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From the Australian Broadcasting Corporation:
Tuesday, 17 February 2015
Penguins can’t appreciate the delicate flavours of the seafood they catch thanks to a lack of taste.
New research published today in Current Biology reveals that penguins don’t have the genes that encode three of the five basic tastes – sweet, bitter, and umami (a savoury flavour), although they are genetically capable of detecting salty and sour tastes.
The study, led by Dr Huabin Zhao from Wuhan University, looked at the recently sequenced genomes of Adelie penguins (Pygoscelis adeliae) and emperor penguins (Aptenodytes forsteri) and 14 other bird species.
They found that while many other birds, such as chickens, finches and Amazon parrots have also lost the ability to taste sweet flavours, unlike penguins they are still able to detect bitter and umami tastes.
“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,” explains study co-author Dr Jianzhi Zhang from the University of Michigan.
He says it’s a puzzling finding.
“Penguins eat fish, so you would guess that they need the umami receptor genes, but for some reason they don’t have them.”
Adding to the puzzle is that taste is a powerful indicator of whether a potential food item is good enough to eat.
“In general, a sour taste helps detect spoiled food and bitter taste helps detect toxic food,” Zhang says. “Presumably, penguins cannot use taste to detect toxins … but we don’t know if penguins have other means of detecting toxins.”
Zhang says that while the research team doesn’t yet have a definitive answer to why penguins are so lacking in taste, they do have some ideas.
Penguins evolved in the chilly Antarctic, and the taste receptors for sweet, umami and bitter tastes (but not salty or sour tastes) are inactive at lower temperatures. It is likely that since they wouldn’t have worked anyway their loss along the evolutionary pathway would have had minimal impact.
Many species of penguins have since spread out into warmer climates. However, Zhang says “if ancestral penguins had lost the receptor genes for the three tastes while in the Antarctic, the genes and tastes cannot be regained even when some migrated away.”
In addition, anatomical studies have shown that penguin tongues seem to lack taste receptors – even for salty and sour flavours. Instead they are covered in stiff, horny bristles designed to help catch and hold their slippery prey, which they then swallow whole, so they may not have much interest in how their food tastes anyway.
But Zhang says he is not sure that penguins completely lack taste buds pointing to the possibility that they yet might be found in the tissues of the pharanx or elsewhere in the palate.
Interestingly, in nectar-loving hummingbirds that have also lost the genes encoding sweet receptors, the umami receptor has been repurposed to detect the sweet tastes of the food source they depend on.
Zhang says that a similar repurposing hasn’t happened in penguins.
“The sweet and umami receptors have a common ancestry and are similar in structure so it is possible for the hummingbird umami receptor to be repurposed to detect sweet,” he says.
“However, the sour and salty receptors are structurally completely different from sweet and umami receptors: There is no way that they can be repurposed to sense sweet or umami.”