Scientists have been puzzled by the fact that hummingbirds maintain such a sugary diet without a sweet-taste receptor. For most mammals, the sweet-taste receptor that responds to sugars in plant-based carbohydrates is made up of two proteins: T1R2 and T1R3. The taste receptor that detects savory, or umami, flavors found in meat and mushrooms is made up of the proteins T1R1 and T1R3.
But after the chicken genome was sequenced in 2004, researchers noticed the birds lacked the gene that encodes T1R2, a crucial component of the sweet-taste receptor. This same pattern was seen in other bird genomes.
"If a species is missing one of those two parts, then the species can't taste sweet at all," said Maude Baldwin, a doctoral student of evolutionary biology at Harvard University and one of the researchers on the study.
When scientists sequenced the genomes of cats, lions, tigers and cheetahs - true carnivores that also don't have a taste for sweets - they found these species still have a nonfunctional "pseudogene" (a nonfunctional gene that's lost its protein-coding powers) for the sweet-taste receptor. But in bird genomes, scientists never even found a trace of a pseudogene for a sweet tooth, Baldwin told Live Science.