Digestion in Giant and Red Pandas

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Welcome to my website on digestion in pandas, created for an Animal Physiology class at Davidson College. This website will investigate diet and digestive mechanisms in the giant panda (Ailuropoda melanoleuca) and the red panda (Ailurus fulgens). While both of these species are classified as carnivores, they are each notable for their tendency to eat bamboo almost exlusively. In fact, the name "panda" comes from the Nepalese word bigalya ponya, meaning "eater of bamboo" (Red Pandas). Due to the low-energy content of their food, pandas spend almost all of their time either eating or sleeping. I will review papers that have studied panda digestion in order to summarize our current level of understanding of how and why these animals survive on such a cellulose-rich diet.


Giant Panda (left) and Red Panda (right). Photos courtesy of Wikimedia Commons.

Pandas

Giant Panda (Ailuropoda melanoleuca)

Typically characterized by their round face with black circles around the eyes, giant pandas are native to Southern China. This endangered species is estimated to have less than 2,000 individuals living in the wild, and efforts to help revive the panda population of China have been hindered by the reluctance of giant pandas to mate in captivity (Number of pandas). Adults weigh 65-110 kg and eat almost 6% of their body mass per day (Johnson, 1988).


Giant Panda feeding on a bamboo shoot. Photo courtesy of Wikimedia Commons.

Red Panda (Ailurus fulgens)

Red pandas, sometimes referred to as "lesser pandas," differ significantly from giant pandas in their size, with adults weighing 3.7-6.2 kg. Like giant pandas, this species is found in Southern China and is listed as endangered. With a fragmented popultation of fewer than 3,000 individuals in the wild, the red panda population is on the decline (Endangered Species). Like the giant panda, red pandas eat almost exclusively bamboo. It is estimated that a red panda can consume up to 45% of its body mass in a single day (Red Pandas). That's over 200,000 bamboo leaves!


Red Pandas feeding on bamboo. Photo courtesy of Wikimedia Commons.
Taxonomy

The classification of the giant panda and red panda has been a source of contention for more than a century. The red panda was described before the giant panda in scientific literature, and at that time it was classified in the raccoon family (Mayr, 1986). Upon the discovery of giant pandas, which were classified in the bear (Ursidae) family, a problem was presented. Obvious similarities existed between the red and giant panda, however, they were classified in different families. Eventually, the red panda and its extinct relatives were given their own family (Ailuridae), since the relationship between giant pandas and red pandas was difficult to quantify (Roberts, 1984).

	Giant Panda	Red Panda
Kingdom	Animalia	Animalia
Phylum	Chordata	Chordata
Class	Animalia	Animalia
Order	Mammalia	Mammalia
Family	Ursidae	Ailuridae
Genus	Ailuropoda	Ailurus
Species	melanoleuca	fulgens

Most of the controversy in taxonomy arose because it was difficult to determine whether similarities in the red panda and the giant panda were the result of a phylogenetic relationship or convergent evolution. Regardless of how these similarities came about, red pandas and giant pandas have many unique characteristics that can be investigated by comparing the two species.

Bamboo Diet

Bamboo makes up over 99% of the diet of both red and giant pandas in the wild (Wei, 1999). A diet of this type would typically be characteristic of a herbivore, however, pandas are members of the order Carnivora. The reason for this distinction in classification is that pandas do not have a specialized digestive tract for digesting plant material. Sheep, cows, deer, and goats have a rumen that allows for the break down of plant matter by microbes. Other herbivores have a large caecum that houses bacteria responsible for semi-digesting cellulose and other plant-like material.

Surpringly, the digestive tract of pandas is relatively short and simple, completely lacking a caecum and only stretching 10 cm in colon-length (Senshu, 2007). In fact, the small intestine of giant pandas is actually smaller than that of most other bear species (Sims, 2005). Studies have shown that the giant panda is only capable of digesting 19.6 +/- 0.12% of the dry matter (DM) in bamboo, while they can digest 100% of gruel DM and 82% of apple DM (Dierenfeld, 1982). This has led researchers to the conclusion that pandas undergo little to no microbial digestion of plant matter in their digestive tract (Dierenfeld, 1982). See the Bacterial Flora section for more on this.


Bamboo Plant.
Photo courtesy of Wikimedia Commons.

How, then, are pandas able to survive on a diet of only bamboo? Even today, no one knows for sure. In 2000, Wei wrote that "it is still unclear how this animal utilizes the energy of its bamboo diet to fulfill its daily energy requirement" (Wei, 2000). While the exact mechanisms are not completely understood, it is clear that the panda has adpated in multiple ways to be able to consume bamboo as its primary foodsource.

One way that pandas compensate for their low energy diet is by simply eating more food. Giant pandas are able to move food through their entire digestive tract in a little over 8 hours, while red pandas can discharge their digesta in 2-4 hours (Dierenfeld, 1982). As a result, even red pandas, which weigh between 4 and 6 kg, are capable of consuming as much as 4 kg of bamboo shoots in a single day (Wei, 2000).

To complement this fast rate of digestion, pandas have developed strong jaw muscles (which account for their rounded faces), a dense skull, and jagged teeth that assist in grinding plant matter (Wei, 2000).

A final adaptation to a bamboo diet is the panda's ability to preferentially select the most nutritious parts of the bamboo plant to eat. In 2000, Wei showed that the red panda changed their diet preferences from bamboo leaves to bamboo shoots in the spring and winter months because the leaves were less energy-rich than the shoots at those times (Wei, 2000). See the Seasonal Variations section for more on this.

Giant vs. Red

Because the giant and red panda both inhabit the same region of the world (Southern China), it is important to look at the resource competition, if any that occurs between the two species. Two studies have been done on this interaction, one in the Yele Natural Reserve in Sichuan, China (Wei, 1999) and one in the Wolong Natural Reserve (Johnson, 1988). I will look at each study separately to provide an overview of their findings.

Giant and Red Pandas in the Wolong Natural Reserve (Johnson, 1988)

This study used radiotelemetry to track the movement of red and giant pandas throughout the Wolong Natural Reserve. This was the first paper to document movements of the red panda, since they are difficult to capture and tag. The results of the study revealed that the two species spent about 85% of their time in the same area, however, there was only a dietary overlap of about 40-43%. The maximum overlap in diet occurred during the summer.

Red pandas were observed to rest about 65% of the day and be actively eating for almost all of their waking hours. Giant pandas, however, were only observed to rest for 43% of the day. This difference could be attributed to the giant pandas higher requirement for energy consumption due to body size. Additionally, this paper notes that, unlike giant pandas, red pandas lowered their metabolic rate significantly in cold temperatures and decreased blood flow to the skin. The authors hypothesized that this behavior, coupled with a thick fur coat, allowed the red panda to survive in cold weather on a relatively low-energy diet. This type of restriction in blood flow to the skin would not be as necessary in giant pandas, given their large body size.


Giant Pandas in the Wolong Reserve. Photo courtesy of Wikimedia Commons.

Giant and Red Pandas in the Yele Reserve (Wei, 1999)

This study used fecal droppings of red and giant pandas to track their movement and their diet. Giant panda droppings are longer and thicker than red panda droppings, making the distinction between the two fairly easy to detect. 5 different species of bamboo were grown in the reserve and fecal matter was tested to determine the dietary matter.

The results of the study showed significant similarities between the diet of red and giant pandas. Both preferred to eat the bamboo species of highest nutritional value (Q. macrophyla and B. fangiana). Additionally, both red and giant pandas preferred to eat leaves (as opposed to shoots). When eating shoots, however, both panda species preferred young, tender shoots.

Even with this extreme overlap in ecological niches, the study also found that the species had apparently adapted for coexistence. Although the red panda's diet consisted of 98% bamboo, the other 2% consisted of various fruits that the giant panda did not eat. Additionally, giant pandas were found to eat the bamboo branches and culms (the woody parts of the plant), while red pandas ate only the shoots and leaves. While giant pandas preferred leaves to other parts of the plant, leaves only consisted of approximately 40% of their dietary intake, while it was over 90% of the red panda's dietary intake. One last way in which giant pandas and red pandas are able to coexist is in their tendancy to roam in different microhabitats. Giant pandas appeared to prefer areas of low density brush, while red pandas preffered areas that were densely covered with fallen logs. The differences in red and giant pandas that were described in these studies may explain how these two species can coexit in Southern China.


Red Pandas playing. Photo courtesy of Wikimedia Commons.

Bacterial Flora

It seems likely that, given their herbivorous nature, the digestive tract of pandas would be ripe with microbes to assist in the break-down of plant matter. However, pandas present a unique case for animals. While they eat almost entirely plant matter (most of which they cannot digest themselves), they also contain few gut organisms to assist in its break-down.


Cows utilize the rumen, above left, to aid in the break down of plant matter. Photo courtesy of Wikimedia Commons.
In 1989, Hirayama looked at the bacterial community in the feces of giant pandas using a culture-dependent approach (Hirayama, 1989). His paper demonstrated that Streptococcus, a facultative anaerobe, was the predominate bacterial species in the gut of pandas. While most plant-eating species contain obligate anaerobes in their gut to help break down plant material, pandas contain few to none. The paper suggests that the lack of obligate anaerobes could be due to the quick rate with which digesta moves through the gut. This would prevent anaerobic conditions from developing inside the digestive tract. The study also noted that in infant pandas, Lactobacillus are present in the highest numbers, likely to assist in the digestion of milk from the mother.


Streptococcus, shown above, are the most abundant gut
microbe in pandas. Photo courtesy of Wikimedia Commons.
In 2007, another study was done on the microbial community in giant panda feces (Wei, 2007). This study had the benefit of powerful DNA-based approaches to characterizing the gut organisms of pandas and provided what is likely to be more accurate data on the gut organisms of pandas. Data was recorded over a 2 year period, during which the flora of the pandas remained relatively constant.

The study found an Escherichia coli relative to be the predominant single organism in the gut. 60% of the organisms sampled appeared to be facultative anaerobes, corresponding to a very different overall population of gut organisms than exist in herbivores. This finding supports the main claims of the Hirayama paper, and the Wei paper actually cites strong correlation with the findings of the Hiryama paper, even though they used a different sampling technique. Notably absent in the gut community were Bacteriodes, the most dominant population in the human digestive tract. Both of these studies confirmed that the microbial flora in the intestine of pandas has not adapted to the cellulose-rich environment that is presented by a bamboo diet.


Figure 1: Phylogenetic tree of the different microbial species detected in the fecal matter of giant pandas using
a 16S rDNA library. Figure taken with permission from Wei, 2007.

Seasonal Variations

Another area of panda digestion and diet that has been studied are the affects of seasonal variation on diet in red and giant pandas. In 1999, Wei reported that in the Spring, giant pandas fed mainly on bamboo stems - the thick woody parts of the plant (about 90% of their diet by mass). However, as the plants began to mature in the late Spring and Summer, pandas ate about 70% shoots - the part of the plant that has leaves coming off of it. Finally in the late Summer and Fall, giant pandas ate over 90% bamboo leaves by mass (Wei, 1999). Obviously, there is a very large difference in the dietary preference of pandas based on the season.


Bamboo stems (thick, woody stalks) with shoots coming off. Leaves are
attached to the shoots. Photo courtesy of Wikimedia Commons.

In 2000, Wei followed up his 1999 paper with a look at seasonal energy utilizaion in the red panda (Wei, 2000). This study compared the changes in energy content of bamboo plant parts as they related to changes in dietary preferences. Wei was able to show that the protein content of immature bamboo leaves in the Winter and Spring was significantly lower than during the Summer and Fall months (P < 0.001). This difference can be observed in Figure 1 below.

Figure 1 also shows that in the Spring and Winter, red pandas digested less total energy from plant leaves than they did in the Summer and Autumn. In Spring this is likely due to the immaturity and unavailability of the bamboo leaves. In the Winter this can be attributed to lower nutritional value, since the energy intake remains high. Wei was able to show that this decrease in energy content of leaves in the Spring and Winter correlated with an increase in bamboo shoot intake. The ability of pandas to preferentially select nutritious parts of the bamboo plant is an important evolutionary adaptation to their bamboo diet.

Season	Crude Protein (% mass)	Energy Intake (kJ/day)	Energy Digested (kJ/day)
Spring	14.93 +/- 0.27	10,145 +/- 142.7	2,738 +/- 69.0
Summer/Autumn	16.57 +/- 0.49	12,045 +/- 3210.6	3,309 +/- 138.5
Winter	14.65 +/- 0.19	12,276 +/- 350.9	2,932 +/- 130.8

Figure 1: Seasonal Nutritional Value of Bamboo Leaves in Red Pandas. (Adapted from Wei, 1999 and Wei, 2000)

Conclusion

All of the information presented in this website leads to the obvious question of why pandas would have ever evolved to be carnivorous bamboo-eaters. While there is no definitive answer to this question, most authorities on the subject agree that it has a lot to due with the lack of competition for bamboo resources. In any case, lack of any definitive answer to this question might explain why pandas are on the brink of extinction. In 2007, Wei wrote that the inability to digest cellulose means that pandas are "balancing on the threshold of nutritional subsistence, making them highly vulnerable to perturbations, such as those imposed by humans" (Wei, 2007).


Giant and red pandas are both endangered species whose populations
are still on the decline. Photo courtesy of Wikimedia Commons.

In order to address the problem of panda extinction, some researchers are investigating the possibility of adding gut organisms to the intestine of pandas to help them break down the plant material (Wei, 2007). However, as we have seen, the small digestive tract of pandas may make this strategy difficult to accomplish.


Pandas do not appear to have a stable ecological niche, which could pose problems
for their surival in the wild and in captivity. Photo courtesy of Wikimedia Commons.

In terms of classifying pandas, it is obvious that putting giant and red pandas into specific taxonomic groups is a difficult challenge, and perhaps impossible. The apparent evolutionary relationship of the panda to the bear and to the raccoon sheds light on the ambiguity that exists in scientific classification as a whole and to the uniqueness of these two species specifically.

References

Journal Articles

1. Dierenfeld, E. S., Hintz, H. F., Robertson, J. B., Vansoest, P. J. & Oftedal, O. T. (1982) Utilization of bamboo by the giant panda. Journal of Nutrition, 112, 636-641.


2. Hirayama, K., Kawamura, S., Mitsuoka, T. & Tashiro, K. (1989) The fecal flora of the giant panda (Ailuropoda-melanolleuca). Journal of Applied Bacteriology, 67, 411-415.


3. Jin, C. Z., Ciochon, R. L., Dong, W., Hunt, R. M., Liu, J. Y., Jaeger, M. & Zhu, Q. Z. (2007) The first skull of the earliest giant panda. Proceedings of the National Academy of Sciences of the United States of America, 104, 10932-10937.


4. Johnson, K. G., Schaller, G. B. & Hu, J. C. (1988) Comparative behavior of red and giant pandas in the Wolong Reserve, China. Journal of Mammalogy, 69, 552-564.


5. Johnson, K. G., Schaller, G. B. & Hu, J. C. (1988) Responses of giant pandas to a bamboo die-off. National Geographic Research, 4, 161-177.


6. Mayr, E. (1986). Uncertainty in science: is the giant panda a bear or a raccoon? Nature, 323, 769-771.


7. Roberts, M. S. and Gittleman, J. L. (1984) Ailurus fulgens. Mammalian Species, 222, 1-8.


8. Senshu, T., Ohya, A., Ide, K., Mikogai, J., Morita, M., Nakao, T., Imazu, K., Jingcao, L., Xuanzhen, L., Wenqi, L. & Lili, N. (2007) Studies on the digestion in the Giant Panda, Ailuropoda melanoleuca, fed feedstuffs including bamboo. Mammal Study, 32, 139-149.


9. Sims, J. A., Parsons, J. L., Bissell, H. A., Sikes, R. S., Ouellette, J. R. & Rude, B. J. (2007) Determination of bamboo-diet digestibility and fecal output by giant pandas. Ursus, 18, 38-45.


10. Tarou, L. R., Williams, J., Powell, D. M., Tabet, R. & Allen, M. (2005) Behavioral preferences for bamboo in a pair of captive giant pandas (Ailuropoda melanoleuca). Zoo Biology, 24, 177-183.


11. Wei, F., Feng, Z., Wang, Z. & Li, M. (1999) Feeding strategy and resource partitioning between giant and red pandas. Mammalia, 63, 417-429.


12. Wei, F. W., Feng, Z. J., Wang, Z. W., Zhou, A. & Hu, J. C. (1999) Use of the nutrients in bamboo by the red panda (Ailurus fulgens). Journal of Zoology, 248, 535-541.


13. Wei, F. W., Wang, Z. W., Feng, Z. J., Li, M. & Zhou, A. (2000) Seasonal energy utilization in bamboo by the red panda (Ailurus fulgens). Zoo Biology, 19, 27-33.


14. Wei, G. F., Lu, H. F., Zhou, Z. H., Xie, H. B., Wang, A. S., Nelson, K. & Zhao, L. P. (2007) The microbial community in the feces of the giant panda (Ailuropoda melanoleuca) as determined by PCR-TGGE profiling and clone library analysis. Microbial Ecology, 54, 194-202.


15. Yu, G. Z., Jiang, Z. G., Zhao, Z. L., Wang, B. F. & Wang, Y. (2003) Feeding habitat of giant pandas (Ailuropoda melanoleuca): why do they prefer bamboo patch edges? Journal of Zoology, 261, 307-312.

Websites

1. Endangered Species. http://www.endangeredspecie.com/specieprofile.htm. October 27, 2008.


2. Red Pandas. http://www.wellingtonzoo.com/net/explore/animals.aspx?id=20. October 27, 2008.


3. Number of pandas bred in China down from last year. http://news.xinhuanet.com/english/2007-11/08/content_7034856.htm. October 27, 2008.