A giant panda munches on a stick of bamboo. Katherine Feng/Minden Pictures

With their sharp canine teeth and a gut built to break down meat, pandas have kept many features of their carnivorous ancestors. But these bamboo-munching vegetarians sport one piece of anatomy that may be evolving at a more rapid clip: their tongue. As ancient pandas switched to a plant-based diet, their bitter taste perception got better, according to a new study, helping them detect the dangerous toxins in bamboo.

Most plants are laced with bitter—and potentially harmful—toxins like cyanide, nicotine, and ricin to deter hungry herbivores. That may explain why plant eaters are more sensitive to bitter flavors than meat eaters, who rarely encounter them. At the extreme lie whales—exclusive carnivores that have gone completely tongue-blind to bitterness.

But as herbivores go, pandas are the new kids on the block, having gone vegetarian just a few million years ago. The ancestors of both the giant panda and the raccoonlike red panda regularly dined on flesh. But sometime in the past 7 million years, for reasons still unknown, they swapped meat for shoots and leaves. Dwindling prey coupled with the massive growth of bamboo forests in what is now southern China may have fueled the shift in diet. Previous genetic research found that soon after giant pandas began chomping on bamboo, they lost the ability to taste umami, the savory flavor in meat.

To find out whether giant and red pandas also gained the ability to taste the bitter toxins in bamboo, researchers led by conservation geneticist Fuwen Wei at the Chinese Academy of Sciences in Beijing screened the genomes of both species. The team looked specifically at genes of the TAS2R family—TAS2R1, TAS2R2, and so on—which encodes the bitter taste receptors inside taste buds. They also screened the genomes of seven carnivorous panda cousins, including polar bears, wolves, tigers, and cheetahs.

Both species of panda possessed 16 intact bitter taste receptor genes, more than their meat-eating relatives, who had between 10 and 14, the scientists will report in Integrative Zoology. In the carnivores, the bitter sensing genes had slowly built up random mutations that rendered many of them useless. Conversely, the panda genomes not only largely preserved the codes of their bitter receptors, but even developed some seemingly useful mutations. One gene in giant pandas, TAS2R42, had accumulated mutations with incredible speed compared with their other genes—a telltale sign that natural selection had favored these mutations. Presumably, these changes to the code generated a superior version of the receptor that helped the pandas detect chemicals in bamboo. “This is the most important evidence that giant pandas evolved adaptively to bamboo,” says Lei Shan, a geneticist and co-author of the study.

Stephen Wooding, an evolutionary geneticist at the University of California, Merced, says the findings are consistent with what he and other researchers believe happened in pandas. He notes that the pandas still have slightly fewer bitter taste receptors than most herbivores, which jives with their former penchant for meat. “They probably started to lose their bitter receptors, but it looks like when their diet shifted, that put the brakes on,” he says. Their meat-loving contemporaries continued to lose their bitter receptors, but pandas began retaining theirs.

The findings are still speculative, says molecular biologist Peihua Jiang at the Monell Chemical Senses Center in Philadelphia, Pennsylvania, who was not involved in the research. He points out that whether the panda receptors can detect the specific set of bitter compounds found in bamboo remains unknown. But as it turns out, Shan and his colleagues are already working on this, testing how panda receptors in living cells react when they get bombarded with bamboo-derived toxins. Their results could further clarify why, for pandas, evolution has such a bitter taste.（Science）