How hummingbirds taste sweetness

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 in Science reveals that hummingbirds have repurposed their umami receptor (which recognises meaty and savoury flavours) 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 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 10 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 hypothesised 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.

Behavioural 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.

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.