Neuroscience Ph.D. Candidate, University of Colorado Anschutz Medical Campus
twitter handle: @courtthewilson
Last week, I cooked up some sugar water and pulled my hummingbird feeder out of winter retirement in preparation for the incoming population of thumb-sized, feathered friends. Every spring, a handful of hummingbird species make the long journey from their winter habitats in Central America to their summer breeding grounds in the Rocky Mountains. I’m hoping some of them will decide to stop in for a sweet snack at my birdfeeder on their way. It’s no great surprise that hummingbirds would be attracted to the sugar water in my feeder—they are, after all, uniquely built to be able to access the sweet nectars of flowering plants. Until a couple years ago, however, hummingbirds’ preference for sugar seemed mysterious in the face of an important feature of bird taste: birds cannot taste sweet.
When you or I eat something sugary, cells in our taste buds recognize sugar molecules via specialized structures called receptors. You can think of a sweet receptor as a lock, and the sugar molecule as a key—if the sugar molecule fits into the receptor, it “unlocks” the cell and it sends a signal to the brain, and the brain says, “Sweet!” We have lock and key type receptors for other tastes too. Your bitter receptors help you recognize the bitterness in foods like coffee and green vegetables. And your umami receptor helps you taste things that are savory, like beef broth and parmesan cheese, for example. What’s umami, you ask? Umami is a Japanese word for the sensation of savory taste—these receptors recognize protein content in the food we eat.
The gene that is responsible for the sweet receptor is so important to survival for so many species that it is nearly identical across the tree of life… except in birds! From chickens to penguins, these guys lack the part of their DNA that includes the sweet receptor. Even my little visiting hummingbirds lack this part of their DNA. So why do they seek out the sugar water in my hummingbird feeder?
The Mystery of Hummingbirds and Sweet Taste
In 2014, taste scientist Maude Baldwin and her colleagues set out to solve this mystery. They isolated various taste receptors from both hummingbirds (who seek out sweet) and chickens (who don’t) and tested how well their receptors could recognize sugars. In a nutshell, they isolated several taste receptors (locks), and tested whether each was activated by sugar, allowing them to see how hummingbirds might be unlocking the sensation of sweet.
They discovered that hummingbirds use an entirely different way to detect sugars—instead of the sweet receptor, they use the umami receptor! In hummingbirds, however, this receptor has evolved to recognize sugars instead of protein. This means that the shape of the hummingbird umami “lock” changed and changed and changed over time. Slowly it went from being unlocked by protein to being unlocked by sugar. So sometime after the loss of the sweet receptor in birds, hummingbird ancestors re-evolved the ability to detect sugars. Something in the environment at the time—a scarcity of other food sources, perhaps—favored the survival of birds that could taste, and thus would eat, the sweet nectars of plants over those that could not. Since they no longer had the original blueprint for the sugar receptor, a functional sweet receptor evolved from the next best thing—the umami receptor.
If my balcony hummingbird feeder manages to attract migrating hummingbirds this year, I’ll be able to witness in person the result of a clever evolutionary taste twist—one that confirms the hummingbird to be nature’s sneakiest sweet taster.