During shoot-eating season, pandas manage to maintain their bodacious body type despite subsisting on a pretty low-fat diet. How? By shifting their gut microbiota, of course.

The discovery, revealed today in a paper in Cell Reports, details how pandas (which are no longer endangered, yay!) manage to not only stay chubby, but actually gain weight faster when shoots are all that’s on the menu. It would be logical to expect that such a diet would result in lean pandas (a harrowing thought indeed) but thanks to these bears’ unique gut microbiota they can continue to gain weight and store fat. Phew!

That the bacterial party goings-on in the gut can change with the seasons isn’t new information, with other examples including humans: specifically, the Hadza people. As modern hunter-gatherers, their diet changes throughout the year as Tanzania’s seasonal offerings shift, and when it comes to the gut microbiome you really are shaped by what you eat.

The novel finding in the case of the pandas' persistent pudginess is that scientists have been able to link the animal’s appearance to its changing internal environment.

“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, Chinese Academy of Sciences, in a statement. “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.”

Reaching their conclusions required panda droppings for a rather unusual cross-species poop transplant. It might sound gross, but poop transplants are a great way to change the gut microbiome and are used in human medicine as a way of restoring a healthy environment in the gut (it can even cure people brewing alcohol in their bodies). In this instance, the poop was transplanted into mice, which are far easier to study under lab conditions than 100-kilogram bears.

“For endangered and vulnerable wild animals, we can’t really run tests on them directly,” explained Huang. “Our research created a mouse model for future fecal transplant experiments that can help study wild animals’ gut microbiota.”

The team on the study collected fecal samples during the shoot-eating season and gave them to mice to see if or how it affected their physiology. Their results showed that mice transplanted with shoot season panda feces grew faster and fatter than mice given leaf season panda poop, despite eating the same.

You might think it’s a lot of effort to go to just to probe the chubbiness of pandas but establishing a connection between the gut microbiome and the ability to gain weight could have implications for human medicine.

“Causal research of host phenotype and gut microbiota in wild animals is just beginning,” said Huang. “Identifying what bacteria are beneficial for animals is very important, because one day we may be able to treat some diseases with probiotics.”

God bless you, chubby pandas.