Seeing a bird eat nectar from a flower is a common sight in our world. The ability to detect sugars, however, is not ancestral in the bird lineage, where most ancient species were carnivorous. In a new study published today in the journal Science, an international team of researchers looked at receptors within the largest group of birds, the passerines or songbirds, and found that the emergence of sweet detection involved a single early sensory shift in a receptor for umami.
Toda et al. identified a single early sensory shift of the umami receptor that conferred sweet-sensing abilities in songbirds. Image credit: Sci-News.com.
Bitter, salty, sweet, sour and umami are the five basic tastes that humans perceive.
The sense of taste has an enormous influence on our diet — what we think tastes good often ends up on our plates.
The rest of the animal kingdom is no different, as taste reliably helps to distinguish what is nutritious from what is poisonous. But what exactly do other animals taste?
It is well known that the sweet taste receptor is widespread among mammals.
Birds, however, descend from carnivorous dinosaurs and lack an essential subunit of this receptor — presumably leaving most unable to detect sugar.
The only known exception is the hummingbirds, who re-purposed their umami taste receptor to recognize carbohydrates.
“Sugar is a vital carbohydrate providing lots of energy and may have had far-reaching effects on songbird evolution,” said Dr. Maude Baldwin from the Max Planck Institute for Ornithology and colleagues.
“Though it is thought that most bird lineages can’t taste sweetness at all, we now believe that songbirds, which account for more than 40% of the world’s bird species, can actually taste sweet, and that sugary food sources may have contributed to their success.”
In the study, the researchers offered two species of songbirds a choice between sugar water and plain water — nectar-taking honeyeaters, as well as canaries, a grain-eating bird not known for consuming sweet foods.
They also examined taste receptor responses sampled from a variety of other species.
Regardless of whether their main diet consisted of seeds, grains, or insects, songbird taste receptors responded to sugars.
“This was a clear hint that we should concentrate on a range of songbirds, not only the nectar-specialized ones, when searching for the origins of avian sweet taste,” Dr. Baldwin said.
The scientists then dug down to the molecular level to understand the modifications to the umami taste receptor that enabled sweet perception among songbirds.
“Because sugar detection is complex, we needed to analyze more than one hundred receptor variants to reveal the molecular mechanisms underlying the sugar responses,” said Dr. Yasuka Toda, a researcher at Meiji University.
“These exact changes coincide only slightly with those seen in the distantly-related hummingbirds, even though similar areas of the receptor are modified.”
“Though songbirds evolved similar workarounds to taste sweetness, they did so at different times, in different places, and in slightly different ways — an example of convergent evolution.”
“Some songbirds, such as honeyeaters, sunbirds, honeycreepers, and flowerpiercers are now just as dependent on nectar as the hummingbirds.”
Exploring the songbird family tree, the researchers conclude that songbirds evolved to sense sweetness approximately 30 million years ago, before the early ancestors of songbirds left Australia, where all songbirds originated.
Even after songbirds radiated across the world, they kept their ability to taste sugar.
“This study fundamentally changes the way we think about the sensory perceptions of nearly half the world’s birds,” said Dr. Eliot Miller, a researcher at the Cornell Lab of Ornithology.
“It demonstrates that most songbirds definitely can taste sweet and got there by following nearly the same evolutionary path that hummingbirds did — it’s a neat story about how convergence happens.”
Yasuka Toda et al. 2021. Early origin of sweet perception in the songbird radiation. Science 373 (6551): 226-231; doi: 10.1126/science.abf6505