Songbirds taste sugar, and hence became ubiquitous

The story: A genetic change happened some 30 million years ago, at the beginning of the evolution of the Passeri. A songbird is a bird belonging to th

Songbirds taste sugar, and hence became ubiquitous

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• The story: A genetic change happened some 30 million years ago, at the beginning of the evolution of the Passeri. A songbird is a bird belonging to the clade Passeri of the perching birds.
• Evolution of birds: Birds evolved from carnivorous dinosaurs called theropods. Meat eaters need not detect sugar in the way that, say, fruit eaters do, and genetic analyses of modern birds suggest their theropod ancestor had lost the ability to taste sweetness.

1. Today many birds have sugar-rich diets of nectar or fruit, so perceiving things as sweet is a useful attribute.
2. Research suggests songbirds can indeed perceive sweetness. This ability may have led to their success. Since almost half the bird species now alive are Passeri, that is clear proof.
3. Vertebrates’ taste-receptor genes normally include three that encode proteins called T1R1, T1R2 and T1R3. The taste receptors themselves are formed from pairs of these proteins. Receptors for sweetness are a combination of T1R2 and T1R3. Birds, however, lack the gene for T1R2. Presumably, it was lost by their theropod ancestors, which did not need it.

• The lost one: A study on hummingbirds, which feed on nectar from flowers, found that hummingbirds regained the ability to taste sugars via mutations in the genes for T1R1 and T1R3. The receptor formed by combining T1R1 and T1R3 normally detects umami, a savoury flavour typical of meat. In hummingbirds, these mutations allow this receptor to detect sugars, too. Was that also the case for songbirds? Scientists 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 songbirds regained perception of sweetness via mutations of the gene for T1R1 and T1R3.

1. 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.
2. 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.

• Summary: When researchers looked at 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. And it was that which resulted in the Passeri’s current diversity. But what about the sweet songs they sing? No one knows the connection, at least not yet.

 

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