By Brendan Moyle 11/02/2019


Giant pandas (Ailuropoda melanoleuca) eat bamboo and not much else. This in evolutionary terms is odd. It’s odd in part because the panda has a short gut typical of carnivores. And it still possesses many of the genes associated with a carnivorous lifestyle. This is largely due to belonging to the bear family Ursidae. This is a group of animals not noted for being herbivores. They are animals that are carnivores or omnivores.

In turns out the giant panda isn’t very good at eating bamboo. It has to eat a lot to make up for the short gut (herbivores have longer guts to give more time to digest the plant-matter). And it has to use conserve energy. So pandas aren’t very active animals and they occur at a low density (this reduces energy-burning conflicts). Like all bears, they don’t have opposable thumbs so grasping bamboo is tricky. The solution to this was an awkward false thumb- an extension of a wrist bone that could grip the bamboo instead. It turns out this though, is not unique to modern pandas. Other extinct bear species have it too [2].

So the two big evolutionary questions are:

  • were the ancestors of the extant Giant panda also vegetarian?
  • when did pandas become specialised bamboo eaters?

Now as a rule diets are hard to reconstruct from fossil evidence. There’s a lot of soft tissue about the digestive system. That does not fossilize well.  Nonetheless it turns out there is a way to do it. That is by using stable isotopes of Carbon and Nitrogen.

So something like Carbon has many isotopes. The most common is 12C – which has 6 protons and 6 neutrons. It is stable. Carbon 14, which is used for dating is unstable. It has 6 protons and 8 neutrons and decays into Nitrogen. A rarer isotope is Carbon-13 (13C) which has 6 protons and 7 neutrons. Similarly Nitrogen-15 has an extra neutron compared to the more common Nitrogen 14, and is also stable. Han et al [2] investigated the proportions of the 13C and 15N isotopes in collagen and teeth from ancient and modern samples. The proportion of these isotopes should inform us about diet.

One of the reasons this should work well with pandas is because bamboo has an odd metabolic trick. It uses C4 photosynthesis. This makes more efficient use of carbon-dioxide so the plants grow more rapidly, even if oxygen and water is limited in availability. In short, it’s why bamboo grows so quickly. Most plants however use C3 photosynthesis. Certainly all the alternative plant-sources available for the ancestors of pandas. Hence a bamboo specialist should have a different distribution of stable istotopes stored in their collagens and teeth enamels.

So comparing carnivores to herbivores in Chinese forests, and to the modern panda showed that carnivores generally had more carbon-13 and nitrogen-15 isotopes than herbivores, but the narrower diet of modern pandas showed even less of the two isotopes. These isotopes therefore could type the diet of these species.

There are two known ancient panda species. These are A. microta and A. baconi. Both have teeth adapted for eating durable food types. So this adaptation would have occurred before extant pandas became specialised for bamboo. (This food has tough fibres that need a lot of grinding too). Importantly these ancient pandas survived into the mid-Holocene (about 6000 years ago).

Comparing the isotopes was interesting. Basically ancient pandas were not bamboo-specialists. Their isotope mix was broader, indicative of a wide browsing habit of plants in closed-canopy forests. They were predominantly vegetarian. Modern pandas however, had a much narrower distribution pattern of the isotopes, indicative of the a specialised diet of C4 plants. Which in this case, means bamboo.

What makes the change so interesting is that it appears to be such a recent change. The mid-Holocene is not that long ago. It may also be linked to the substantial contraction in range of pandas. Originally pandas were distributed as far north as Beijing and as far south as the countries contiguous with Yunnan. Now they are limited to forests in Sichuan, Shaanxi and Gansu (with reports some trickle into Yunnan occasionally).

 

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References:

[1] Han, H., Wei, W., Hu, Y., Nie, Y., Ji, X., Yan, L., Zhang, Z., Shi, X., Zhu, L., Luo, Y., Chen, W., and Wei, F. Diet Evolution and Habitat Contraction of Giant Pandas via Stable Isotope Analysis. Current Biology 29, 1-6.

[2] Salesa, M.J., Anton, M. Pegine, S., and Morales, J. Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas.  Proc Natl Acad Sci U S A. 2006 Jan 10; 103(2): 379–382.

* One of the odd twists to my work in China has been developing some expertise with bears, qualifying as possibly the only New Zealander as a member of the IUCN Bear Specialist Group

(Header image taken by me at Beijing Zoo)