How the Giant Panda Evolved to be Vegetarian

Giant pandas—bamboo-chomping, sleep-loving, rolling balls of fur—are among the most adorable animals alive today. Unlike their relatives, such as the grizzly and polar bear, the panda doesn’t eat meat but is content surviving on a diet of only bamboo. Their anatomy seems more suited toward a carnivorous or omnivorous nature, so why and how have pandas abandoned their taste for meat to pick up a vegetarian diet?

Pandas today don’t eat meat. However, it was only two million years ago when pandas stopped eating meat and switched to an exclusive bamboo diet. Hence, they’ve retained much of their meat-eating adaptations from times past. This inefficiency means they have to eat a lot of bamboo while living a low-energy, lazy lifestyle.

How Pandas Stopped Eating Meat

Pandas Weren’t Always Vegetarian

The giant panda, usually referred to as just the ‘panda’, is a species of bear native to certain mountain ranges in Sichuan, China. It belongs to the family Ursidae (true bears), sharing much of its genetic material with other well-known bear species such as the American black, brown (grizzly) and polar bears. Despite sharing many characteristics with these bears, one thing that sets pandas apart is their very peculiar diet.

As far as we know, giant pandas today survive on a diet of specifically bamboo shoots and leaves. But this wasn’t always the case. Using fossil dating techniques to analyze isotopes in teeth and bone samples, researchers have found that pandas used to eat mainly meat, with bamboo only a small part of their diet¹. Only around 2 million years ago, they drastically switched to total vegetarianism.

They Are Still Built for Eating Meat

The trouble, it turns out, is that pandas aren’t suited to a plant-based diet. Their digestive systems have not changed much from their meat-eating days, so they cannot metabolize much of the bamboo they eat. Many of the enzymes produced in its digestive tract are specialized to break down meat.

In fact, humans too suffer from this effect. Our digestive systems aren’t equipped to break down vegetables to efficiently extract energy from them, although the high fiber content helps prevent constipation.

Unsurprisingly, much of the bamboo that giant pandas eat exits the other end as waste. Out of the whopping 12.5 kg of bamboo they eat in a single day, only about 17% of it is digested³. To survive, pandas eat vast amounts of bamboo while maintaining a low-metabolic, lazy lifestyle to compensate for the poor energy return².

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Loss of Tas1r1: The Meat-Eating Gene

The giant panda’s shift to a vegetarian diet coincides with the inactivation of a specific gene—Tas1r1— that codes for the umami taste receptor⁴. This G protein-coupled receptor is present in carnivores and omnivores, providing us with the ability to taste certain amino acids abundant in meat, such as glutamic acid. Bamboo has very little amino acid content and hence Tas1r1’s inactivation in pandas would have coincided with their dietary change.

It has been hypothesized that the panda’s switch to bamboo occurred at a time when meat was scarce. The incorporation of bamboo in their diet meant they were less reliant on meat, rendering the umami taste receptor less important. Once the Tas1r1 gene was inactivated, the lack of the receptor meant that giant pandas were less attracted to, and less likely to return to, a meat-based diet.

Since the region that they lived in had an abundance of bamboo, this change would have been beneficial to its survival. Over time, the pandas gradually lost this umami taste receptor (with it went the taste for meat) and eventually survived on eating bamboo almost exclusively.

Interestingly, while other bears have kept the Tas1r1 gene intact, several herbivores like cows and horses also possess this gene. Our knowledge of taste and diet in evolution is far from complete. Or cows and horses secretly view their fellow farm animals as delicacies.

Panda Evolution Strategies

A Bamboo-Gripping Thumb

Despite everything they have going against them, it wouldn’t be fair to suggest that pandas are bumbling toward extinction. In fact, pandas have evolved to a great extent to cope with their relatively recent bamboo-eating lifestyle.

Unlike humans, other bears such as the grizzly have no need for a gripping ability provided by opposable thumbs. Their paws work just fine to provide grip with the ground. Pandas are different, however, as they must grip long and thin pieces of bamboo shoots.

To assist with this, they have developed an elongated ‘pseudo-thumb’ from an existing bone at the side of their paws⁵. Along with their other digits, giant pandas have six digits in each paw! Although this pseudo-thumb cannot move independently, it provides better support for the panda to grip objects such as bamboo in one paw quickly. In other bears, such as the grizzly, this bone remains undeveloped.

Getting Bacteria to Help with Digestion

As mentioned, pandas suffer from having a carnivorous digestive system as they cannot break down cellulose, the energy component of plants. Most herbivores can break down cellulose into sugar by producing certain enzymes, which is how they obtain their energy. Despite not being able to produce cellulose-digesting enzymes, pandas rely on bacteria present in their intestines to do this job.

Using RNA gene sequencing, it was found that the panda gut is host to many bacterial cells. Among them were many bacteria from the Clostridium genus, possessing genes coding for cellulose and hemicellulose-digesting enzymes³. Despite their poorly-suited digestive tract, giant pandas can at least partially digest bamboo fibers by harboring these bacteria.

A Tongue for Both Plants and Meat?

Although the panda has jaws and teeth designed for tearing meat from the bone and a digestive system more than capable of digesting it, it has a somewhat less well-defined tongue. Carnivores generally have long tongues with rounded tips, which is the case with giant pandas. However, researchers have found that while the overall structure is retained, the surface of the panda’s tongue is different.

While other bears and many carnivores have smooth tongues, pandas have evolved tiny projections on the surface of their tongue that serve to file down food⁶. This is especially useful for pandas as they remove the outer layer of the bamboo sheath before consuming the shoots.

This feature is also seen in other mammals such as the omnivorous lemurs and treeshrews, where these tongue projections serve a secondary purpose of cleaning. The small projections fit between their teeth and—like bristles on a toothbrush—mechanically remove food trapped in the spaces.

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Will Meat Be Back On the Panda’s Menu?

Giant pandas in the wild are endangered because deforestation has shrunk their habitat and destroyed much wild bamboo, the panda’s sole source of food. Challenges in mating and reproduction—male giant pandas possess a very small baculum in relation to their size—also seem to work against them. Together with an anatomy that is more suited to eating meat, we can easily see why their survival is highly dependent on protection and breeding programs.

Despite these conservation efforts, giant pandas have a lot of evolution ahead if they are to optimize their bodies for digesting plants. Time will tell if pandas will adapt and survive, go back to eating meat, or perish in the wild. Whatever the future holds, we can be grateful that, for the time being, we get to share our planet with these docile, clumsy, simply adorable fluffballs.

Reference

1. Han, H., Wei, W., Hu, Y., Nie, Y., Ji, X., Yan, L. I., … & Chen, W. (2019). Diet evolution and habitat contraction of giant pandas via stable isotope analysis. Current Biology, 29(4), 664-669.
2. Nie, Y., Speakman, J. R., Wu, Q., Zhang, C., Hu, Y., Xia, M., … & Zhang, J. (2015). Exceptionally low daily energy expenditure in the bamboo-eating giant panda. Science, 349(6244), 171-174.
3. Zhu, L., Wu, Q., Dai, J., Zhang, S., & Wei, F. (2011). Evidence of cellulose metabolism by the giant panda gut microbiome. Proceedings of the National Academy of Sciences, 108(43), 17714-17719.
4. Zhao, H., Yang, J. R., Xu, H., & Zhang, J. (2010). Pseudogenization of the umami taste receptor gene Tas1r1 in the giant panda coincided with its dietary switch to bamboo. Molecular biology and evolution, 27(12), 2669-2673.
5. Endo, H., Yamagiwa, D., Hayashi, Y., Koie, H., Yamaya, Y., & Kimura, J. (1999). Role of the giant panda’s ‘pseudo-thumb’. Nature, 397(6717), 309-310.
6. Pastor, J. F., Barbosa, M., & De Paz, F. J. (2008). Morphological study of the lingual papillae of the giant panda (Ailuropoda melanoleuca) by scanning electron microscopy. Journal of Anatomy, 212(2), 99-105.