Hummingbirds are drawn to nectar in an unusual way. Instead of depending on a sugar sensor found in many vertebrates, the flitting, frenetic birds use a repurposed sensor that normally responds to savory flavors, scientists report in the Aug. 22 Science.
Researchers led by Maude Baldwin of Harvard University and Yasuka Toda of University of Tokyo studied the genomes of 10 bird species and found no hint of the gene that encodes the sweet detector that most vertebrates rely on. Like those birds, hummingbirds probably also lack the gene, the researchers reasoned. But experiments on cells in dishes revealed that hummingbirds’ umami receptors, which normally detect savory amino acids, pick up the slack and detect sucrose, glucose and fructose.
In both lab tests and the wild, hummingbirds preferred liquid sweetened with sucrose over water. The birds lapped up a low-calorie sweetener called erythritol that the umami receptor can sense, but didn’t enjoy the sugar substitute aspartame, which didn’t get a response from the receptor.
SWEETEN THE DEAL A ruby-throated hummingbird takes only 162 milliseconds to reject water in the top feeder but settles in for a long meal of sweet sucrose at the bottom feeder.
François Peaudecerf and M. Baldwin
Hummingbirds’ ability to sense sweets seems to set them apart from other birds, including their closest living relatives, insect-eating chimney swifts. The researchers suggest that a dinosaur ancestor of modern birds lost the sweet detector common to other vertebrates, and that hummingbirds later regained a sweet tooth. This distinction is what allows them to feast on nectar while other birds ignore it, the researchers say.