Since the 1830s, ornithologists have assumed that hummingbirds drink by capillary action (wicking), the passive process of a fluid rising into a narrow tube because of forces attracting the liquid to the tube’s solid internal surface. A new study, led by Dr Alejandro Rico-Guevara from the University of Connecticut, debunks the ‘wicking theory’ and shows that the hummingbird’s tongue actually works as an elastic micropump.
A male Allen’s hummingbird (Selasphorus sasin) near Santa Cruz, CA. Image credit: Shravans14 / CC BY-SA 4.0.
Using high-speed cameras, Dr Rico-Guevara and co-authors filmed the tongue-nectar interaction in 18 hummingbird species, from seven of the nine main hummingbird groups throughout the Americas.
The results were published online today in the journal Proceedings of the Royal Society B: Biological Sciences.
“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,” Dr Rico-Guevara explained.
“Rather than wicking, 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 liquid 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.
“Our discovery of this elastic tongue micropump could inspire applications, and the study of flow, in elastic-walled tubes in both biological and artificial systems,” the scientists concluded.
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Alejandro Rico-Guevara et al. Hummingbird tongues are elastic micropumps. Proceedings of the Royal Society B: Biological Sciences, published online August 19, 2015; doi: 10.1098/rspb.2015.1014
