Hummingbird tongues may work like micropumps

A new way of looking at hummingbird tongues sees them mainly 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 functional morphologist Alejandro Rico-Guevara of 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 up 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 stayed open would, the researchers report 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, with no muscles in them, have a semicircular groove on each side. The tongue forks into fringed halves at the tip. 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.”

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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,” proposed Wonjung Kim, now at Sogang University in Seoul, South Korea, Bush and collaborators in 2012.

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 videos showed that 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 the tongue. Even under ideal conditions, a simple capillary rise would draw in nectar much slower, only about 36 centimeters per second, the new paper reports.

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 prematurely before touching the nectar. In this instance, nectar did a typical capillary rise — but moved more slowly than nectar in the groove on the opposite side of the tongue that sprang open later.

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 videos, Rubega says.

Kim says the videos used to develop the capillary-siphon model only showed nectar rising in already open tongue grooves. Even in the new grooves-closed model, “I think that capillary force is still at work,” Kim suggests. This force of water molecules gripping each other might explain why the wet, compressed tongue grooves stay closed until the tongue dips into nectar. In the new model, nectar isn’t rising like water in some tiny laboratory tube. But that doesn’t mean capillarity is irrelevant, he says.