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Pandas develop a supercharged gut for part of the year, allowing them to gain weight when they chow down on the most nutritious parts of bamboo.
The gut bacteria of the giant panda changes when bamboo shoots are available, with scientists suggesting this allows them to build up fat reserves for leaner times ahead.
For four months each year pandas put on weight as their favourite food, bamboo shoots, becomes available. Researchers have now found that the reasons for this go beyond a healthy appetite.
Scientists from China found that certain gut bacteria become more prevalent in giant pandas when the bamboo shoots are available and suggest this allows them to store more fat. This may allow them to survive in the leaner parts of the year when less nutritious bamboo leaves are their main source of food.
First author Dr Guangping Huang, says, 'We've known for a long time that these pandas have a different set of gut microbiota during the shoot-eating season, and it's very obvious that they are chubbier during this time of the year.
'This is the first time we have established a causal relationship between a panda's gut microbiota and its phenotype.'
The findings of the study were published in Cell Reports.
Giant pandas are something of an oddity within the animal kingdom. Despite being a part of the order Carnivora, a group of mammals which mostly eat flesh, they consume almost nothing but bamboo.
While the giant panda's behaviour may have changed, the same can't be said for its digestive system. Unlike the red panda, the giant panda is a true bear having split off from the common ancestor of all other ursids around 19 million years ago.
While its relatives have generally stuck to a carnivorous or omnivorous lifestyle, the panda eats a high fibre diet, but without many of the necessary adaptations. Many of its genes produce enzymes which digest meat rather than plant material, while its gut is not long enough to ferment the bamboo to obtain more nutrition from it.
Instead, the giant panda relies on gut bacteria to help it digest its meals. While producing some enzymes that help it consume bamboo it is still relatively poor at digesting plants, especially when compared to specialist herbivores such as the ruminants.
However, young bamboo is much easier to eat as it contains less fibrous materials like cellulose. When bamboo shoots are readily available, generally between April and August, giant pandas will eat this preferentially as they contain large amounts of sugars and carbohydrates.
Other animals (including humans) experience changes in their gut microbiome to adapt to seasonal shifts in food. Having studied giant pandas for many years, researchers wanted to look into how their digestive bacteria changed over time, and how these vulnerable animals might be affected.
To investigate the changes brought on by bacteria, the researchers transplanted microbes from panda guts during the bamboo shoot season and during the leaf season into mice. These were then compared with a control group, where the process was carried out with saline.
'For endangered and vulnerable wild animals, we can't really run tests on them directly,' Guangping says. 'Our research created a mouse model for future faecal transplant experiments that can help study wild animals' gut microbiota.'
The researchers found that the bacterium Clostridium butyricum was relatively more abundant in mice which had received a faecal transplant from bamboo shoot season pandas than the bamboo leaf group.
This bacterium produces a fatty acid known as butyrate, which is one of a group of compounds used to make lipids. Mice with bamboo shoot bacteria started putting on weight more quickly than the other groups of mice and developed a higher proportion of fat in their bodies.
These changes were associated with higher levels of phospholipids, compounds which are used to build cell membranes, circulating in the blood. The scientists linked this to changes in the expression of certain genes, including those involved with regulating the body clock.
A further behavioural experiment found that pandas which had a higher level of butyrate moved further than those without, suggesting that the increased amount of this fatty acid from their gut bacteria causes them to maximise the amount of time looking for bamboo shoots to build up their reserves for winter.
While the results are suggestive of a causal link, the use of mice for many of the experiments means that this correlation cannot be confirmed at this point.
However, the researchers hope to find out more about the giant panda microbiome, and potentially develop new ways of improving the animals' health through their gut bacteria.