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Researchers Discover Biological Mechanism

Unlike many types of birds, hummingbirds not only have the ability to detect sweetness, they also have a craving for sugary substances, and now the authors of a new Science study have discovered the biological reason why these tiny flying creatures differ from their avian counterparts.

According to experts from the Harvard University Department of Organismic and Evolutionary Biology, the University of Tokyo Graduate School of Agricultural and Life Sciences, the Dublin City University Bioinformatics and Molecular Evolution Group, the University of California Department of Animal Science and the Harvard Medical School Department of Cell Biology, the reason resides in a taste receptor typically used to detect savory-type flavors.

As Live Science News Editor Megan Gannon explains, in many creatures the sweet-taste receptor that responds to sugars in plant-based carbohydrates is comprised of the T1R2 and T1R3 proteins, while the taste receptor that detects savory or umami flavors in meat and mushrooms is made up of the T1R1 and T1R3 proteins.

In 2004, researchers sequenced the genome of the chicken and its DNA revealed it was missing the T1R2 gene. The discovery suggested that chickens could not taste sweet flavors, and since it was the first bird to have its genome fully sequenced, Harvard University’s Maude Baldwin and Yasuka Toda of the University of Tokyo wondered if the same was true of other birds. They went on to sequence the genomes of 10 other birds, none of which had T1R2.

Alligators do, and they’re some of the closest living relatives of birds. So at some point, as birds evolved from small dinosaurs, they lost their sweet tooth,” explained National Geographics Ed Yong. “What about hummingbirds? Hummingbirds feed largely on nectar, the sweet liquid that flowers produce… the sweeter the better; they’ll actually reject flowers whose nectar isn’t sweet enough. They lack the T1R2 gene, but they can clearly taste sugar.”

Baldwin, Toda and their colleagues have now discovered that the savory sensors on a hummingbird’s tongue also double as sugar sensors. The researchers figured that if hummingbirds had lost T1R2, it was possible that the T1R1-T1R3 savory sensor could detect sugars instead, Yong said. They tested their hypothesis on the Anna’s hummingbird, a medium-sized hummingbird native to the western North America, and discovered they were right.

Anna’s hummingbirds, as it turns out, can detect simple sugars like glucose and fructose, as well as some sweeteners like sorbitol and erythritol. They can also detect amino acids, according to Yong – it simply gained a new ability sometime during the last 42 to 72 million years, thanks to dramatic changes to the T1R1 and T1R3 proteins.

In order to figure out just how drastic those changes were, and which ones were important, the study authors spliced together both the chicken and hummingbird versions of the proteins in different combinations, then tested their responsiveness to sugars. They identified 19 amino acids located in one region of T1R3 that had changed during hummingbird evolution, warming the shape of the proteins and allowing them to stick to sugars, Yong noted.

In a statement, Baldwin said that this marks “the first time that this umami receptor has ever been shown to respond to carbohydrates.” She went on to tell National Geographic that she and her colleagues are investigating to see whether the 19 mutations happened all at once or in batches, and whether or not they are all directly involved in detecting sugars. “This has the possibility of answering bigger questions in evolutionary biology,” she added.