Hummingbird tongues may be tiny pumps

Hummingbirds flit from flower to flower to drink sugar-laden nectar. The birds have a long tongue that lets them lap up their meals. Scientists had thought they understood how the tongues work. Nectar was thought to flow up open grooves in the tongue the way water rises inside thin capillary tubes. But a new way of looking at hummingbird tongues instead sees them as long, skinny pumps.

This view challenges the old notion of how hummingbirds sip, notes Alejandro Rico-Guevara. He is an ornithologist, or bird biologist, at the University of Connecticut in Storrs. His team’s claim is the latest in a lively debate over just how hummingbird tongues work.

Rico-Guevara and his Connecticut colleagues propose that the hummingbird tongue is an “elastic micropump.” Their theory relies on the same tendency of water molecules to grip each other that lets water rise up an open tube. But high-speed videos of the hummingbirds show the tube does not start out open.

In the wild, the videos show, the birds 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. That pulls up a column of nectar as the grooves expand. This pulling, or pumping, slurps nectar faster than grooves that stayed open would. The researchers report this online August 19 in the Proceedings of the Royal Society B.

Hummingbirds do this tongue dipping fast. Rico-Guevara says he has clocked 23 licks per second.

The skinny, translucent tongues have no muscles in them. But they have a semicircular groove on each side. The tongue forks into fringed halves at the tip. Rico-Guevara first started studying the tongue tips. Those tips are not so much capillary tubes as traps 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.”

Other scientists countered with computer simulations and their own videos of birds in the lab. They argued that, regardless of what happens at the tip, capillary suction is important in drawing nectar up the grooves.

To make his own study of the grooves, Rico-Guevara went looking for birds with Kristiina Hurme. She studies bird behavior. The pair coaxed 18 hummingbird species in the wild to sip on camera. The videos showed that the birds’ tongue grooves mostly stayed closed when waiting for nectar. And when tongue met nectar, the fluid moved fast. It averaged nearly 1 meter (3 feet) per second as it rose up the tongue. Even under ideal conditions, a simple capillary rise would draw in nectar much more slowly. That would max out at only about 36 centimeters (14 inches) per second, the new paper reports.

In the course of filming, one accident turned into a “perfect experiment” to compare plain capillary rise and pump action, Rico-Guevara says. A bird bumped one side of its tongue against a feeding tube. The tongue’s compressed groove opened early, touching the nectar. This time, the nectar indeed rose by the typical capillary action. But it moved more slowly than did nectar in the groove on the opposite side of the tongue. That groove sprang open when it touched the nectar. And the nectar raced up.

To describe the process, coauthor Tai-Hsi Fan, who studies fluids, developed the concept of the tongues as elastic micropumps. With computer simulations, he predicted such details as nectar uptake speeds. “We’re pretty excited about how the mathematical model matches the data” from the videos, Rubega says.

Power Words

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behavioral ecologist A scientist who studies animal behavior in a natural setting.

capillary action  This is the force that governs the movement of a liquid along the surface of a solid. Because molecules of the liquid are attracted to the surface and to each other, they can pull each other along. Capillary action explains how sponges wick up liquids.

ecology  A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

model  A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.

molecule  An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O₂), but water is made of two hydrogen atoms and one oxygen atom (H₂O).

nectar A sugary fluid secreted by plants, especially within flowers. It encourages pollination by insects and other animals. It is collected by bees to make into honey.

ornithology The scientific study of birds. Experts who work in this field are known as ornithologists.

simulate  To deceive in some way by imitating the form or function of something. A simulated dietary fat, for instance, may deceive the mouth that it has tasted a real fat because it has the same feel on the tongue — without having any calories. A simulated sense of touch may fool the brain into thinking a finger has touched something even though a hand may no longer exists and has been replaced by a synthetic limb. (in computing) To try and imitate the conditions, functions or appearance of something. Computer programs that do this are referred to as simulations.