Typically, vertebrates have five common taste receptors: sweet, salty, sour, bitter and umani. According to new genetic research, penguins have lost three of the five; sweet, bitter and umani, the savory taste associated with meat.

Because penguins are almost exclusively fish eating animals the revelation was especially perplexing to the leader of the study Jianzhi “George” Zhang, a professor in the University of Michigan Department of Ecology and Evolutionary Biology.

“Penguins eat fish, so you would guess that they need the umami receptor genes, but for some reason they don’t have them. These findings are surprising and puzzling, and we do not have a good explanation for them. But we have a few ideas,” he said in a statement.

Zhang’s best hypothesis is that the taste receptors were lost during climate cooling events in Antarctica where penguins originated about 60 million years ago. According to Zhang it is possible that the temperatures, plummeting into a brutal cold, interfered with taste perception.

Over the last 15 years, scientists have learned a great deal about the molecular basis of taste. Based on this knowledge they have been able to infer taste abilities from genetic data for a number of species.

All birds are thought to be poor tasters compared to mammals. They have no teeth for chewing, fewer taste buds overall and the sweet taste receptor is missing from all birds examined to date.

The study on penguins, by Zhang and two Chinese colleagues in the February 16 edition of the journal Current Biology, was prompted by an email from a Chinese genomics institute (BGI). Researchers there had sequenced genomes from emperor and Adelie penguins. They couldn’t find some of the taste genes and wanted help determining whether there was a problem with incomplete sequencing or whether the genes were actually missing.

Zhang and his team examined the data as well as tissue samples from king, rockhopper and chinstrap penguins and eight closely related, non-penguin, species. Additionally they examined genomes for 14 more non-penguin bird species.

The researchers found that all five penguin species lack genes for the sweet, bitter and unami tastes. In emperor and Adelie genomes the bitter and unami receptor genes have come “pseudogenes”. Although they possess sequences that resemble genes, pseudogenes lack the ability to encode proteins.

All other birds examined to date contain the bitter and unami genes, but lack the sweet gene.

“Taken together, our results strongly suggest that the umami and bitter tastes were lost in the common ancestor of all penguins, whereas the sweet taste was lost earlier,” the authors wrote.

Penguins originated in Antarctica about 60 million years ago after separating from tuberose seabirds. The major penguin groups because distinct species about 23 million years ago. The taste loss must have occurred at some point in the 37 million year interval between the two dates.

Zhang suspects that the protein Trpm5, responsible for the transduction of bitter, sweet and unami taste signals to the nervous system in vertebrates. Earlier research on mice showed that the protein does not function well in cold temperatures.

“This give us a hint, perhaps, that this loss of taste genes has something to do with the inability of this protein to work at lower temperatures,” said Zhang.

Huabin Zhao, first author of the study and a University of Michigan postdoctoral student under Zhang when the research was conducted is planning follow-up experiments to see how well the protein Trpm5 functions at the temperature of Arctic seawater.