A genetic study of penguins revealed they lost the ability to experience three of the five basic vertebrate tastes over 20 million years ago.
The findings show the flightless birds are no longer able to taste sweet, bitter and umami (savory), the University of Michigan reported.
"Penguins eat fish, so you would guess that they need the umami receptor genes, but for some reason they don't have them," said Jianzhi "George" Zhang, a professor in the U-M Department of Ecology and Evolutionary Biology. "These findings are surprising and puzzling, and we do not have a good explanation for them. But we have a few ideas."
The researchers believe the genes responsible for the perception of these tastes were lost during a period of extreme cooling in Antarctica. To make their findings, the researchers analyzed tissue from a number of penguins and related bird species. They found all penguin species lacked functional genes for the receptors of sweet, bitter, and umami tastes; the other bird species only lacked the genes for sweet perception, which is believed to be true for all bird species.
In the Adelie and emperor genomes, the umami and bitter taste perception genes have become "pseudogenes," which lack the ability to encode proteins.
"Taken together, our results strongly suggest that the umami and bitter tastes were lost in the common ancestor of all penguins, whereas the sweet taste was lost earlier," the authors wrote.
The protein Trpm5 is required for the transduction of these taste signals to the nervous system in all vertebrates. Past studies involving mice have suggested Trpm5 does not function well at extremely low temperatures. Past studies have also shown that penguin tongues have only a single type of lingual papillae, which holds taste buds in humans. These papillae are stiff and sharp, suggesting penguin mouths are designed more for catching fish than tasting them.
"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 concluded.
The findings were published in a recent edition of the journal Current Biology.
