Wednesday, February 18, 2015

#Penguins love eating fish but probably can't taste them

(BMJ / Shutterstock)
The evolutionary peril of not chewing your food
You'd think that penguins would love the meaty taste of fish, but it turns out that they may not be able to taste their food much at all. A new genetic study out of the University of Michigan finds that penguins appear to have long ago lost the ability to taste sweet and bitter flavors, as well as the savory, meaty flavor known as umami. Together, sweet, bitter, and umami make up three of the five basic tastes. The other two, sour and salty, may still be present in penguins. The findings are being published today in Current Biology.

The fish may have been too cold for penguins to taste

So what led to penguins' believed loss of these tastes? "This is the most difficult question, especially because penguins eat fish and fish have the umami taste, so you would have predicted that the umami taste would have been useful to penguins," George Zhang, the paper's corresponding author, tells The Verge. Zhang's hypothesis — and he notes "it is still a hypothesis" — is that it's too cold down in Antarctica for these taste receptors to matter.

Prior research has shown that the tongue's receptor channels, which are what react to tastes, function poorly at lower temperatures when it comes to detecting sweet, bitter, and umami. The Michigan researchers speculate that the environment may have been so cold that this receptor was "effectively non-functional" in penguins' ancestors. "Those three tastes … would not be useful any more because the channel is not functioning," Zhang says. "So gradually mutations would accumulate in those genes and eventually they would become lost."

The study is based entirely on genetic findings, so another group of researchers will have to actually run tests with penguins to confirm that they can't taste these foods. But the study's authors are quite confident that future work will show penguins can't taste sweet, bitter, and umami flavors based on the genetic findings. "It's very clear," Zhang says. If the genes aren't there, he says, then the animals don't have those tastes.

The genes for tasting sweet, bitter, and umami were broken or missing

"The neighboring genes were all there, it's just that those specific taste genes were missing," Zhang says. "So we know it's not because the quality of the genome sequence." Zhang's research group was able to locate the genes for tasting bitter and umami flavors in around 20 other birds, but in the five penguin genomes that they looked at, including those from Adélie and emperor penguins, the genes were either broken or missing. Birds as a group are known to have lost their ability to detect sweet tastes, so that gene was absent in every sample that was tested.

The absence of bitter and umami tasting genes in the five penguins leads the researches to believe that penguins as a group likely lost this taste in their common ancestor, while the ability to detect sweet tastes appears to have been lost much farther back. Because penguins seem to have originated in the Antarctic, the researchers believe that penguins that live elsewhere should also be unable to detect these tastes, too.

penguin high five (shutterstock / Anton_Ivanov) (Anton_Ivanov / Shutterstock)

Though the researchers believe that penguins should still be able to detect sour and salty tastes, David Yarmolinsky, a taste researcher at Columbia who was not involved with the study, says that's still up in the air. "Really this paper can only make a statement about those three taste qualities," he says. That's because while the absence of a gene typically says something, its presence doesn't always mean the same thing.

"They did find a channel that's necessary for salt taste … but that doesn't mean a whole lot because that channel is required for your kidneys to work," he says. "If they didn't have that, they'd be dead penguins whether or not they can taste. So just because they have it in their genome doesn't mean they have it on their tongue." Yarmolinsky also says that the gene associated with the ability to detect sour tastes has complications that prevent that tasting ability from being a certainty, too.You really have to take the penguin and see how it reacts when you give it something sour to eat," he says. "That's really the test to see what they can or cannot taste."

Earlier research suggests that what taste buds penguins do have are fairly limited, if they have them at all. That makes sense, since the main function of penguins' tongues appears to be allowing them to capture prey; penguins also swallow their food whole. "Given the way their mouths are oriented, and how they hunt, capture, and consume their prey, it’s not surprising that penguins may have limited taste perception," Michael Polito, an ecologist with Louisiana State University who was not involved with the study, writes in an email to The Verge. "Other senses, especially vision and possibly even smell, may be more important when it comes to how penguins find and identify their favorite foods in the ocean."

Whales and dolphins are also believed to have lost most of their taste, with only the salt channel remaining. Yarmolinsky notes that researchers have been looking at taste loss and its consequences in other animals lately. This study, he says, "really fits into the story that's coming out the last few years that sensor receptors are adaptive to the lifestyles of animal diet." It's also believed that taste receptors may play other roles inside the body, so a penguin's inability to detect sweet, bitter, and umami flavors could have deeper implications than just what's on its tongue.


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