WASHINGTON, Feb. 16 (Xinhua) -- Penguins apparently can't enjoy the savory taste of the fish they eat, according to a genetic study conducted by Chinese and U.S. researchers that suggested the flightless, waddling birds' adaptation to an extreme cold environment may come at a price.

The study, published Monday in the U.S. journal Current Biology, found that penguins have lost three of the five basic tastes -- sweet, bitter and the savory, meaty taste known as umami -- more than 20 million years ago and never regained them.

For these birds, it appears that food comes in only two flavors: salty and sour.

"Penguins are often viewed as a success story of adaptation to a very harsh environment," study leader Jianzhi Zhang, professor of the University of Michigan, told Xinhua. "Apparently there were failures in this adaptation."

Because penguins are fish eaters, the loss of the umami taste is especially surprising, said Zhang.

"Penguins eat fish, so we expected that they have the umami taste genes," he continued. "But for some reason they don't have them ... we do not have a good explanation."

Compared with mammals, birds are thought to be poor tasters, due in part to the observations that they have fewer taste buds on their tongues and lack teeth for chewing food.

Previous genetic studies showed that the sweet taste receptor gene is absent from the genomes of all birds examined to date.

The penguin study was prompted by an email from BGI, a genomics institute in China, where researchers had sequenced genomes from Adelie and emperor penguins and could not find some of the taste genes, Zhang said.

They took a closer look at the penguin DNA to find that all penguin species lack functional genes for the receptors of sweet, umami, and bitter tastes.

Penguins originated in Antarctica after their separation from tubenose seabirds around 60 million years ago, and the major penguin groups separated from one another about 23 million years ago. The taste loss likely occurred during that 37-million-year span, which included periods of dramatic climate cooling in Antarctica, Zhang said.

The researchers suggested that genes encoding those taste receptors may have been lost in penguins not because they weren't useful, but rather because of the extremely cold environments in which penguins live.

Unlike receptors for sour and salty, the taste receptors required for detecting sweet, umami, and bitter tastes are temperature sensitive and don't work when they get really cold. In other words, even if penguins had those taste receptors, the receptors wouldn't be much use to many of them, the researchers said.

Penguin tongues are also unusual in other ways, the researchers noted.

Previous studies have suggested that some penguins lack taste buds -- the primary location for taste receptors -- altogether. Instead, penguin tongues have stiff, sharp papillae covered by a thick, horny layer, suggesting that their tongues are used not so much to taste food, but rather to catch and hold onto it, they said.

Penguins also have a habit of swallowing their food whole, which might leave them less concerned about what their food actually tastes like.

"Their behavior of swallowing food whole, and their tongue structure and function, suggest that penguins need no taste perception, although it is unclear whether these traits are a cause or a consequence of their major taste loss," Zhang said.

Although all penguins originated in Antarctic, some subsequently migrated out and now live in the temperate zone.

"I have been asked whether these temperate penguins have the sweet, umami and bitter tastes," Zhang said. "The answer is no, because these tastes were already gone in the common ancestor of all penguins and cannot be regained once lost." Enditem