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Hummingbirds pump up nectar: new videos spark debate over how tongue slurps up fluid.

A new way of looking at hummingbird tongues sees them as long, skinny pumps.

This view challenges an old notion of how hummingbirds sip--that nectar flows up open grooves in the tongue the way water rises inside thin capillary tubes, says Alejandro Rico-Guevara, a functional morphologist at the University of Connecticut in Storrs. It's the latest in a lively debate over just how hummingbird tongues work.

The "elastic micropump" theory that Rico-Guevara and his Connecticut colleagues propose relies on the same tendency of water molecules to grip each other that creates capillary rise in an open tube. But Rico-Guevara's high-speed videos show that hummingbirds in the wild rarely dip open grooves into nectar. Instead, bird bills squash the tongue and its grooves flat. When the tongue tip touches nectar, the grooves spring open, pulling up a column of nectar as they expand. This pulling, or pumping, slurps nectar faster than grooves that had been open all along would, the researchers report in the Aug. 22 Proceedings of the Royal Society B.

The skinny, translucent hummingbird tongue, with no muscles in it, has a semicircular groove on each side. The tongue forks into fringed halves at the tip. That tip is not so much a capillary tube as a trap for nectar, Rico-Guevara and colleague Margaret Rubega proposed in 2011. Based on high-speed video, they argued that as the squashed grooves touch nectar and spring open, the fringe helps capture nectar. Proposed as an alternative to capillary rise, "it was going against what everybody believed," Rico-Guevara says. "It got a lot of attention, but also a lot of skepticism."

John W.M. Bush, who studies fluid dynamics at MIT, and his colleagues countered with computer simulations and their own videos of birds in the lab. The team argued that, regardless of what happens at the tip, capillary suction is important in drawing nectar up the grooves. The tongue is really a "self-assembling capillary siphon," Bush proposed in 2012 with Wonjung Kim, now at Sogang University in Seoul, South Korea, and collaborators.

To make his own study of the grooves, Rico-Guevara with ethologist Kristiina Hurme coaxed 18 hummingbird species in the wild to sip on camera. The birds' tongue grooves mostly stayed closed waiting for nectar. And when tongue met nectar, the fluid moved fast--averaging nearly 1 meter per second as it rose up. Even under ideal conditions, simple capillary rise would draw in nectar much more slowly, only about 36 centimeters per second, the researchers report.

In the course of filming, one accident turned into a "perfect experiment" to compare capillary and pump action, Rico-Guevara says. A bird bumped one side of its tongue against a feeding tube and the tongue's compressed groove opened before touching the nectar. In this instance, nectar did a typical capillary rise--and moved more slowly than nectar in the groove on the opposite side of the tongue that sprang open later.

Study coauthor Tai-Hsi Fan, who studies fluids, developed the concept of the tongue as an elastic micropump. With computer simulations, he predicted such details as nectar uptake speed.

Even in the new grooves-closed model, "I think that capillary force is still at work," Kim says. This force of water molecules gripping each other might explain why the wet, compressed tongue grooves stay closed until the tongue dips into nectar.

Caption: A new way of looking at hummingbird tongues suggests that they work like tiny pumps, challenging the familiar idea that nectar rises as if the tongue were an open tube.


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Title Annotation:LIFE & EVOLUTION
Author:Milius, Susan
Publication:Science News
Date:Sep 19, 2015
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