In their “tuxedo suits,” penguins may appear dapper. Yet these birds have little taste, a new study finds.
Penguins can’t taste bitter, sweet or the meaty flavor known as umami. That’s what researchers in China and Michigan reported February 16 in Current Biology. Still unclear, they say, is whether the birds might sense salty and sour flavors. So while these birds can down big meals of fish, they will never fully taste what they’re eating.
Taste is one of the body’s major senses. For animals to register taste, the chemicals that make up food must make contact with certain types of cellular proteins. These are known as receptors. Most of the receptors reside in the outer wall of cells in the taste buds. (Some can be found elsewhere in the body, even in the guts of some animals.)
Once receptors detect food molecules, they send out a message. Nerves ferry this signal to the brain, describing the flavors of what’s tripping across the tongue.
In the past, scientists uncovered genes — inherited instructions — for building the taste receptors. Researchers know which genes make the sweet, bitter, umami and salty-detecting proteins. (A gene for tasting for sour has also been found. And scientists suspect there are more they have not yet discovered.) These genes are but a few of the thousands in any organism’s full genetic instruction book. Scientists call that full playbook an individual’s genome.
Huabin Zhao of Wuhan University in China and Jianwen Li at BGI-Shenzhen in China began combing through the genomes of Adélie and emperor penguins. They were looking for the genes that create taste-receptor proteins. Salty and sour genes showed up. But the scientists found none for the other three tastes.
That’s when they called on Jianzhi Zhang. He’s an evolutionary geneticist at the University of Michigan in Ann Arbor. Zhang has studied taste receptors in mammals. So he helped his Chinese colleagues search for the receptors in penguins.
Zhang was skeptical that fish-eaters, like penguins, would lack the umami receptor. After all, fish are quite savory. To his surprise, he found the umami receptors “are truly lost in the two penguins.” Adélie and emperor penguins also lacked receptors for sweet and bitter.
The researchers then started probing genes of other penguin species. Chinstraps, rockhoppers and king penguins also lack working genes for umami, bitter and sweet receptors, they found. This suggests that those genes already were broken or missing in some common ancestor of all penguins, Zhang says.
The missing sweet receptor came as no surprise. Biologists already knew all birds lack this (so you’ll never entice them with a spoonful of sugar). But bitter’s absence was a harder pill to swallow. Many plant toxins — poisons — are bitter. Indeed, their harsh taste can serve as a warning, notes Peihua Jiang. He’s a neurobiologist at the Monell Chemical Senses Center in Philadelphia. But as penguins mostly eat fish, Jiang says, they may have relatively little need for the bitter receptors used to sense plant poisons.
Penguins aren’t the only animals unable to taste bitter, sweet and umami, he notes. Dolphins, sea lions and whales can’t sense them either.
Zhang, at Michigan, has an idea about why those tastes might have been no big loss for the penguins’ common ancestor. All three tastes rely on a protein called Trpm5 to send flavor signals to the brain. Previous research had shown this flavor messenger doesn’t work well when it’s cold. So swimming in freezing water could have essentially killed this protein’s action even if it had been present, he suspects.
Interestingly, fish possess all five taste receptors. And, in a cruel twist of fate, many fish wear taste buds on their gills, skin or fins. So even though a penguin probably can’t taste the fish it swallows whole, fish may be able to taste their predator as they slide down its gullet.
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evolutionary genetics A field of biology that focuses on how genes — and the traits they lead to — change over long periods of time (potentially over millennia or more). People who work in this field are known as evolutionary geneticists.
gene (adj. genetic) A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
genome The complete set of genes or genetic material in a cell or an organism. The study of this genetic inheritance housed within cells is known as genomics.
gullet The tube that carries food from the mouth to the stomach.
neurobiologist Scientist who studies cells and functions of the brain and other parts of the nervous system.
penguin A flightless black-and-white bird native to the far Southern Hemisphere, especially Antarctica and its nearby islands.
proteins Compounds made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. The hemoglobin in blood and the antibodies that attempt to fight infections are among the better known, stand-alone proteins.Medicines frequently work by latching onto proteins.
receptor (in biology) A molecule in cells that serves as a docking station for another molecule. That second molecule can turn on some special activity by the cell.
rookery A place where birds or other animals, such as penguins and seals, nest or breed. The term stems from the word "rook," which is an ancient bird that looks like the North American crow and nests in colonies near the tops of trees.
taste buds A collection of 50 to 100 or so taste receptors, they’re found on the tongues of land animals. When certain chemicals in food or other materials trigger a response in these receptors, the brain detects one or more flavors — sweet, sour, salty, bitter or umami.
toxin A poison produced by living organisms, such as germs, bees, spiders, poison ivy and snakes.
umami One of the five major tastes (along with sweet, sour, salty and bitter). It has been described as savory but most people find the flavor hard to characterize. It is particularly prized as a flavor in Japanese cuisines.
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Original Journal Source: 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.