Australia’s unique forests are the birthplace of birdsong. The plants there are drenched in sunlight and can readily mass-produce sugars through photosynthesis. But with few nutrients in the soil, they struggle to convert those sugars into leaves, seeds, and other tissues. They end up with excess, which they simply give away. Flowers overflow with nectar. Eucalyptus trees exude a sweet substance called manna from their bark. Even insects that suck plant sap are forced to excrete surplus sugars, in the form of liquids known as honeydew or lerp. As the biologist Tim Low once wrote, Australia has “forests that exude energy.”
In his book Where Song Began, Low reasoned that Australia’s birds have benefited from the island’s free-flowing calories, becoming unusually large, aggressive, intelligent, and vocal. They are also extraordinarily successful. Genetic studies show that the largest group of birds—the oscines, or songbirds—originated in Australia before spreading worldwide. That group now contains about 5,000 of the 10,000 known bird species, including robins, cardinals, thrushes, sparrows, finches, jays, and starlings. All of these birds descended from an ancestor whose voice lilted through Australian trees and whose taste buds were tickled by sweet Australian nectar.
But this story has a catch. An animal should obviously be able to sense the food that it eats. And until recently, it didn’t seem as if songbirds could even taste sugar.
Humans and most other mammals detect sugars with a sensor called the sweet receptor, which is found in taste buds and recognizes the shape of sugar molecules. Two genes, called T1R2 and T1R3, each build one half of the receptor. But in cats, hyenas, seals, dolphins, and vampire bats, the T1R2 gene is faulty, and the sweet receptor doesn’t work. When animals eat meat (or blood) and nothing else, they no longer need the ability to taste sugar, and quickly lose it. The same fate probably befell the small predatory dinosaurs that were the ancestors of birds. They may have lost T1R2 entirely, which is why no modern birds have the gene. When birds first evolved, sweetness wasn’t part of their palate.
But what about hummingbirds? They are a distinct group from songbirds, and specialize in drinking nectar. They are so drawn to sweetness that they’ll avoid flowers that are insufficiently sugary. Like other birds, they lack T1R2. But as Maude Baldwin, from the Max Planck Institute of Ornithology, showed in 2014, they have evolved a work-around. Together with Yasuka Toda, of the University of Tokyo, Baldwin found that hummingbirds transformed a different taste receptor that normally detects savory umami into one that also detects sugar. In doing so, they regained the sensory ability that their dinosaur ancestors had lost. (The savory receptor of some hummingbirds can still detect umami, which means that “they may not be able to distinguish between sweet and savory,” Baldwin told me; imagine if soy sauce and apple juice tasted identical to you.)
Baldwin and her colleagues went on to study other nectar specialists such as honeyeaters—large songbirds that are common to Australia. (Fans of the board game Wingspan and its Oceania expansion will be familiar with the importance of nectar to Australian birds.) The team found that, like hummingbirds, honeyeaters also have savory receptors that respond to sugar. But unexpectedly, so do other songbirds, including canaries, which mostly eat grain, and great tits, which mostly eat insects. By comparing the savory receptors of these modern species, Baldwin’s team could project back in time, and figure out what the receptors of the early songbirds would have looked like. They could even re-create these ancient sensors in their lab and show that they also react to sugar. Their research suggests that songbirds have been tasting sweetness for almost as long as songbirds have existed. “That was really surprising,” Baldwin told me. And when she looked more closely at the songbird savory receptor, her surprise deepened.
Like the sweet receptor, the savory receptor consists of two halves that are built by different genes—T1R1 and T1R3. Hummingbirds mostly repurposed the receptor by changing the T1R3 half. But songbirds did it by mostly altering T1R1. Imagine these halves as two open hands that are touching at the wrist, with fingers that are specifically splayed to grab molecules of a particular shape. Both songbirds and hummingbirds changed the position of those fingers so that they could better seize sugars, but one group did so on the right hand and the other did so on the left. They achieved the same end through radically different means.
These songbird discoveries represent at least six years of research and “an astounding amount of work,” says Heather Eisthen, a sensory biologist from Michigan State University. “It’s kind of amazing what they were able to learn.” For example, the team found that songbirds needed 16 mutations to convert their savory receptor into a sweet one. None of these mutations does much on its own, and Baldwin suspects that they amassed slowly and randomly, neither disabling the savory receptor nor giving it new properties. Only with combinations of them, and perhaps even the full set of 16, did the receptor react to sugar. Only then did songbirds gain a sense of sweetness.
This evolutionary journey is so complicated that it’s no wonder more birds haven’t completed it. And maybe that’s why those that did flourished. Songbirds probably evolved sweet perception about 30 million years ago, when Australia was much wetter. As the climate dried, the soils became poorer and the eucalyptus trees expanded. The forests abounded with new sources of sugar such as manna, which the songbirds were already primed to find and exploit. Perhaps the extra energy from these abundant calories allowed them to migrate over long distances and travel to other continents. Perhaps they could thrive in their new homes by finding flowers that were already baiting insects with nectar. “They are the most successful group of birds,” Eisthen told me. “You have to wonder how much of their success is due to this hidden talent, which allows them to invade new niches and feed on food sources that other animals are not exploiting.”
Baldwin is trying to find out. She wants to know whether other birds have also evolved a sweet tooth, and whether those that did diversified more quickly into new species. She wants to know whether any songbirds, like the meat-eating shrikes, have lost their sense for sweetness. And she wants to know whether a taste for sugar can fuel complex courtship rituals such as energetic dances.
Meanwhile, Sushma Reddy, an ornithologist at the University of Minnesota, points out that hummingbirds, songbirds, and parrots, three groups of birds with lots of nectar-eating species, “are also the same lineages that have convergently evolved vocal learning”—the ability to make new songs and sounds after listening to other individuals. Could these traits be related? Perhaps there’s a hidden connection between the sugary riches of Australia’s forests and the beautiful tunes that fill the air of every continent—between sweetness of palate and sweetness of voice.