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Pandas' Preference for Bamboo Comes with Caveat

Posted By Prucia Buscell, Thursday, October 20, 2011
It's Plentiful, But They Have to Eat 16 Hours a Day

Pandas are anomalies of evolution. They illustrate unusual evolutionary adaptations that may have both helped and hindered survival. They are descendants of meat-loving carnivorous bears, and their digestive systems are built to process meat, yet they rarely eat it.

In fact, they eat almost nothing but bamboo. "Microbes Help Giant Pandas Overcome Meat-Eating Heritage," a story by Ewen Callaway, tells how these endangered creatures have evolved to live on plants.

Bamboo contains protein, fats and sugars, but most of its nutrition is locked in the cellulose fibers the make up the plant cell walls. Most carnivores don't produce the enzymes necessary to digest cellulose. Callaway describes the work of Fuwen Wei, an ecologist at the Institute of Zoology at the Chinese Academy of Sciences and his colleagues who studied microbes in the guts of giant pandas. They found that the guts of wild and captive giant pandas contained previously unknown genes produced by Clostridium bacteria, which were similar to known genes for enzymes that break down cellulose. The research is published in the Proceedings of the National Academy of Sciences.

Ling-Ling and Hsing-Hsing
Ling-Ling and Hsing-Hsing, 1985

A 1982 study of Ling-Ling and Hsing-Hsing, the two pandas the Chinese government presented to President Richard Nixon in 1972 as a gift after his visit to China, found that the pandas can't digest most of the bamboo they eat - 92 percent of the cellulose and 73 percent of the chemically similar hemicelulose passes right through their systems.

Wei and colleagues say the newly discovered microbial enzymes may help the pandas get extra energy from the bamboo they do process. He says the microbes are "part of a suite of evolutionary adaptations" that include powerful jaws and teeth and thumb-like bones in their paws that help them grip the plant stalks.

A SciTech story by Chi-Chi Zhang reports that pandas probably roamed the earth as long as three million years ago, and says researchers believe pandas may have stopped eating meat because mutations in their taste genes made them unable to experience its flavor. Ruth Ley, a microbiologist at Cornell University, is quoted in the Nature story as saying the panda is pretty poorly adapted, because the main way it gets its nutrients is to eat for 15 to 16 hours a day. The need to eat almost constantly is apparently one of the reasons pandas don't hibernate like other bears. They can't pile on enough body fat to last for months.

Pandas are endangered - only some 1,600 are known to live in the wild, mostly in China, because of loss of habitat, poaching, and slow reproduction. Wild pandas produce only one cub every two or three years, and captive pandas reproduce even less.

Ling-Ling and Hsing-Hsing have both died after relatively long lives. The Washington National Zoo has two more pandas from China, but this time they weren't gifts. They arrived in 2000 under a 10 year $10 million loan agreement with China. The agreement was renewed in January 2011 so the Zoo can keep them another five years.

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