Scientists have released a study showing how hummingbirds' ability to detect sweetness evolved from an ancestral savory taste receptor that is mainly tuned to flavors in amino acids.
Everything about hummingbirds is rapid. They're so quick, their movements can only be captured with clarity by high-speed video. When researchers slow down hummingbirds on replay, they're able to see the birds using its tongue to lap up sugar from a feeding station.
Their tongues can dart 17 times a second to get sugar from a feeding station.
It takes just three licks of their forked, tube-like tongues to reject water when they expect nectar, according to a Harvard Medical School press release.
They pull their beaks away, shake their heads, and spit out the tasteless liquid. They are also not fooled by the sugar substitute that sweetens most diet sodas, and when they are tricked they show their displeasure.
Hummingbirds' preference for sweetness is plain, but now scientists can explain the biology behind their taste for sugar.
It took Harvard University biologist Maude Baldwin and her colleagues more than three years to answer the sweetness question.
Their research required an international team of scientists, fieldwork in the California mountains and at Harvard University's Concord Field Station, plus collaborations from Harvard labs, according to the release.
The scientists were able to show how hummingbirds' are able to detect sweetness evolved from an ancestral savory taste receptor that is mainly turned to flavors in amino acids.
The tiny birds expanded throughout North and South America living off of nectar and insects. More than 300 species over the 40 to 72 million years could be found in those two locations since they branched off from their closest relative, the swift, according to the release.
Baldwin reached out to Stephen Liberles, a HMS associate professor of cell biology, and they started researching how hummingbirds developed a taste receptor that wasn't present in the genome of other birds.
"It's a really nice example of how a species evolved at a molecular level to adopt a very complex phenotype," said Liberles, according to the Harvard release. "A change in a single receptor can actually drive a change in behavior and, we propose, can contribute to species diversification."
After cloning the genes for taste receptors from hummingbirds, chickens, and swifts, Baldwin needed to test what the proteins expressed by these genes were responding to, according to the release.
She then teamed up with Yasuka Toda, a graduate student of the University of Tokyo and co-first author of the paper, who devised a method for testing taste receptors in cell culture.
They were able to show that in chickens and swifts the receptor responds strongly to amino acids, but in hummingbirds only weakly. The receptor in hummingbirds responds strongly to carbohydrates, the sweet flavors, according to the release.
"This is the first time that this umami receptor has ever been shown to respond to carbohydrates," Baldwin said.
Toda was able to mix and match different subunits of the chicken and hummingbird taste receptors into hybrid chimeras to understand which parts of the gene were involved in this change in function.
She found 19 mutations, but there are probably more contributing to the switch.
"If you look at the structure of the receptor, it involved really dramatic changes over its entire surface to accomplish this complex feat," Liberles said. "Amino acids and sugars look very different 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."
Once the mutations were discovered, the researchers set out to figure out if they matter. After bringing the birds back to the feeding stations, they spat out that water, but the siphoned up both the sweet nectar and one artificial sweetener that evoked a response in the cell-culture assay.
"That gave us the link between the receptor and behavior," Liberles said. "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."
Their work was published this week in the journal Science.
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