Birds as they evolved from dinosaurs lost the ability to perceive the sweet taste of sugar, with one notable exception -- hummingbirds -- and now, scientists say the understand the biology behind the hummers' sweet tooth.

Birds don't possess genes that code for a receptor known as T1R2, which along with another receptor, T1R3, gives vertebrate a taste for sugar. The genes are present in lizards, however, suggesting birds lost them at some time in their evolution from dinosaurs.

However, it turns out hummingbirds re-evolved the capacity to taste sugar by repurposing receptors used by other vertebrates to taste savory foods, Harvard University researchers working as part of an international team report in the journal Science.

A hummingbird's ability to perceive sweetness is linked to an ancestral savory receptor known as T1R1, they say, and that evolution has led to their success, with more than 300 species found today in North and South America.

Other vertebrates utilize the T1R1 receptor to taste savory foods, but hummingbirds have apparently been able modify proteins on the surface of the receptors so they react to sugars and sweet tastes instead.

The resulting sweet tooth is a powerful one, the researchers said, with tests showing hummingbirds strongly preferring sweet-tasting food such as nectar over any possible blander alternatives.

"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," Harvard researcher Maude Baldwin says.

The evolution of hummingbirds from their ancestors has seen many physiological and behavioral changes, she says, including "small body size, a long bill and changes in the wing which allowed them to hover."

The re-evolution of the birds' sugar receptors may have occurred multiple times, the researchers say, noting that the loss of certain tasting abilities by species is common in nature.

Pandas that feed exclusively on bamboo lack receptors for savory tastes, while many carnivores, especially cats, are mostly uninterested in sweet tastes in food.

They have the gene for sensing sweetness but it has become nonfunctional, the researchers explain.

In the study of hummingbirds, the researcher found 10 mutations that likely led to turning on of a "sweet switch" despite the absence of the usual sweet receptor gene.

"This dramatic change in the evolution of a new behavior is a really powerful example of how you can explain evolution on a molecular level," says Harvard cell biology Professor Stephen Liberles.