For members of the taxonomic order Carnivora, giant pandas (Ailuropoda melanoleuca) eat surprisingly little meat. They're nearly entirely herbivorous, subsisting almost exclusively on bamboo. What's strange about the panda is that it isn't really very well designed for a plant-based diet. At least not at first glance.
Other herbivorous mammals have specialized adaptations in their digestive systems to break down fibrous plants. Often those critters have stomachs that are separated into compartments, like kangaroos, hamsters, cows, or even some primates. They also usually have gut microbiota that are uniquely suited to break down plant-based foods. But the giant panda's gastrointestinal tract is more like those of their nearer taxonomic relatives, the carnivorous dog, cat, and raccoon. How do they get away with just eating bamboo?
Make no mistake: pandas eat a lot of the stuff. One 1982 study conducted by Cornell University researchers at the National Zoo found that 75% of a male panda's diet consisted of bamboo, while it was 54% for a female. That meant that the male ate 5.6% of his body weight - 7-9 kilograms! - in bamboo every day. It was 3%, or 2-5 kilograms, for the female. (In the wild, both sexes eat only on bamboo, since there's no rice, apples, or carrots to be found in the bamboo groves of China.)
Digestion is the process through which food is broken down into smaller and smaller bits. There are two ways that food can be digested: mechanically or chemically, either with acids or with the aid of microbes. "The extremely rapid transit of digesta through the gastrointestinal tract of the panda, a direct consequence of its simple gut architecture, [limited] any potential microbial degradation of bamboo that might have otherwise occurred," they wrote. "Since pandas in the wild live on a diet consisting almost entirely of bamboo, they must employ means other than efficient fiber digestion to fulfill their nutritional requirements." In other words, they're not getting any help from their gut microbiome.
If they're not getting nutrients from the bamboo thanks to microbial digestion, then what's their secret? Rather than having evolved adaptations in their digestive system, giant pandas instead have skeletal adaptations.
To start with, the panda's skull is denser than those of other members of the order Carnivora, such as bears. A denser skull can better support the muscles surrounding the jawbone, resulting in a more powerful bite. Their teeth, large and flat, are covered by lots of ridges (like our molars, but more so) to better pulverize their food. Combined, their super-strong skull and teeth designed for efficient crushing allows them to mechanically, rather than chemically, break down their food, increasing the availability of the nutrients locked within those plant cells.
But that's not all. They also have an elongated wristbone, the radial sesamoid. The muscles that are attached to the first finger in other species (like our thumb) are instead attached to the radial sesamoid. That allows them far more dexterity and a pincer-like grasp of objects. It allows them to be more explicit about what they eat, opting primarily for nutrient-rich leaves which they strip off of stems. Only after the leaves are gone will they even consider eating the more fibrous pith or stems.
Basically, pandas survive because they're expert masticators and extremely good with their hands.
Header photo: Giant panda at the San Diego Zoo, copyright Jason G. Goldman.