Hummingbirds are rare among birds in their ability to taste sweetness. Avians tend to only have the ability to detect savory flavors. A new study shows hummingbirds evolved new forms of these receptors, able to taste nectar and other sugary food sources.
Taste bud receptors called T1R's bind to molecules in food, providing a sense of taste for the sustenance. In humans and other vertebrates, this group of receptors includes T1R2 and T1R3 varieties, which combine to provide a sense of sweetness. Birds do not possess T1R2 receptors, nor do they have genes that code for them.
Hummingbirds use incredible amounts of energy during their day-to-day life. Their hearts beat up to 20 times each second. Nectar, high in calories, is a natural food source for the small, energetic birds. When they feed, hummingbird tongues dart in and out of the bird's mouth 17 times each second.
Hummingbirds expecting nectar and given water will turn away from the liquid after just three licks, spitting out whatever water is in their mouths. Sugar substitutes were also rejected by the energetic birds during experiments.
The T1R2 receptor missing in birds is present in lizards, suggesting the modern feathered animals may have lost their ability to taste sweet foods after evolving from dinosaurs.
Maude Baldwin at Harvard University discovered that T1R3 receptors work together with T1R1's to produce a sugary sense in the darting avians.
Swifts are the closest living relative to hummingbirds. The two species divided somewhere between 40 and 72 million years in the past. Since that time, more than 300 species of the animal have evolved, filling large portions of North and South America.
The T1R1 receptors are sensitive to amino acids in most animals, including swifts and chickens. This taste, called umami, was one of the last of the basic tastes to be identified. Molecular structures of amino acids and carbohydrates (sugars) are dissimilar to each other, meaning evolution of T1R1 receptors capable of detecting sugary snacks was highly complex.
"If you look at the structure of the receptor, it involved really dramatic changes over its entire surface to accomplish this complex feat. Amino acids and sugars look very different structurally so in order to recognize them and sense them in the environment, you need a completely different lock and key. The key looks very different, so you have to change the lock almost entirely," Stephen Liberles, associate professor of cell biology at Harvard Medical school, said.
Taste, and how it can be driven by evolution, may be better understood from this new discovery.
Study of hummingbirds and their ability to taste sweet foods was profiled in the journal Science.
