To find out, they cloned T1R1-T1R3 receptors from a variety of songbirds and tested their responses to sugar.All the receptors they tested―from birds with sugar-rich and sugar-poor diets alike―interacted strongly with sugar molecules.This confirmed that, as with hummingbirds, songbirds regained perception of sweetness via mutations of the gene for T1R1 and T1R3.By contrast, umami receptors cloned from the Tyranni, a sister group to the Passeri, did not interact with sugars, though they did so strongly with amino acids typical of meat.The mutations in the songbird lineage must thus have happened after the Passeri and Tyranni lines diverged, but before the Passeri themselves began proliferating into their current variety.Intriguingly, when Dr Toda and Dr Baldwin looked at the molecular modifications which allowed the T1R1-T1R3 receptors of hummingbirds and Passeri to detect sweetness, they found them to be completely different.Both, though, involved numerous changes to the underlying DNA, suggesting a strong evolutionary pressure to optimise them.This pressure was probably a consequence of competition to fill the new ecological niches opened up by an ability to recognise sweet things as both edible and nutritious.
