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(CNN)Pandas are famously picky eaters. They only consume bamboo -- a poor quality diet low in fat.
But the creatures appear to have evolved to get the most out of what they do eat, according to a new study.
Their gut bacteria changes in late spring and early summer when bamboo is at its most nutritious -- while it's sprouting protein-rich green shoots. The bacteria makes the bear gain more weight and store more fat, which researchers said may compensate for a lack of nutrients later in the year, when bamboo plants have only fibrous leaves to chomp on.
"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," said lead study author Guangping Huang, a researcher for the Institute of Zoology at the Chinese Academy of Sciences in a news release.
The study was published on Tuesday in the journal Cell Reports.
To investigate how the gut bacteria could affect a panda's metabolism, the team first collected the feces of eight wild giant pandas in China's Qingling mountains during both leaf-eating season and shoot-eating season and then examined how the poop samples differed.
They found that a bacterium called Clostridium butyricum was more abundant in the pandas' guts during the season when they enjoy the fresh bamboo shoots.
To understand whether this bacteria helps the bears gain and store weight, the researchers performed a fecal transplant, putting the panda poop they collected into lab mice. Then they fed the mice for three weeks with a bamboo-based diet that simulated what pandas eat.
While mice are very different from pandas, it wasn't possible to run such tests on endangered and vulnerable animals, said Wei Fuwen, a coauthor of the study and a professor at the Key Lab of Animal Ecology and Conservation Biology in the Institute of Zoology at the Chinese Academy of Sciences. Mice were a good substitute.
"Mice have proven to be an effective model for studying the function of the gut microbiome in humans," Wei said via email.
Researchers found that mice transplanted with panda feces collected during shoot-eating season gained significantly more weight, despite consuming same amount of food.
"The gut bacteria was the only variable in this research," Wei explained.