Contrary to conventional thought, songbirds can taste sugar — even though songbirds are the descendants of meat-eating dinosaurs and are missing a key protein that allows humans and many other animals to taste sweetness.
An international team investigated how many bird species can taste sweet and how far back that ability evolved. Their work was published in the journal Science.
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.
Sugar is a vital carbohydrate providing lots of energy and may have had far-reaching effects on songbird evolution. Though it is thought that most bird lineages can’t taste sweetness at all, the scientists 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.
Baldwin and Toda dug down to the molecular level to understand the modifications to the “umami” taste receptor that enabled sweet perception among songbirds.
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.
Researchers in Japan, Germany, the United States, Hong Kong, and Australia participated in this study. Future studies will explore how sweet perception has co-evolved with other physiological traits, such as changes in digestion and metabolism, across bird evolution.
Illustrations, from above: New Holland Honeyeater by Gerald Allen, Macaulay Library at the Cornell Lab of Ornithology; and model showing the convergent evolution of sweet perception courtesy Science magazine.