It’s sensational! A recent paper describing the taste ability of penguins was sensationalized in title and claim by the popular media. Don’t get me wrong, it’s a super cool story, but what this science coverage was missing was the opportunity for discussion of the author’s results. We tend to be so quick to simplify science to a black and white statement, e.g. “Penguins can only taste sour and salt!” This can make our scientific research relevant to a broader audience, but it often removes the opportunity for the public to engage in a dialogue about science and data.
To put it plainly, oversimplification may have the consequence of misleading the public to believe that the end claim for a study is the end of the story. Instead, we should aim to share and discuss in our scientific coverage that each piece of data brings more info to the table, and no data is the end all of the discussion.
With that in mind, I asked PhD candidate, Courtney Wilson, a graduate student in the University of Colorado Denver Neuroscience Program, to respond to both the paper and the need to share more beyond the headline. Courtney is in Dr. Sue Kinnamon's lab at the Anschutz Medical Campus, and her dissertation on taste has a focus on sour-sensing cells.
Thanks to Courtney and here’s to all of you out there who choose to think critically and for yourself.
The ability to sense a variety of compounds in food is critical for the survival of all species. Most vertebrates accomplish this with taste receptors—membrane proteins in taste buds that allow us to detect sweet, bitter, umami, salty, and sour substances in our food. While we’re still figuring out how salty and sour detection works, the receptors for bitter, sweet, and umami are known and well conserved across species; most vertebrates have similar gene sequences for these receptors!
Last week, Current Biology released a paper titled “Molecular evidence for the loss of three basic tastes in penguins” by Huabin Zhao, Jianwen Li, and Jianzhi Zhang.
The authors compared taste receptor gene sequences from a variety of penguins as well as other bird species, and found that the receptors for bitter, sweet, and umami were either absent or psuedogenized (mutated so that the resulting protein is no longer functional) in all examined penguin sequences. The loss off functional bitter, sweet, and umami receptors suggests that penguins may not be able to detect these compounds the way many other animals do.
The idea that taste ability differs across species is actually well established in the taste field! For example, cats and several other carnivores have lost their ability to taste sweet substances over time. In 2012, scientists at the Monell Chemical Senses Center in Pennsylvania showed that several carnivores have all lost functional sweet receptors in their genomes. Behaviorally, cats, Asian otters, and sea lions have no preference towards sweet substances while carnivores that still have functional sweet receptors (i.e. the spectacled bear) show a preference for sweet substances (Jiang et al, 2012).
While the loss of functional bitter, sweet and umami gene sequences in penguins suggests a loss of taste ability, these penguins may have evolved different ways of detecting important compounds in their environment. Hummingbirds, for example, also have a pseudogenized, non-functional sweet receptor gene. But they can still taste sweet nectar thanks to their umami receptor, which has evolved to detect sweet substances (Baldwin et al, 2014). Perhaps penguins too have evolved “replacement receptors” that we haven’t yet discovered. To prove that penguins have truly lost the ability to taste bitter, sweet, and umami compounds, we will need to test penguin taste behaviorally.
Penguins may have lost the known taste receptors for bitter, sweet, and umami because of their cold environments and feeding techniques. At the cold arctic temperatures in which penguins evolved, an important taste ion channel for bitter, sweet, and umami detection, (called TRPM5), does not function very well—making those methods of taste less useful. Additionally, penguins swallow most of their food whole, as opposed to chewing, so very little of the food content contacts the tongue. Thus, there may be little evolutionary need for taste buds to provide information about ingested food. Bottlenose dolphins eat the same way, and also show taste receptor psuedogenization.
If the original taste system becomes less useful for such environmental or behavioral reasons, then the pressure to maintain its components in the genome lessens considerably. All animals have evolved to survive in their respective environments. The receptors that most animals use for bitter, sweet, and umami detection weren’t very useful in the arctic environment where penguins evolved, so they gradually lost them. They may not be able to taste bitter, sweet, or umami compounds the same way we do (or at all!), but that doesn’t mean they are “bad tasters!” Penguins, like all creatures, are well suited to the environment in which they evolved.