How Hummingbirds Evolved a Taste for Sugar: The Sweet Switch

Hummingbirds are some of the fastest birds on Earth. In fact, their movements can only be captured with clarity with high-speed video. Now, scientists have learned a bit more about these iridescent birds; scientists have found out how these creatures developed their taste for sugar.

When scientists slow down hummingbirds on replay, they can see a hummingbird using its tongue to lap up sugar from a feeding station; in fact, their tongues can dart out at 17 times a second. Yet it takes only about three licks of their forked, tube-like tongue to reject water when they expect nectar. They pull their breaks back, shake their heads and spit out the tasteless liquid. In fact, they're not even fooled by sugar substitutes.

The search to find out about a hummingbird's taste for sweetness all started with the chicken genome. Before scientists sequenced the chicken genome, most thought that all birds tasted things the same way that mammals do-with sensory receptors for salty, sour, bitter, sweet and savory. Yet it turns out that it's not the case; the chicken genome has no trace of a sweet-taste receptor gene.

"The immediate question to ornithologists or to anybody who has a birdfeeder in the backyard was: What about hummingbirds?" said Maude Baldwin, co-author of the new paper, in a news release. "If they are missing the single sweet receptor, how are they detecting sugar?"

The researchers cloned the genes for taste receptors from chickens, swifts and hummingbirds. Then they tested the taste receptors in cell culture. In the end, they found that in chickens and swifts the receptor responds strongly to amino acids-the savory flavor. But this is response is weak in hummingbirds. Instead, hummingbirds respond strongly to carbohydrates-the sweet flavors.

The scientists then looked at these receptors a bit more closely. In the end, they found 19 mutations that were involved in the switch between tasting savory and sweet.

"If you look at the structure of the receptor, it involved really dramatic changes over its entire surface to accomplish this complex feat," said Stephen Liberles, one of the researchers. "Amino acids and sugars look very differently structurally so in order to recognize them and sense them in the environment, you need a completely different lock and key. The key looks very different, so you have to change the lock almost entirely."

The findings reveal a bit more about how taste evolved in hummingbirds and how they can sense sugar in the first place. This, in turn, should give insights into the brain, which defines the world around us with the help of sensory systems.

The findings are published in the journal Science.