Birds, the descendants of carnivorous dinosaurs, lack part of the sweet receptor found in mammals. This should leave them insensitive to sugars. However, recent studies have shown that both hummingbirds and songbirds have regained the ability to sense sugar by repurposing their savory receptor to now detect carbohydrates in fruits and nectar. How other birds sense sugars, and the extent to which taste receptor responses track the immense dietary diversity of birds, is unclear. To investigate this question, Julia Cramer and Maude Baldwin from the Research Group Evolution of Sensory Systems at the Max Planck for Ornithology/Max Planck for Biological Intelligence, together with colleagues from the Cornell Lab of Ornithology, Meiji University, the Swedish University of Agricultural Science and our lab focused on woodpeckers. Although primarily insectivorous, this group of birds also contains multiple species that include sugar-rich sap, nectar, and fruits in their diets.
Using behavioral tests of wild birds, the team showed that woodpeckers clearly prefer sugar and amino-acids over water. Surprisingly, wrynecks – a member of the woodpecker group whose diet is almost exclusively composed of ants – displayed preferences for amino acids but not sugars. Functional analyses of taste receptors confirmed that woodpecker receptors were sensitive to sugars, whereas those of wrynecks were not. Interestingly, ancestral reconstructions indicated that the common ancestor of wrynecks and woodpeckers already possessed a modified savory receptor capable of responding to sugars. A meticulous dissection of differences between wryneck and woodpecker receptors revealed unexpectedly that changes in only a single amino acid in the wryneck receptor selectively turned off sugar-sensing: the birds kept their ability to taste savory, which is likely important for insect-specialist birds that consume a protein-rich diet.
These results trace an evolutionary history in which an early gain of sugar sensing in woodpeckers —possibly arising in an earlier ancestor and therefore older than woodpeckers themselves — was followed by its reversion when the wryneck receptor was later altered. Further investigation will be required to describe how specific changes in taste receptors, and in other physiological and sensory systems, are related to the rich dietary diversity across birds.
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