How the Giant Panda Evolved to be Vegetarian

giant panda biting on bamboo shoot

Giant pandas – being bamboo-chomping, sleep-loving, rolling balls of fur – are among the most adorable animals alive today. Unlike their mainly carnivorous close relatives, such as the grizzly and polar bear, the panda is content surviving on a diet of only bamboo. Their anatomy seems more suited toward a carnivorous or omnivorous nature, so how and why have pandas abandoned their taste for meat to pick up a vegetarian diet?

How Pandas Stopped Eating Meat

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 its very peculiar diet.

Pandas and Bamboo – A Fairly Odd Pairing

As far as we can tell, giant pandas today survive on a diet of plants, 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 in the past enjoyed a more varied, omnivorous diet that might have included a small amount of bamboo1. It was only around 2 million years ago that they made this drastic switch to vegetarianism.

But it turns out that pandas aren’t really suited to a plant-based diet. Their digestive systems have not changed much from their meat-eating days, which means they cannot metabolize much of what they eat. Many of the enzymes that are 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 simply 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 huge amounts of bamboo while maintaining a low-metabolic, lazy lifestyle to compensate for the poor energy return2.



Loss of Tas1r1 – The Meat-Eating Gene

The giant panda’s shift to a vegetarian diet coincides with the inactivation of a 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. 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 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, suggesting that our knowledge of taste and diet in evolution is far from complete.

Panda Evolution Strategies

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.

A Bamboo-Gripping Thumb

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 onto 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, this means giant pandas have 6 digits in each paw! Although this pseudo-thumb cannot move on its own, it provides better support for the panda to easily grip objects such as bamboo in one paw. In other bears such as the grizzly, this bone remains undeveloped.

evolution of false thumb in pandas
The elongation of the radial sesamoid bone in its paw gives the giant panda better support when gripping bamboo (Source)

Harboring Cellulose-Digesting Bacteria

As mentioned, pandas suffer from having a carnivorous digestive system as they cannot break down cellulose, the key component of plants. Most herbivores are able to break down cellulose into sugar by producing certain enzymes, which is how they obtain their energy. Despite not having genes that code for cellulose-digesting enzymes, pandas instead shift this responsibility to bacteria present in their intestines.

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

An In-Between Tongue

Although the panda has jaws and teeth that are designed for tearing meat from bone and a digestive system more than capable of digesting it, it has a rather less well-defined tongue. In general, carnivores 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 unlike other carnivorous mammals.

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 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.



What Next for the Giant Panda?

Giant pandas in the wild are endangered, where deforestation has severely shrunk their natural habitat. Challenges in mating and reproduction – male giant pandas possess a very small baculum in relation to their size – also seem to work against them. Add their extremely narrow diet to this list, and we can understand why their survival is extremely dependent on conservation efforts in the form of protection and breeding programs.

It is clear that pandas have a long way to go in their evolutionary journey before they are able to thrive on a solely plant-based diet. Time will tell if the species will adapt and survive, abandon their vegetarian lifestyle, 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 floofballs.

giant panda just laying on a rock

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 Biology29(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. Science349(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 Sciences108(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 evolution27(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’. Nature397(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 Anatomy212(2), 99-105.

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