By Susanne Clara Bard

Hummingbirds sip nectar from flowers, and nectar is made of sugar. It powers their famously high metabolisms, allowing them to beat their wings rapidly to hover in midair. If they don’t get enough nectar in a day, they’ll drop dead of hunger.

Other birds eat seeds and insects and fish and sometimes even other birds – but not nectar. So how did hummingbirds develop their taste for the sweet substance? Scientists have hit upon the unexpected answer. Animals — including us — have taste receptors for different types of foods. That’s how we can tell if something is sweet, sour, bitter, salty or savory (otherwise known as umami). If you have just eaten cake, for example, molecules of the dessert interact with the sweet taste receptors in your mouth. Those receptors send a signal to your brain, which then perceives – ding – ding – ding – sugar! If an animal species doesn’t have a receptor for a one of these tastes, it can’t taste the food. Cats – lions, tigers, and your house pet included, have no sweet taste receptors at all, one reason they stick to a pure meat diet.

But the scientists couldn’t find any genes for sweet taste receptors in the hummingbirds, or any other birds either, for that matter. So they turned their attention to other possible receptors. The closest relatives of hummingbirds, called swifts, eat all kinds of insects, from dragonflies to ants. The scientists found that swifts have umami taste receptors that respond to amino acid molecules in the insects they eat. So do chickens. But give a hummingbird amino acids and not much happens in their umami receptors. Give them simple carbs, like sugar, though, and – boom! Those umami receptors go ding – ding – ding – sugar!

Taking a closer look, the research team found 19 gene differences between the umami receptors of chickens and hummingbirds. Enough to change them from detecting amino acids to detecting sugar.

So the next time you refill the sugar water in your backyard feeder, think about how evolution shaped the hummingbird’s sweet tooth to make it the nectar-sipper it is today.

The research was led by Harvard zoologist Maude W Baldwin and colleagues, and is published in the journal Science.