Many birds like to taste nectar and sweet fruits. How they perceive this taste, however, has long been a mystery to researchers. Unlike mammals, birds do not have a clear receptor for sweets. A new study has now clarified how songbirds can still taste sweet: According to this, their umami receptor, which is actually responsible for savory flavors, has changed in the course of evolution in such a way that it reacts to sugar. The preference for sweets may have allowed songbirds to occupy new ecological niches and spread around the world, say the researchers.
We humans can perceive five flavors: bitter, salty, sweet, sour and umami. The sense of taste has a major influence on our diet, as it shapes our food preferences and helps us to differentiate between nourishing and poisonous things. But what do animals taste like? Genetic analyzes of their taste receptors provide clues about this. But of all birds, genome analyzes had suggested that they lacked a sweet tooth. So why do many of them still prefer sugary foods?
Grain eaters also prefer sugar water
A team led by Yasuka Toda from the University of Tokyo in Japan has now found a solution to this puzzle: According to this, songbirds can perceive a sweet taste – with the receptor that actually reacts to something hearty. This conversion was already known from hummingbirds. “It was not yet clear whether other birds that eat fruit and nectar can taste sweets,” write Toda and her colleagues.
To clarify this question, the researchers first systematically examined the eating habits of various bird species. They found out that not only typical nectar eaters prefer sweet food. Grain eaters who only occasionally consume fruit or nectar also preferred sugar water to normal water in behavioral experiments. “This was the first indication that we should concentrate on a number of songbirds and not just on nectar specialists when searching for the origins of the sweet sense of taste,” explains co-author Maude Baldwin from the Max Planck Institute for Ornithology in Seewiesen .
Modified receptors
Apparently, not only hummingbirds, but also numerous other birds can perceive a sweet taste. The researchers investigated the molecular basis of this ability by reconstructing the taste receptors of various birds in the laboratory and testing how the receptors react to substances such as proteins and carbohydrates. The result: Although all birds lack the receptor subunit T1R2, which is normally responsible for sugar, other receptor subunits take over this task in songbirds and hummingbirds. In both cases, the subunits T1R1 and T1R3 are modified in such a way that they can bind not only hearty proteins but also sweet carbohydrates.
However, further molecular analysis revealed that the receptors in hummingbirds and songbirds come to the same conclusion in different ways. “This means that hummingbirds and songbirds independently changed the binding sites on their umami receptors,” the authors explain. Since when songbirds have had a sweet sense of taste, Toda and her colleagues researched by reconstructing the umami receptors of various representatives in the songbirds family tree and testing the extent to which they react to sugar. It turned out that the early ancestors of the songbirds already had the sweet sense of taste before they emigrated from their native Australia and spread all over the world. “As a result, the sweet sense of taste arose very early in songbirds and was also preserved in species that are not primarily dependent on sugary food,” says Baldwin.
Successful evolution thanks to a new sense of taste
“The early evolution of sweet perception probably played an important role in the diversification of songbirds, which today make up the largest group of all living birds,” writes evolutionary biologist F. Keith Barker of the University of Minnesota in a comment on the study, also published in the journal Science was published. Perhaps the focus on a sugary diet helped early songbirds occupy new ecological niches, create new species, and spread successfully around the world.
Future studies should now decipher how the taste of sugar has developed together with other physiological characteristics, such as digestion or metabolism, in the course of evolution.
Source: Yasuka Toda (University of Tokyo, Japan) et al., Science, doi: 10.1126 / science.abf6505