It's been thought that songbirds were unable to taste sugar; they lack a protein that's crucial for many animals including humans to be able to sense sweetness, and songbirds trace their evolutionary history all the way back to carnivorous dinosaurs. But new research has assessed sweet sensing in birds and how long birds have been able to taste sweetness, and the work has suggested that it was incorrect to assume that songbirds couldn't taste sugar.
In this study, which was reported in Science, honeyeaters and canaries were able to select either plain water or sugar water. Both are songbird species, but honeyeaters consume nectar, while canaries eat grain and are not known to seek out sugary foods. When the researchers examined the responses of taste receptors in the birds, both species were found to react to sugar.
"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," noted senior study author Maude Baldwin of the Max Planck Institute for Ornithology in Germany.
The researchers assessed taste receptors in the birds at the molecular level. "Because sugar detection is complex, we needed to analyze more than one hundred receptor variants to reveal the molecular mechanisms underlying the sugar responses," noted first study author Yasuka Toda of Meiji University.
Different species of songbirds seem to have independently evolved the ability to taste sweet foods at different times and places and in various ways, a phenomenon known as convergent evolution.
All songbirds originated in Australia, and they moved around the world. The researchers estimated that the ability to taste sweetness arose in songbirds about 30 million years ago.
Some songbirds have grown highly dependent on sugar. It can be a vital source of energy for many animals, and might have affected the evolution of songbirds, the researchers suggested. Abut 40 percent of the birds in the world are thought to be songbirds, and sweet foods may have helped them along the way; many retained this sense.
"This study fundamentally changes the way we think about the sensory perceptions of nearly half the world's birds," said study co-author Eliot Miller of 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."
Interestingly, another recent study in Science has indicated that birds are also much better at smelling things than we assumed.