A hummingbird at a feeder filled with sugar water. Scientists observed that the birds happily consumed liquids with varying sugar content but would spit out those with artificial sweeteners. - Filepic
Scientists have found a receptor that's sensitive to sweetness.
FOR birds that don’t have teeth, hummingbirds have quite a sweet tooth. Although they eat insects to get essentials like protein and fat, most of their diet consists of sugary nectar.
This has puzzled scientists. Humans and other animals that prefer sweet tastes are able to recognise them thanks to a pair of sensory receptors known as TIR2 and TIR3. But birds don’t have the gene that codes for TIR2. How, then, do hummingbirds know that nectar and sugar water are sweet?
An international team of biologists and their colleagues think they have found the answer: over millions of years, genetic mutations converted a receptor that used to help hummingbird ancestors detect umami (a savoury taste) into a receptor tuned to sweets. A report on their discovery appeared in last Friday’s edition of the journal Science.
The team conducted a series of experiments involving the gene for TIR3 and identified a group of 19 mutations that allowed the taste receptor to recognise sugars. However, there are probably additional mutations – in the TIR3 gene as well as the related TIR1 gene – that help make hummingbirds sensitive to sweet tastes, the study authors wrote.
And sensitive they are! In field tests conducted in Southern California’s Santa Monica Mountains and at other feeding stations, hummingbirds required as few as three quick licks to determine that a feeder contained plain water instead of a sweet solution.
They didn’t seem pleased. After tasting the water, they “often displayed a characteristic behavioral pattern involving beak withdrawal, head shaking, and/or spitting”, the researchers reported. This is how hummingbirds react when they taste something bitter, the scientists added.
The birds that visited the feeding stations seemed happy to drink sweet liquids, including ones made with regular sugar, pure glucose and pure fructose. They also eagerly drank solutions with the sugar alcohols sorbitol and erythritol, but they spat out liquids containing the artificial sweeteners sucralose, aspertame, acesulfame potassium and cyclamate.
Scientists have long recognised that some animals have dumped a taste receptor or two along the way. Cats and other creatures that get all their nutrients from meat have lost the TIR2 receptor. Giant pandas, who rely on bamboo for nearly all of their diet, lost TIR1. And sea-dwelling mammals such as dolphins and whales no longer have any functional TIR receptors.
“This dramatic change in the evolution of a new behaviour is a really powerful example of how you can explain evolution on a molecular level,” Harvard Medical School biologist Stephen Liberles, who oversaw the study, said in a statement from Harvard.
A similar process probably gave humans and other mammals the ability to taste sweets, the researchers wrote. – Los Angeles Times/McClatchy-Tribune Information Services