Why pandas eat bamboo

Pandas have long confounded biologists because they have a digestive system which resembles that of carnivores, yet the bear species relies mostly on bamboo – a grass – for food.

Bucking evolutionary trends, giant pandas (Ailuropoda melanoleuca) are one of just a few examples of a species which turned away from its carnivorous ancestry to become almost totally herbivorous.

Other bears – such as grizzly and black bears – are well known omnivores while polar bears are almost exclusively carnivores.

Pandas, too, have the gastrointestinal tract of a carnivore. So why do they subsist on bamboo?

New research published in the journal Frontiers in Veterinary Science suggests that the answer is to be found in the RNA of bamboo.

DNA encodes the genetic information of organisms in the cell. RNA carries and transfers this information. MicroRNA (miRNA) are small segments that play an important role in gene expression.

The authors of the study explored whether the miRNA of bamboo could have entered the bloodstream of the giant panda’s ancestors and regulated gene expression to cause the lineage to adapt to a bamboo-based diet.

To test this theory, the microbiologists took blood samples of 7 giant pandas to see if bamboo miRNA was present.

“We showed that plant-derived miRNAs are present in the blood of giant pandas,” says senior author Feng Li, a researcher at China West Normal University. “Our study proved that bamboo used as food for giant pandas does affect the change of giant pandas’ feeding habits.”

They found 57 miRNAs likely derived from bamboo.

“MiRNA in bamboo can enter giant pandas’ bodies through diet, be absorbed by the intestine, enter the blood circulation, and then regulate when the giant panda’s RNA transfers information, thus playing a role in regulating the gene expression of giant pandas,” Li explains.

The miRNAs can regulate growth and development, biological rhythms, behaviour, and immune responses in the pandas.

“MiRNA in bamboo is also involved in the regulation of smell, taste, and dopamine pathways of giant pandas, all of which are related to their feeding habits,” Li says.

The researchers believe that certain miRNAs accumulate as the pandas grow, altering gene expression. These miRNAs might also influence giant pandas’ sense of smell, allowing them to pick out the freshest and most nutritious pieces of bamboo plants.

“Only miRNAs that can specifically play a role in regulating gene expression can remain in the body, and those that do not play a role will be expelled,” says Li. For example, some miRNAs regulate reproductive processes and can only be found in the blood of pandas of a certain sex or age.

The researchers say that miRNA transfer from plants to animals may open doors for studying the treatment and prevention of animal diseases.

It could also help to assess and improve the safety of plant-based foods for animals and humans. But more research is needed to assess the plausibility of these applications.