Pandas have been munching on bamboo for 7 million years and exclusively for 2 million years, so it was a big surprise when researchers analyzed 121 fecal samples from 45 giant pandas and found gut bacteria not at all designed to process the animals' food of choice. Though pandas developed "powerful jaws and teeth" to break down up to 28 pounds of bamboo per day, the samples showed pandas process only 17% of the bamboo they eat—meaning they can't extract maximum energy—mainly because their gut bacteria is meant to process meat, the Los Angeles Times and BBC report. Rather than house bacteria common in herbivores that need to break down fiber, the pandas' guts featured Escherichia/Shigella and Streptococcus bacteria, similar to bacteria found in meat-eating bears, used to process protein.
"Unlike other plant-eating animals that have successfully evolved anatomically specialized digestive systems to effectively deconstruct fibrous plant matter, the giant panda still retains a gastrointestinal tract typical of carnivores," lead researcher Zhihe Zhang says. Over time, other herbivores developed longer guts or produced enzymes that helped them break down plants. But whereas cows have four-chambered stomachs that help in plant digestion, pandas have only simple stomachs and short intestines, the Guardian reports. They also have "extremely low" diversity in their gut microbiome, which researchers think could indicate an animal is less resilient to changes in their environment. That's especially bad news for pandas that lead a "highly fragile lifestyle" in a shrinking habitat. This combination of factors "may have increased their risk of extinction," Zhang says. There are thought to be just 1,600 giant pandas left in the wild. (Read a case for why we should let them die out.)