By Dr. Becker
Somewhere along their evolutionary history, birds lost their ability to taste sugar. But according to new research, hummingbirds have been able to restore their sweet tooth by repurposing a specific type of receptor used to detect salty or spicy flavors.
A team of scientists from Harvard University, Harvard Medical School, the University of Tokyo, Dublin City University, and the University of California, Davis conducted the study, which was published last year in the journal Science.1
Most Birds Lack Taste Buds for Sweet Flavors
The tongue contains surface receptors (taste buds) known as T1Rs that differentiate between tastes. In vertebrates, including humans, the T1R2 and T1R3 receptors work together to give us the ability to taste sweet flavors. Birds don’t have the genes that code for T1R2, however, lizards do. This suggests that sweet receptors disappeared at some point during the evolution of birds.
Hummingbirds, as it turns out, have evolved a workaround to the lack of a sweet tooth. Not only do the tiny birds feast on nectar, given the option, they will choose sweet tasting foods over non-sweet offerings. Hummingbirds in both the lab and in the wild showed a preference for liquid sweetened with sucrose over water.
And the researchers discovered why. It seems that in hummingbirds another pair of receptors – T1R1 and T1R3 – work together to detect sweet tastes. Interestingly, while the birds enjoyed a low-calorie sweetener called erythritol, their sweet taste receptors didn’t respond to aspartame.
Other vertebrates use the T1R1 receptor to taste savory (spicy or salty) foods, but it seems that in hummingbirds, the proteins on the surface of T1R1 and T1R3 have been altered to detect sweet foods instead.
Hummingbirds Occupy a Unique Environmental Niche
Hummingbirds spend their lives vacillating between a sugar high and starvation. Their hyperactive metabolisms often require that they eat more than their weight in food each day to survive.
Hummingbirds occasionally eat insects, but their primary food source is the nectar from flowers. Since nectar is not a food source for most other birds on the planet, the hummingbird occupies a unique space in the environment.
The little birds are quite adaptable, living throughout North and South America in habitats as varied as high-altitude mountainous regions to tropical rainforests. They have also diversified into more than 300 species.
The Change in Hummingbird Taste Receptors May Have Happened Repeatedly
According to Maude Baldwin of Harvard University, and lead study author:
"’The change in the taste receptor was certainly not the only factor or aspect of hummingbird biology that was important [for them to feed on nectar], but it seems like it played an important role,’ says Baldwin. ‘There are many behavioral and physiological changes that have occurred between hummingbirds and their ancestors: small body size, a long bill, and changes in the wing which allowed them to hover.’"2
The researchers believe the re-emergence of sugar receptors in hummingbirds may have occurred several times. She believes:
"Birds are the descendants of carnivorous dinosaurs, so maybe this gene was lost early on because of the diet of their ancestors," Baldwin said. "That would be very cool, but we're still not sure."3
The researchers look forward to future studies on other species of nectar loving birds to see what similarities, if any, exist between their evolution of a sweet tooth and that of hummingbirds.