To sense the taste of flower nectar, hummingbirds had to adapt a taste receptor that was originally not designed for sweet perception.
The metabolism of hummingbirds is remarkably fast, even among birds. This can be easily explained by looking at the frequency of their wing beats. If you measure a hummingbird's heart rate, you can find that their heart beats at a rate of up to 20 beats per second. Such physiology requires a constant supply of energy, that is, nutrients, and although hummingbirds can catch insects, the basis of their diet is flower nectar, rich in sugars. In this sense, their choice is truly unique — no other bird has 'mastered' nectar as hummingbirds have.
Hummingbirds have achieved significant evolutionary success: over 300 species of these birds have spread across North and South America, from tropical forests to the high Andes. However, to choose sweet and carbohydrate-rich nectar, hummingbirds must be able to sense its sweetness. And here arises one of the most interesting evolutionary and genetic puzzles: birds lack sweet taste receptors.
It is known that there are only five basic tastes: bitter, sour, sweet, salty, and umami, also known as the taste of proteins. Each of these tastes has its own receptors. For example, in mammals, sweet taste is recognized by the receptor proteins T1R2 and T1R3, while umami taste is 'sensed' by proteins T1R1 and T1R3. However, when the chicken genome was sequenced in 2004, no traces of the T1R2 gene, necessary for sweet perception, were found. This gene is also absent in other bird genomes. T1R3 is present, but it is not sufficient for sensing sweet taste. By the way, cats also do not sense sweetness, but they have a non-functional 'sweet taste gene' that once worked but no longer does. Birds, however, lack even such a variant. And the question arises: how could hummingbirds come to love sweetness if they cannot taste it?
This puzzle was attempted to be solved by researchers from Harvard in collaboration with colleagues from the University of Tokyo. They decided to directly test how the taste receptor proteins of birds respond to different tastes. For this, cells synthesizing receptor proteins for umami taste (T1R1 and T1R3) were exposed to various taste stimuli; the genes for the proteins were obtained from chickens, hummingbirds, and swifts, from which hummingbirds are believed to have evolved. The results of the experiments were published in the journal Science.
It turned out that the receptors of chickens and swifts actively respond to umami taste, or the taste of protein. At the same time, the same receptor in hummingbirds responded weakly to the taste of protein but actively reacted to carbohydrates. This means that the hummingbird's taste receptor has, so to speak, changed its specialization. Obviously, this happened due to mutations — the authors of the study counted 19 mutations that could change the chicken umami receptor to perceive sweetness, but there may actually be more, and a detailed study of the hummingbird receptor proteins is needed to fully determine them.
The results obtained not only open new horizons in understanding the amazing hummingbirds and their evolution. The story of how a taste receptor unexpectedly changed its function underscores the importance of a comprehensive study of genes — it is not enough to know their sequence and origin; it is also necessary to investigate the function of the protein, the information about which is contained in the gene.
