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Hummingbird Tongue Design Gets an Upgrade

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One of the memorable moments in Illustra Media’s documentary Flight: The Genius of Birds is the hummingbird tongue animation (see it on YouTube). The unique nectar-trapping mechanism of unfurling flaps (lamellae) on supporting rods that automatically fold over to seal in the nectar was discovered by two biologists at the University of Connecticut in 2011 (see the paper in PNAS). This was cutting-edge science at the time the film was made, because most biologists previously had assumed the tongue worked by simple capillary action.

The two biologists have continued their work since then, spending five years filming hummingbirds in the wild. Now, along with a mechanical engineer who is an expert in fluid mechanics, they have published a new paper in the Proceedings B of the Royal Society that should increase our admiration for the design of this structure. It’s not only a nectar trap; the hummingbird tongue is a micropump!

News from UConn Today includes a video clip of the tongue in action (see it by clicking image above; nice soundtrack too). The new findings debunk the notion that capillary action called “wicking” draws nectar up the tongue.

Rico-Guevara explains that a hummingbird’s tongue, which can be stuck out about the same length as its beak, is tipped with two long skinny tubes, or grooves. Rather than wicking, he says, the nectar is drawn into the tongue by the elastic expansion of the grooves after they are squeezed flat by the beak.

The tongue structure is collapsed during the time it crosses the space between the bill tip and the nectar pool, but once the tip contacts the nectar surface, the supply of fluid allows the collapsed groove to gradually recover to a relaxed cylindrical shape as the nectar fills it.

When the hummingbird squeezes nectar off its tongue during protrusion, it is collapsing the grooves and loading elastic energy into the groove walls. That energy subsequently facilitates the pumping of more nectar. [Emphasis added.]

This pumping action apparently works in concert with the lamellae (flaps) shown in the Illustra film. What’s new is that the cylinders are in a flattened shape when they enter the nectar. Having been compressed by the beak, they store elastic energy that makes them rapidly expand in the fluid as they unfurl. This expansion helps to pump the fluid into the cylindrical cavity upward from the lamellae. That way, more nectar can be delivered into the bird’s mouth.

Figure 1 shows the beak in cross-section from a CT scan. It looks beautifully designed to squeeze the tongue’s cylinders during protrusion, with the lower bill fitting into spaces in the upper bill that spread laterally to flatten the tongue as it exits the bill tip. This design probably also squeezes the previous load of nectar into the mouth at the same time. “After complete loading, the grooves filled with nectar were brought back inside the bill and squeezed for the next cycle, all in less than a tenth of a second,” they observed. The caption for Figure 1 explains how these two mechanisms (pumping and trapping) work together:

The hummingbird tongue fills with nectar even when only the tip is immersed. (a) Hummingbirds can drink from flowers with corollas longer than their bills by extending their bifurcated, longitudinally grooved tongues to reach the nectar. During protrusion, the tongue is compressed as it passes through the bill tip, which results in a collapsed configuration of the grooves (cross-section). (b) Upon reaching the nectar, the tongue tips fringed with lamellae roll open and spread apart, but some of the grooved portions of the tongue will never contact the nectar pool. For the grooves to fill with nectar, they must return to their uncompressed, cylindrical configuration.

Why doesn’t the collapsed tongue rebound immediately after it leaves the beak and enters the air? That would result in open tubes that would need to fill by capillary action when they enter the nectar. But capillary action is much slower than the observed filling. Apparently the tongue material is designed to expand upon contact with the nectar. “After contacting the surface, the grooves expanded and filled completely with nectar,” they found.

All hummingbirds have this mechanism. They filmed 32 wild birds, representing18 species (in 7 of the 9 main hummingbird clades), with a high-speed camera in natural wild habitats, undergoing hundreds of feeding cycles — all with the same results. This allowed them to falsify the “century-old paradigm” of the capillary hypothesis and shed new light on this rapid, dynamic process.

All observed licks followed the same pattern: tongue thickness was stable during protrusion of the tongue, and rapidly increased after the tongue tips contacted the nectar… After complete loading, the grooves filled with nectar were brought back inside the bill and squeezed for the next cycle, all in less than a tenth of a second.

Capillary action could not have filled the cylinders this rapidly. In addition, no meniscus (diagnostic of capillarity) was observed to form in any of the 96 video sessions. The pumping action, by contrast, fills the entire tongue in just 14 milliseconds. Here’s how it works, according to their new model:

We suggest that while squeezing nectar off the tongue during protrusion, the bird is collapsing the grooves and loading elastic energy into the groove walls that will be subsequently used to pump nectar into the grooves. The collapsed configuration is conserved during the trip of the tongue across the space between the bill tip to the nectar pool. Once the tongue tips contact the nectar surface, the supply of fluid allows the collapsed groove to gradually recover to a relaxed cylindrical shape until the nectar has filled it completely; hereafter, we refer to this previously undocumented mechanism as ‘expansive filling’.

The tongue stays flattened and sealed, in short, until it hits the nectar pool. Then, inside the fluid, the tongue’s twin cylinders rapidly expand, pumping nectar up into the tongue as it darts into the flower at speeds of a meter per second. As the tongue is withdrawn, the lamellae then seal the cylinder tightly shut for delivery into the bird’s mouth, as shown in the Illustra animation (fluid trapping). This is a wonderful dual mechanism that results in much more efficient food capture in far less time.

Fluid trapping is the predominant process by which hummingbirds achieve nectar collection at small bill tip-to-nectar distances, wherein tongue grooves are wholly immersed in nectar, or when the nectar is found in very thin layers. Expansive filling accounts for nectar uptake by the portions of a hummingbird’s tongue that remain outside the nectar pool. The relative contributions of the two synergistic mechanisms (fluid trapping and expansive filling) to the rate and volume of nectar ultimately ingested are determined by the distance from the bill tip to the nectar surface during the licking process.

In other words, these two “synergistic mechanisms” give the hummingbird the biggest possible nectar bang for the buck, regardless of how deep the nectar pool is. The new model explains how the tongue can fill up even in a short flower. Since hummingbirds already “have remarkably high metabolic rates, amazing speed and superb aeronautic control,” it is essential they get the optimum return on investment of feeding energy.

All these traits result from the ability of hummingbirds to feed on nectar efficiently enough to fuel an extreme lifestyle out of a sparse, but energetically dense, resource. Therefore, the way in which they feed on nectar determines the peaks and span of their performance, and thus their behaviour (and evolutionary trajectory), across a range of environmental axes.

But did evolutionary theory contribute anything to this study? The authors speculate briefly about “co-evolution” of flowers and their pollinators, but do not offer any “trajectory” by which a bird could evolve either of these mechanisms from ancestors lacking them. How useful is it to offer up evidence-free promissory notes like this?

The new explanation of the mechanics of nectar uptake we provide here suggests that physical constraints are the main determinants of the relationship between pollinator type and nectar concentration, and can guide us through alternative hypotheses of hummingbird-flower coevolution.

By contrast, they save their best lines for what might be termed (though not by the authors) intelligent design. The paper begins:

Pumping is a vital natural process, imitated by humans for thousands of years. We demonstrate that a hitherto undocumented mechanism of fluid transport pumps nectar onto the hummingbird tongue.

This implies a seamless connection between human design and biological design. They conclude on the design theme:

Our discovery of this elastic tongue micropump could inspire applications, and the study of flow, in elastic-walled (flexible) tubes in both biological and artificial systems.

You see, not only does a design focus inspire study of biological systems, it leads to better designed applications. Everyone can agree on this: hummingbirds are inspiring!

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