Birds lack the genes to taste the fish they gulp
Hubertus Kanus/Science Source
Despite their dapper appearance, penguins have remarkably little taste, a new study finds.
Penguins can’t taste bitter, sweet or the savory taste known as umami, researchers report February 16 in Current Biology. The jury is still out on whether the birds can taste salty and sour.
The ability to taste depends upon proteins called receptors. Those receptors nestle in the outer membrane of cells in the taste buds. Once receptors detect the molecules responsible for a particular taste, the receptors send a message to the brain about what’s tripping across the tongue.
Researchers Huabin Zhao of Wuhan University in China and Jianwen Li at BGI-Shenzhen in China began combing through the genomes of Adélie penguins and emperor penguins looking for the genes that create taste receptor proteins. The pair found salty and sour genes but none for the other three tastes. That’s when they called on evolutionary geneticist Jianzhi Zhang of the University of Michigan in Ann Arbor. Zhang has studied taste receptors in mammals, and helped the Chinese researchers search for the receptors in penguins.
Zhang says he was skeptical that fish-eaters like penguins would lack the umami receptor. After all, fish are quite savory. But to his surprise, the umami receptors “are truly lost in the two penguins,” he says. The Adélie and emperor penguins were also missing receptors for sweet and bitter, the researchers discovered. Chinstrap, rockhopper and king penguins also lack functional umami, bitter and sweet receptor genes, suggesting that the genes were already broken or missing in the common ancestor of all penguins.
The missing sweet tasters were no surprise; researchers already knew that all birds lack sweet receptors. But bitter’s desertion is a harder pill to swallow. Many toxins and poisons made by plants are bitter and the taste serves as a warning, says Peihua Jiang, a neurobiologist at the Monell Chemical Senses Center in Philadelphia. Since penguins eat mostly fish, they may not need bitter receptors as much for detecting poisonous plant compounds, Jiang says.
Penguins aren’t the only animals that lack the ability to taste bitter, sweet and umami, Jiang adds. Previous studies have shown that dolphins, sea lions and whales are all missing those taste sensations, too.
Zhang has an idea about why those tastes might have been dispensable for the penguins’ common ancestor. All three tastes require a protein called Trpm5 to send messages to the brain. Previous research has shown that the messenger protein, unlike the proverbial postal worker, is deterred by cold temperatures. Swimming in freezing water may have essentially killed penguins’ taste anyway, making it no big deal when the genes themselves were damaged and eventually lost.
In a cruel twist of fate, fish possess all the taste receptors and often have taste buds on their gills, skin or fins. So even though penguins probably can’t taste the fish they swallow whole, fish may be able to taste the penguins as they slide down the birds’ gullets.
H. Zhao, J. Li and J. Zhang. Molecular evidence for the loss of three basic tastes in penguins. Current Biology. Published online February 16, 2015. doi: 10.1016/j.cub.2015.01.026.