Few sights capture nature's elegance and inventiveness as keenly as a flitting hummingbird pausing momentarily in midair to feed on a flower's nectar. The stillness of the hovering itself demands awe, and the biomechanics of hummingbird flight have rightly been at the center of a number of recent high-profile studies. But if you can look beyond the aerodynamic intricacies involved, another mystery becomes apparent; that of nectar extraction itself. Recent work published in the Proceedings of the National Academy of Sciences by Alejandro Rico-Guevara and Margaret Rubega from the University of Connecticut, USA, is soon to radically change our understanding of how hummingbirds feed.

Tongues are important for fluid consumption in a number of tetrapods (consider a lapping cat or dog), and nectar-feeding hummingbirds are no exception. For nearly 200 years the tongue's role in hummingbird feeding was recognized as crucial, but the mechanism involved was debatable and theories invoked suction and, more popularly, capillary action as potential driving forces. Hummingbird tongues possess two grooves running from their tip back toward their base, and the idea behind the capillary model is that when the tongue tip is placed into nectar the fluid is passively drawn through the grooves toward the mouth via capillarity. This passive mechanism involving the fluid's own properties (e.g. surface tension) is central to foraging and energy balance models used by physiological ecologists studying hummingbirds; however, recent empirical work is inconsistent with capillary action as the driving force behind nectar feeding.

To find a new explanation, Rico-Guevara and Rubega performed detailed morphological studies of tongues from 20 species of hummingbird and used high-speed video to record tongues from animals of 10 hummingbird species moving in and out of nectar in vivo and post-mortem. Hummingbird tongues are bifurcated at the tip, which is covered with lamellae that help to form the previously mentioned grooves on either side of the tongue. Prior to feeding, the tongue tips are held together and the lamellae are tightly furled, making the groove appear like a flattened tube in cross-section. Upon entering nectar, the tongue tips spread apart and the lamellae unfurl, essentially opening the tongue grooves wide, allowing them to fill with nectar. When pulled out of the liquid, the tongue tips twist about their long axes, and the lamellae roll inward, furling back up again and creating a tube-like structure with the nectar trapped inside it. This re-furling of the lamellae happens just as they pass across the liquid–air boundary. The authors suggest that these lamellar movements create a `lingual seal', which prevents nectar from leaking out of the tongue as it is pulled back into the mouth.

This whole process can happen in about 50 ms and the movements at the tongue tip appear to require no muscular work. Post-mortem tongues perform just as well as those in vivo, indicating something special about the structure of the tongue itself and the way it responds to physical forces at the air–nectar interface as it is pulled back into the mouth. The authors show that the structural bases of this nectar-trapping mechanism are found in species from all major clades of the hummingbird family, indicating the likelihood of its widespread presence as a means of feeding. Whether this fascinating fluid-trapping design can be incorporated into biomimetic technology remains to be seen, but until then at least we should appreciate that hummingbirds can take advantage of it.

M. A.
The hummingbird tongue is a fluid trap, not a capillary tube
Proc. Natl. Acad. Sci. USA