Researchers debunk hummingbird theory

By Christine Buckley

Since the 1830s, scientists have thought they had hummingbird feeding figured out.

It went something like this: The little guys would hover in front of a colorful flower, stick their tube-like tongue into the flower’s pool of nectar, and poof: through a trick of physics, the sugar-loaded fluid would rise into the tongue’s tubes, ready to be converted into the energy needed to buzz those tiny wings.

But there was one problem. This hypothesis was never tested, but taken as fact for more than 180 years.

Now, a graduate student in the ecology and evolutionary biology department in the College of Liberal Arts and Sciences has proven this theory false. Using high-speed, high-definition video, Alejandro Rico-Guevara has shown that instead of using this mechanism, known as capillary action, hummingbird tongues instead trap fluids by dramatically changing their shape.

Rico-Guevara published his results on Monday in the scientific journal Proceedings of the National Academy of Sciences.

“Hummingbirds are tiny, fast and they feed on flowers, which are hard to see into,” says Rico-Guevara. These three factors prevented scientists from closely observing hummingbird feeding until the advent of modern technology.

In the early 19th century, biologists proposed that hummingbirds drank nectar from flowers using capillary action, the passive process of a fluid rising into a narrow tube because of forces attracting the liquid to the tube’s solid internal surface. The idea was at first controversial, says Rico-Guevara. But in part because it was so difficult to test, the theory was eventually accepted.

Fast-forward about 150 years, and biologists were using computer programs to model what they saw in nature. Using the capillary action theory, a group of scientists predicted that hummingbirds should prefer thin, watery nectar to thicker fluids. This finding, says Rico-Guevara, made him skeptical, since many birds in fact prefer thicker nectars.

“The challenge was: How do you determine what’s going on if you can’t see inside the bird’s mouth?” says Associate Professor Margaret Rubega, Rico-Guevara’s PhD advisor, who worked with him on this paper.

“We didn’t want to just accept what was written,” adds Rico-Guevara. “Capillary action seemed possible, but it couldn’t be the whole story.”

So, Rico-Guevara set about testing the theory on 30 species of hummingbirds, many in the Andes Mountains of his native Colombia. He used high-speed video to record hummingbirds feeding from nectar feeders with clear walls so that he could observe their tongues as they drank.

What he found was very different than the previous theory predicted. Rico-Guevara observed that when in contact with a fluid, the tubes separate from each other, appearing much like a snake’s forked tongue. The tubes expand to expose tiny elongate fringes that trap nectar and then retract, pulling the fluid with them into the bird’s mouth.

Rico-Guevara thinks that this new concept could be more far-reaching than just hummingbirds – it’s possible that others of the more than 200 types of nectar-feeding birds with similar tongues also use this process. If so, this could change the way ecologists think about the behavior, ecology and evolution of these birds.

Additionally, the researchers point out that this novel fluid-gathering process could be useful to engineers. A feature of this mechanism, says Rubega, is that it requires no energy on the part of the bird; all movements are spurred by changes in pressure and molecular interactions between the bird’s tongue and the surrounding fluids. This method could be instructive for creating low-energy fluid trapping and transporting instruments.

Since it’s not every day that a PhD student debunks a 180-year-old scientific hypothesis, Rico-Guevara says that his own reaction to his findings has been mostly positive, but at times nerve-wracking.

“I’m starting to tell my colleagues about this, and it’s kind of scary,” he says. “But it’s also super exciting.”

To read the paper about hummingbirds click here.

To follow some of the news coverage, go to: New York Times, Nature , Wired.

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