New York, Feb 17 (IANS): Penguins lost three of the five basic tastes - sweet, bitter and the savoury meaty taste known as umami - more than 20 million years ago and have never regained them, reveals a University of Michigan-led study of penguin genetics.

"Because penguins are fish eaters, the loss of the umami taste is especially perplexing," said study leader Jianzhi "George" Zhang, professor in department of ecology and evolutionary biology.

Penguins eat fish so people would guess that they need the umami receptor genes -- but for some reason, they do not have them.

"These findings are surprising and puzzling and we do not have a good explanation for them. But we have a few ideas," Zhang added.

He suspects the sensory changes are tied to ancient climate cooling events in Antarctica where penguins originated.

The leading hypothesis is that the genes were lost after cold Antarctic temperatures interfered with taste perception.

Vertebrates typically possess five basic tastes: sweet, sour, salty, bitter and umami.

Previous genetic studies showed that the sweet taste receptor gene is absent from the genomes of all birds examined to date.

For the study, Zhang and his colleagues took a closer look at the Adelie and emperor penguins data.

In addition, they analysed bird tissue samples (chinstrap, rockhopper and king penguins, plus eight other closely related non-penguin bird species).

They also reviewed publicly available genomes of 14 other non-penguin bird species.

They found that all penguin species lack functional genes for the receptors of sweet, umami and bitter tastes.

In the Adelie and emperor genomes, umami and bitter taste receptor genes have become "pseudogenes", genetic sequences resembling a gene but lacking the ability to encode proteins.

The genomes of all non-penguin birds studied -- including egrets, finches, flycatchers, parrots, macaws, falcons, chickens and mallards -- contain the genes for the umami and bitter tastes but, as expected, lack receptors for the sweet taste.

"The results strongly suggest that 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 after their separation from tubenose seabirds around 60 million years ago and the major penguin groups separated from one another about 23 million years ago.

"The taste loss likely occurred during that 37-million-year span which included periods of dramatic climate cooling in Antarctica," Zhang said.

The paper is forthcoming in the journal Current Biology.