New high-speed videos of hummingbirds overturn nearly two centuries of conventional wisdom on how they drink.
Researchers previously thought tube-like channels in their tongues sucked up fluid by capillary action. But the new analysis shows that their tongues actually trap nectar by curling around it.
“The first time I saw these videos, they blew my mind,” said ornithologist Alejandro Rico-Guevara of the University of Connecticut, co-author of a study published May 2 in the Proceedings of the National Academy of Sciences. “I had studied the tongue’s structure in detail, but had no idea it could do something like this. We had to create a new model to explain it.”
Photo: Dave Mosher/Wired.com
With wings that flap up to 90 times per second and heart rates exceeding 1,200 beats per minute, hummingbirds depend on calorie-rich nectar for fuel. They can easily consume their own body weight in the stuff each day; one study described a 3-gram hummingbird drinking 43 grams of sugar water in one day, a full 14 times its body weight.
Researchers have long believed that capillary action — the phenomenon that causes liquid to rise up the sides of a narrow tube — enabled high-speed drinking. That idea was supported by the shape of the hummingbirds’ tongues, and its efficiency made intuitive sense.
There was, however, one major problem with the models. Capillary physics dictate that hummingbirds should prefer liquid with a sugar concentration of about 20 to 40 percent, as more sugary solutions would be too thick to rise quickly. But hummingbirds routinely choose fluids with double the predicted sugar levels.
To discover how hummingbirds drink, Rico-Guevara and colleague Margaret Rubega built see-through flowers that allowed them to take high-speed, high-magnification video of hummingbird tongues, which flick into nectar up to 20 times per second.
The video (above) shows that, instead of simply drawing in liquid, a hummingbird tongue’s tubes open down their sides when hitting nectar. When the tongue pulls back, the tubes zip closed, carrying nectar back into the beak.
“This overturns a lot of energetics work done on hummingbirds, especially assumptions on what sugar concentrations they should prefer. Those assumptions are flat-out wrong,” said ornithologist Christopher Clark of Yale University, who lent Rico-Guevara his high-speed digital camera but was otherwise uninvolved in the work. “[Rico-Guevara] shows, for the first time, an empirical model that matches basic behavioral observations.”
The tongue, however, is where Rico-Guevara and Rubega’s understanding ends. “To drink you have to swallow. No one has tried to explain how this works yet. It’s considered magic right now,” he said.
Rico-Guevara hopes to solve that mystery with X-ray microtomography — a miniaturized version of the 3-D medical scans used to image diseased tissues in people. “If we reconstruct the bill in three dimensions, we might see exactly how the tongue moves into it, and how the nectar is extracted,” Rico-Guevara said.
Because the hummingbird’s tongue is so efficient, the researchers think self-assembling electronics, fluid-power microchips and perhaps liquid-sipping robots could benefit. Clark suspects simpler applications may also be at hand.
“Perhaps understanding the physics behind this will enable engineers to design a more efficient mop or sponge,” he said.