Many mammals have a sweet tooth, but birds have lost their sweet receptor during evolution. Although hummingbirds and songbirds have independently redirected their salt receptor to detect sugars, the sweetness of other birds is unclear. Now, an international team led by researchers from the Max Planck Institute for Biological Intelligence (in foundation) shows that the woodpeckers have also regained a sweet taste. Interestingly, anteater necks, specialized peaks in the anthill, selectively reversed this attainment by an unexpected basic change in receptor. These results demonstrate a new mechanism of sensory reversion and highlight how sensory systems adapt to the food needs of different species.
The birds, the descendants of carnivorous dinosaurs, lack part of the sugar receptor found in mammals. This should make them insensitive to sugars. However, recent studies have shown that hummingbirds and songbirds have regained the ability to sense sugar by repurposing their salt receptor to now sense carbohydrates in fruit and nectar. How other birds perceive sugars and the extent to which taste receptor responses follow birds’ immense dietary diversity are unclear. To investigate this issue. Although primarily insectivorous, this group of birds also contains several species that include sap, nectar, and sugar-rich fruits in their diet.
Using behavioral checks of wild birds, Baldwin’s group showed that woodpeckers clearly prefer sugar and amino acids to water. Surprisingly. “Our next issue was whether the observed preference for sugar is reflected by bird receptors,” Baldwin summarizes.
Common ancestor possessed a sugar receptor
whereas those of the torcols were not. Interesting fact. “This finding unveiled a third case of independent evolution of sugar sensing through modification of the salt receptor in birds,” says Cramer, the study’s first author. “Yet what was even more exciting was the implication that the torcols had subsequently lost the new receptor function.”
Cramer’s meticulous dissection of the differences between wryneck and woodpecker receptors unexpectedly revealed that changes in a single amino acid in the wryneck receptor selectively turned off sugar sensing: birds retained their ability to taste salty, which is likely important for birds. insect specialist birds that consume a high protein diet.
“We were very surprised to find that this reversal is caused by changes in a single amino acid, acting as a molecular switch to selectively regulate sugar sensitivity in wrynecks,” says Cramer. “Unexpectedly, the result of this small change is that the tortcols are once again unable to detect sugar in their food, but have retained the receptor’s ability to gather information about specific amino acid content. This makes a lot of sense when most of your diet is made up of ants.”
Further investigation will be needed to describe how specific changes in taste receptors and in d other physiological and sensory systems are linked to the rich dietary diversity of birds.
This study was conducted in collaboration with researchers from the Max Planck Institute for Ornithology, now the Max Planck Institute for Biological Intelligence (in foundation), the Cornell Lab of Ornithology, the University of Vienna and University of Veterinary Medicine, Vienna, Meiji University and Swedish University of Agricultural Sciences.