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Scientists Solve Mystery of How Hummingbirds Taste Sweetness

A photo of a male ruby-throated hummingbird.
A ruby-throated hummingbird feeds on mimosa flowers. Photograph by George Grall, National Geographic Creative

Hummingbirds feel the sweet lure of nectar, but they taste it in the most unexpected of ways.

This group of feathered friends doesn’t have a sweet taste receptor, which means they shouldn’t be able to taste sweet at all. But a new study published Thursday in Science reveals that hummingbirds have repurposed their umami receptor (which recognizes meaty and savory flavors) to be able to taste nectar’s sweetness.

“There are not many cases of such a complicated function being regained over the course of evolution,” which makes it a unique example to study, said Maude Baldwin, a doctoral student at Harvard University and lead author of the new study.

The mystery surrounding the hummingbird’s ability to taste began a decade ago, when researchers first published the complete sequence of the chicken genome. The chicken was the first bird to have its complete genome sequenced, and the data revealed many of its secrets. The chicken, the researchers found to their surprise, didn’t have a functioning sweet taste receptor. While sugar might not taste bad to chickens, they didn’t seek it out, either.

Because this trait is shared by many different groups of birds, scientists believe that some of the birds’ ancestors, such as the small, four-legged dinosaurs from which they likely descended, also lacked a sweet taste receptor.

Baldwin and other ornithologists noticed a problem. Lacking the ability to taste sugars might not be a problem for birds that eat bugs, but plenty of birds, such as hummingbirds, live entirely on nectar, which is almost completely sugar. Without sweet taste receptors, how could they find food?

Using new genetic-sequencing techniques, Baldwin and colleagues hunted through the genomes of ten types of birds, including hummingbirds. Indeed, all of these species lacked the standard sweet taste receptor, and all also carried the closely related umami taste receptor. When the researchers looked more closely at one of the umami taste receptor genes, they found quite a few mutations in hummingbirds. Their work revealed that more than 19 amino acids, the building blocks of a protein, had changed in hummingbirds.

Baldwin hypothesized that these mutations might have enabled the umami receptor to taste the sugar in nectar. First, she measured how the actual umami receptor protein reacted to amino acids and sugars in three different species: chickens, Anna’s hummingbirds (Calypte anna), and the closely related, insect-eating chimney swift (Chaetura pelagica). Whereas the umami receptor in both the chicken and the chimney swift failed to respond to sugars, the receptor in the hummingbird did. It also responded to several different artificial sweeteners. The umami receptor in all three continued to respond to amino acids.

Behavioral tests confirmed the cellular work. Both ruby-throated hummingbirds (Archilochus colubris) and Anna’s hummingbirds spent significantly more time drinking sugar water than plain water. These tests were repeated in several other hummingbird species with the same results.

In an accompanying perspective piece, Peihua Jiang, a researcher at the Monell Chemical Senses Center, points out that the work is remarkable in several ways. “While many species have lost the use of their taste receptor, there haven’t been any examples where it has been repurposed like this,” he said.

Baldwin is currently interested in determining whether the umami receptor is repurposed in all species of hummingbird, and whether other nectar-eating birds, such as honeyeaters, have similar adaptations. She also wants to see whether hummingbirds are able to distinguish between umami and sweet tastes. With just one receptor for both, she says, it could be like soy sauce tasting the same as a glass of soda.

Follow Carrie Arnold on Twitter and Google+.

Comments

  1. Har Doody
    August 26, 9:40 am

    Rob, bears and badgers, like humans, are not true carnivores, but omnivores.

  2. Kelli Mc
    August 25, 6:20 am

    Hummingbirds do not survive on Nectar alone, they drink the nectar for the energy they need to fly around to catch the insects (Protein) they need! They eat Gnats and flies and other insects, I believe they also eat horn worms the ugly green worms you find on your tomato plants! I would have hoped that someone Studying Science and the evolution etc of species would know this about Hummingbirds!!!

  3. Brent
    Calgary
    August 22, 2:17 pm

    Beverly, I would blame her editor. He or she should have read that and said “Are you sure about that statement? To give credibility to your piece you might want to research that further.”

  4. Jade
    August 22, 1:35 pm

    Hummingbirds do not “live entirely on nectar” as the article says. They also catch insects, they need some protein and nectar does not have any.

  5. Rob
    IL
    August 22, 9:23 am

    Interesting article, but if carnivores can’t taste sweetness, why do bears and Honey Badgers go for honey? Perhaps overgeneralization based on testing a few species, or families of Carnivora?

  6. Janet Kieffer
    Albuquerque, NM
    August 22, 4:21 am

    “Baldwin and other ornithologists noticed a problem. Lacking the ability to taste sugars might not be a problem for birds that eat bugs, but plenty of birds, such as hummingbirds, live entirely on nectar, which is almost completely sugar. Without sweet taste receptors, how could they find food?”

    Hummingbirds do not live entirely on nectar. They consume it as an energy source. Their protein source is bugs.

  7. Beverly Moody
    California
    August 22, 2:07 am

    How can I believe anything in this article which includes this glaringly untrue statement: “Lacking the ability to taste sugars might not be a problem for birds that eat bugs, but plenty of birds, such as hummingbirds, live entirely on nectar…”