Researchers have discovered how pandas are able to gain weight even though they only eat bamboo. It turns out the animals' gut bacteria changes in the season when nutritious bamboo shoots become available. This helps the bears store more fat and could compensate for the lack of nutrients in seasons when there are only bamboo leaves to chew on, a study by the Institute of Zoology at the Chinese Academy of Sciences suggests. Pandas eat between 26 to 84 pounds of bamboo each day depending on what part of the plants they are consuming. Guangping Huang, one of the study's authors, said, "This is the first time we established a causal relationship between a panda's gut microbiota and its phenotype. 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." A newborn panda is about the size of a stick of butter – but females can grow up to about 90kg, while males can reach about 136kg. During late spring and early summer, bamboo shoots the animals consume are protein-rich newly sprouted bamboo shoots – a change from the usual fibrous bamboo that is available. The experts, who studied wild pandas in the Qinling Mountains in central China, found that they have a significantly higher level of a bacterium called clostridium butyricum in their gut during shoot-eating season.