By Dr. Becker
Humans and (most) other vertebrae possess the ability to detect at least five well-defined taste sensations – sweet, bitter, salty, sour, and the newest cuisine-related buzzword, umami, aka savory. Birds, because they have fewer taste buds on their tongues and no teeth to chew food, are usually able to detect four flavors, sans the sweet acuity.
But even though they have feathers, lay eggs, and are warm blooded like other birds, penguins have even more taste restrictions, according to recent genetics studies at the University of Michigan. They're unable to distinguish bitter, sweet, or umami – only salty and sour.
The study noted:
"Remarkable progress in understanding the molecular basis of taste has opened the door to inferring taste abilities from genetic data. Based on genome and relevant gene sequences, we infer that the sweet, umami, and bitter tastes have been lost in all penguins, an order of aquatic flightless birds originating and still occupying the coldest ecological niche on Earth, the Antarctic."1
Although penguins might seem as if they really enjoy eating fish, which exude the umami flavor, they probably can't really taste them. For that reason, the absence of the umami taste receptors was particularly mystifying to the study team, led by Jianzhi "George" Zhang, professor of Ecology and Evolutionary Biology at U-M.
Evidence of Evolution or Incomplete Genome Sequencing?
Zhang's involvement in the scientific conundrum began when he received an email from colleagues at the Beijing Genomics Institute (BGI) in China, requesting help in finding out if the penguins' missing tasting abilities had to do with incomplete genome sequencing, or was a real, live example of evolutionary process.
But they were genuinely bemused when, in subsequent tests on other penguin species including those from warmer climates, all seemed to have the same issue. Zhang said it seemed logical that because they eat fish, penguins would need the umami taste receptors, but for some reason they don't. He wanted to get to the bottom of the puzzle.
"These findings are surprising and puzzling, and we do not have a good explanation for them. But we have a few ideas," Zhang said.
The science, with its ever-shifting paradigm, concentrated on the penguins' DNA, and determined that perhaps the taste-detecting restriction was less about the foods penguins eat, and more about the chilly climate penguins generally live in.
The taste buds themselves were another clue, Zhang said. The Current Biology article, the first to publish the study's findings, explained:
"Those for sweet, umami, and bitter flavors are temperature sensitive… They don't work well, if at all, if the individual is eating cold food in a cold environment. That's one reason why people can't taste the flavors in ice cream very well after a while. The cold dampens their taste buds, as well as ability to smell, a bit.
Penguins or their ancestors probably had all of the usual taste receptors at one point but lost them when they moved to cold environments. Some penguins have since moved to warmer areas, but because all penguins trace their roots to Antarctica, penguins the world over are still hardwired, taste-wise, for eating in a colder habitat.2"
Researchers at BGI had already established that Emperor and Adelie penguins, which hail from Antarctica, were both missing the three taste receptors. Zhang and company's subsequent testing in the U-M lab was conducted using the Beijing team's data and tissue samples from the rockhopper, chinstrap, and king penguins.
They also analyzed the genomes for 14 other types of birds, including finches, flycatchers, egrets, parrots, falcons, chickens, macaws, and mallards and established that while they possessed the genes for the umami and bitter tastes, none were able to distinguish the sweet taste.
The scientists finally concluded that all penguins and not just the Edelie and emperor genomes were missing the functional receptor genes to help them discern a broader palate, and that they had essentially become "pseudogenes." The genetic sequences, they wrote, resulted from an accumulation of multiple mutations over time, resembling a gene but without the ability to encode proteins.
An article in The Verge quoted Zhang as saying that while his initial premise was only a hypothesis:
"It's very clear… If the genes aren't there, then the animals don't have those tastes. The neighboring genes were all there, it's just that those specific taste genes were missing. So we know it's not because the quality of the genome sequence."3
Birds of a Feather… Aren't Necessarily the Same
Besides Zhang, the team included the study's first author, Huabin Zhao, now a professor at Wuhan University in China, but a postdoctoral researcher under Zhang at U-M for most of the study, and Jianwen Li from the China National GeneBank at BGI in Shenzhen.
The trio surmised that the primordial forebears of these flightless birds "lost" the bitter, sweet, and umami taste receptor genes around 20 million years ago when they migrated to colder climates.
Because all penguins originated in Antarctica, Zhang believes the missing taste capabilities of penguins have to do with prehistoric cooling events, which resulted in a change in penguin DNA.
They "probably had the usual taste receptors at one point," he added. A University of Michigan News article explained:
"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."4
Smithsonian Magazine noted:
"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 the University of Michigan outlines, penguins evolved in Antarctica around 60 million years ago and split off into different species around 23 million years ago. 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."5
Sweet, bitter, and umami flavors are best savored when they're warm, not cold; those essences are hard to detect after refrigeration. It's why cheese is usually better at room temperature and steaming hot coffee provides more satisfaction than when it's lukewarm.
Of Mice and Penguins
A protein called TRPM5, needed to signal sweet, bitter, and umami flavors to the nervous systems of vertebrates, may also play a principal role in the poser, especially since mice studies back up the premise that the protein can't do its job in lower temperatures. Zhao, the study's fist author, plans to conduct experiments to see how TRPM5 behaves in temperatures equal to the seawater that penguins in Antarctica are used to.
According to the Smithsonian article:
"In mice, the protein Trpm5 is also involved in insulin secretion and the detection of pheromones. If the same is true in penguins, then Trpm5 is essentially being asked to work simultaneously at a warm body temperature and at the frigid ambient temperature, which may not be possible. When such a dilemma arises, the more important function is retained by natural selection, while the less important one is sacrificed…"6
The study concluded, "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." Zhang added that he was unclear if the anatomical adaptations occurred first or the sensory changes.
Human taste buds include miniscule sensory organs on not only the tongue but the back of the throat and roof of the mouth. Penguin tongues have sharp projecting appendages probably designed to easily nab and ingest slippery food for simple sustenance without the need for taste buds or even the perception of taste.
"It's unclear whether these traits are a cause or a consequence of their major taste loss," Zhang said.
In any case, penguins still eat their share of food when it's available, ingesting krill and squid, as well as fish, which they swallow whole, incidentally. They may not even be aware they're missing out on something all the other birds have.