Many mammals have a sweet tooth, but birds lost their sweet receptor during evolution. Although hummingbirds and songbirds independently repurposed their savory receptor to sense sugars, how other birds taste sweet is unclear. Now, an international team lead by researchers from the Max Planck Institute for Biological Intelligence (in foundation) shows that woodpeckers also regained sweet taste. Interestingly, wrynecks, specialized ant-eating woodpeckers, selectively reversed this gain through a simple and unexpected change in the receptor. These results demonstrate a novel mechanism of sensory reversion and highlight how sensory systems adapt to the dietary needs of different species.

 

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 and colleagues from other universities 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, Baldwin's group 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. "Our next question was whether the observed sugar preference is mirrored by the birds' receptors," recaps Baldwin.