Genome sequencing over the last decade has revealed birds to lack the gene T1R2, one of two that combine to allow animals to taste sugar. Alligators, on the other hand -- one of birds' closest relatives -- have both the necessary sweet tooth genes. The discrepancy suggests that as birds split off from dinosaurs on the evolutionary family tree, they lost their taste for sugar. Yet, hummingbirds are nectar fiends -- they can't get enough. But why? And how?

Most vertebrates have three taste-related genes; they combine in different ways to allow for different tastes. Now, new research suggests hummingbirds have evolved their T1R1 and T1R3 sensors -- typically used to detect umami or savory flavors -- to pick out sugary notes.

Scientists confirmed this by cloning T1R1 and T1R3 sensors from chickens, hummingbirds and finches. With these taste sensors isolated in a lab setting, researchers Maude Baldwin from Harvard University and Yasuka Toda from the University of Tokyo presented the receptors with various stimulants.

Their experiment confirmed that the same receptors that are excited by savory amino acids on the tongues of chickens and finches are titillated by simple sugars like glucose and fructose on a hummingbird's tongue. The researchers estimate hummingbirds got their sweet tooth back sometime between 72 and 42 million years ago.

But that still leaves the question: why?

Baldwin thinks hummingbirds may have begun hanging out near flowers to hunt after insects. At some point, an adventurous specimen took a sip of a flower's nectar. And because nectar provides a dense pack of nutrients, the decision paid off.

"You don't know how it begins," Baldwin told National Geographic. "But once it does, there's selection to reinforce it and make it stronger."

Baldwin and Toda's work is detailed in the latest issue of the journal Science.