bacteria: “For endangered and vulnerable wild animals, we can’t really run tests on them directly. Our research created a mouse model for future fecal transplant experiments that can help study wild animals’ gut microbiota,” he said.

After the bacteria transplant, the mice were fed on a bamboo-rich diet for three weeks. As expected, C. butyricum was significantly more common in the gut of mice transplanted with shoot-eating panda bacteria. After three weeks of their panda-like diet, these mice were also heavier and fatter than their leaf-diet bacteria counterparts, despite both groups eating the same amount of food.

Alteration of circadian rhythms

To explore the method by which the bacterium might have an effect on panda and mouse metabolism, the researchers examined a metabolic product of C. butyricum, butyrate. They identified that alterations to gene networks involving circadian clocks, a highly conserved set of genes that regulate physiological processes in almost every cell of the body, might be involved.

The team showed that butyrate was able to boost the expression of a circadian gene, Per2, that can alter fat levels through phospholipid metabolism genes that are found in Per2’s regulatory network. The researchers concluded that these seasonal changes in the microbiota prepare the panda’s body to better metabolize fat and increase body mass.

The researchers’ next steps are to further explore the panda’s gut environment and track down other bacteria that might subtly alter these big bears’ unusual lifestyle. “Causal research of host phenotype and gut microbiota in wild animals is just beginning. Identifying what bacteria are beneficial for animals is very important, because one day we may be able to treat some diseases with probiotics,” Huang concluded.

Reference: Huang G, Wang L, Li J, et al. Seasonal shift of the gut microbiome synchronizes host peripheral circadian rhythm for physiological adaptation to a low-fat diet in the giant panda. Cell Rep. 2022;38(110203). doi: 10.1016/j.celrep.2021.110203