Pity poor penguins鈥� paltry palates. All they can taste, it seems, is the saltiness or sourness of their food.
Lost forever is the ability to savour the sweet, bitter or umami flavours enjoyed by most animals. And the reason could well be that receptors for these three tastes simply don鈥檛 work at the chilly temperatures at which penguins thrive 鈥� so they were consigned to evolutionary oblivion.
Researchers discovered penguins鈥� taste deficits after screening their DNA for genes coding for the proteins that taste buds use to pick up the five different tastes. They drew a blank when they looked for the usual genes for receptors registering sweet, bitter and umami tastes.
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鈥淎s far as we know, penguins are the only birds that have lost three of the five basic tastes,鈥� says Jianzhi George Zhang of the University of Michigan in Ann Arbor, who led the study.
The key, he believes, is the loss of the gene that makes Trpm5, a protein vital for registering the three lost taste types, but not the surviving two. 鈥淧revious studies showed that Trpm5 doesn鈥檛 function at low temperatures, so my hypothesis is that the cold Antarctic essentially made the three tastes that require Trpm5 unusable,鈥� says Zhang. 鈥淭hus, the taste receptor genes gradually degenerated and got lost.鈥�
Made redundant
Another possibility, he says, is that taste genes became redundant because penguins tend to swallow their prey whole. But in that case, why would salt and sour perception have survived? Zhang believes that sour was retained because it helps penguins avoid rotten food, and the ability to taste salt helps them keep track their salt intake. 鈥淪o it鈥檚 likely that having sour and salty tastes is still beneficial to penguins,鈥� he says.
Zhang previously showed that the giant panda has lost the ability to taste umami, probably when switching from being a meat-eater to bamboo-muncher. But no animal has yet been discovered lacking all five taste receptors, he says.
Birds lost their sweet receptors 100 million years ago 鈥� although hummingbirds have since repurposed other taste receptors to savour nectar. Cats, too, lack sweet receptors, and vampire bats have lost umami and sweet tastes. The most limited palates of all, however, are those of dolphins and whales, which can only taste salt.
鈥淭his is an exciting finding and raises a host of new questions,鈥� says of Harvard University, lead author of the study on hummingbirds rejigging their taste buds. 鈥淚t will be revealing to see if other polar vertebrates also lack these tastes, and to further investigate the behavioural and anatomical consequences of widespread taste loss in penguins.鈥�
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