Most vertebrates can taste five different flavors—sweet, bitter, sour, salty and the savory taste, umami. Most birds are limited to just four flavors, omitting the sweet sense. But penguins have an even more limited palate. Scientists at the University of Michigan have found the aquatic birds can only taste two flavors: sour and salty.

"These findings are surprising and puzzling, and we do not have a good explanation for them. But we have a few ideas," said Jianzhi "George" Zhang, a co-author of the study recently published in the journal Current Biology.

Genetic analysis shows that all five penguin species are limited in flavor receptor abilities, which indicates that the birds’ common ancestor had lost them, too. As University of Michigan outlines, penguins evolved in Antarctica around 60 million years ago and split off into different species around 23 million years go. According to Zhang, the taste loss likely took place during the 37 million years between these developments, “which included periods of dramatic climate cooling in Antarctica."

And that serious coldness might be the key to understanding this development. Unlike the taste receptors for sour and salty flavors, bitter and umami receptors don’t fully function in low temperatures—so even if penguins had them, they wouldn’t be of much use. Zhang theorizes that this likely played a role in penguin palate limitations.

But there are other strange features of the black and white birds’ tongues and eating habits that suggest there may be more to the puzzle. Penguins swallow their food whole, for one thing. And their tongues, rather than having taste buds, a primary flavor receptor, are laden with sharp, spike-like papillae, which serve as grippers for catching prey. But, as Zhang says, “it is unclear whether these traits are a cause or a consequence of their major taste loss.”

Luckily, penguins don’t seem to miss the other flavors. However they perceive the taste of their primary diets of fish, krill, they don’t appear to have a problem choking it down. When preparing to molt, a single adult Emperor Penguin, for example, is known to eat up to 13 pounds of fish a day.