Posted by Douglas Everett on
Hummingbirds have long, thin bills and tongues with channels, bristles, and papillae.
Historically (or for over 184 years), scientists and biologists (Jardine & Martin 1833) believed that hummingbirds stretched their tongues to extract nectar from flowers or feeders through capillary action. The idea was that their tongues would fill with nectar in the same way a small glass tube fills passively with water.
The physics of capillary action is based on two significant forces. Adhesion of the liquid molecules to the inner tube walls makes the liquid climb the sides. Surface tension holds the liquid together and drags the whole fluid column upwards.
The capillary action theory made sense since a hummingbird's tongue has two tube-like grooves. It would be a simple, passive way for nectar to travel up the tongue. (Scharnke, 1931; Klasing, 1998).
Read more at: https://phys.org/news/2015-08-hummingbird-tongues-tiny-nectar.html#jCp
- Jardine, William, Martin, W. C. L. 1833. The Naturalists Library: A General History of Hummingbirds of the Trochilidae, Kessinger Publishing, 276pp
- Klasing K C. 1998. Comparative Avian Nutrition. CAB International, Wallingford, OX, UK. 350 pp.
- Scharnke H. 1931. Beitrage zur morphologie und entwicklungsgeschichte der zunge der Trochilidae, Meliphagidae und Picidae. Journal of Ornithology, 79: 425-491.
Alejandro Rico-Guevara dis-proves that theory.
“The best and most recent 2014 explanation via Rico-Guevara who 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.
The hummingbird tongue fills with nectar even when only the tip is immersed.
- 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.
- 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.
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.
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’.
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”. (Ewald & Williams 1982)
- Alejandro Rico-Guevara B.S. Biology, Universidad Nacional de Colombia, 2005 A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy at the University of Connecticut 2014
- Ewald, P.W.& Williams, W. The Auk Vol. 99, No. 3 (Jul., 1982), pp. 573-576
- Rico-Guevara, A. Morphology and Function of the Drinking Apparatus in Hummingbirds, University of Connecticut, 2014
And Chapter 2
- Rico-Guevara, A., & Rubega, M. A. (2011). The hummingbird tongue is a fluid trap, not a capillary tube. Proceedings of the National Academy of Sciences, USA, 108(23), 9356-9360.