This post contributed by Monica Kanojia, Administrative Assistant/Governance for ESA
While a vegetarian lifestyle is a choice made by omnivorous humans, the panda population may have been forced to convert to a vegetarian diet between 2 and 7 million years ago to ensure survival. The preference for bamboo is unusual for pandasbecause they are classified as carnivores even though their diet is 99% bamboo. Even more unusual is the fact that their digestive system is unable to process cellulose, the major component of plant cell walls. According to research published in Nature, the bamboo diet is both influenced by genetics, and it depends on the digestive microbes present in the panda gut.
Everything from what we eat, to what we taste, to how we eat is determined by our genetics. Umami—the basic taste associated with an amino acid common in protein heavy foods like meat—is sensed through the T1R gene family in carnivores. But in pandas, the T1R gene family has experienced mutations causing the inactivation of the T1R1 gene, making it a pseudogene.
Pseudogenes have either lost protein coding ability or are no longer expressed in the cell. Ruiqiang Li and the team who sequenced the genome found that the malfunction of the T1R1 gene occurred relatively recently in the panda lineage: Estimated loss was about 4.2 million years ago. The malfunction of the umami taste receptor may explain why pandas have a preference for bamboo versus meat.
Gene mutations are random and can change the habits of an organism, affecting its entire existence. In the case of the pandas, it changed the way pandas perceived meat. Despite the loss of taste for meat the digestive system of the pandas remained able to process it because all the enzymes required to were still present in their system. The ability to process plant material on the other hand was not natural. According to Li et al.’s research pandas do not have the necessary enzymes to digest bamboo, hinting at the idea that their ability to do so is dependent upon their gut microbes.
Luckily for the endangered pandas, according to a molecular analysis conducted by Li and his colleagues, they have a very high rate of genetic variation in spite of their low population numbers. The abundance of some genetic changes within the gene pool can be reduced by natural selection, while other “more favorable” mutations may accumulate and result in adaptive changes; this may be part of the reason why the panda population converted from meat eaters to plant eaters as well. Logically, it would go as follows: The panda population experienced a mutation affecting taste buds which caused, or contributed to, a switch to a bamboo diet. If the bamboo diet increased their survival rate (which would have increased their reproductive levels), over time, the mutation of their taste buds would comprise the majority of the population, if not all of it.
The question then becomes what happened first: Did pandas confront environmental factors which limited meat, causing nonfunctioning genes? Or did they lose their T1R1 function, leading to the dietary change? Since the exact dietary conversion date is still unknown, it’s a bit more difficult to determine whether the meat went first, or the taste for it.
Monica Kanojia is a George Mason University graduate with a B.S. in Biology. She has interned at NSF and contributed articles to LiveScience.com. She has also interned with EarthShot Foundation, an environmentally based NGO focused on driving a clean energy revolution.
Photo Credit: Frank Peters