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It's not unusual for birds to lack taste receptors for sweet foods, but penguins may be unique in their inability to taste bitter and umami flavors.
This may be because of the chilly environment from which all penguins -- even the ones who now live further south -- once hailed. The receptors for the three missing tastes don't work very well in the cold, so it's possible that penguins never really had much use for them in the first place.
If the environment doesn't let an animal take advantage of one of their senses, they're not going to rely on it for anything important. So over time, instead of natural selection honing that ability to taste sweets and meats, the ability could actually get weaker: If there's no evolutionary downside to being horrible at tasting your fish, then penguins are just as likely to pass on bad taste genes as they are to pass on the good ones.
This study only looked at the genes that allow taste to exist, so researchers will have to do more hands-on research to confirm what tastes penguins can and can't detect.
But it's possible that the birds are even worse at tasting than these results suggest. From The Verge:
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.
So maybe penguins can taste salty and sour foods, but maybe they can't. It's possible that they rely solely on sight and smell to decide which food is good to eat. This would make sense, given the unusual anatomy of many penguins' tongues: Instead of taste buds, some penguins have sharp protrusions designed to grip their food. In other words, tongues aren't really for tasting in the penguin world -- they're for grabbing food and swallowing it whole.