Hummingbirds have incredibly high metabolic rates. Their hearts can beat more than 1000 times per minute, they inhale more than 200 times per minute and their oxygen consumption during flight can be 10 times grater than that of an elite marathon runner. Clearly all this work requires a lot of energy, which the hummingbirds get by feeding on nectar and consuming more than their own weight of it each day. As quick and efficient acquisition of nectar is essential for hummingbird’s survival their tongues have shape that allows to preform its function perfectly. The tongue of a humming bird consists of two rods and each rod has many hook shaped hairs (called lamella or lamellae for plural) attached to it (figure 1 and video 1).
Video 1. Hummingbird’s tongue. If you look closely you’ll see the forked tongue made of two rods extending from the hummingbird’s bill.
This unique architecture allows the hummingbird to reach and “sip” the nectar from flowers. The mechanics of how the nectar is taken up are quite remarkable. As the bird protrudes the tongue from its bill all lamellae are curled so that the rods together with lamellae essentially form two cylinders. The lamellae are kept in this configuration by the cohesive and adhesive forces maintained by the residing nectar in the tongue from the previous feeding episode (figure 2 A and B).
As the tongue reaches the fluid/air interface and enters the nectar the forces that keep each lamella curled are cancelled out by the opposing surface tension of the liquid on the outside of the tongue. This means that when the tongue is in the nectar the lamellae partially unfold to allow the maximum volume of nectar to enter the semi-cylindrical grooves formed between lamellae and the rods of the tongue (figure 2 C and D).
As the hummingbird retracts its tongue reverse process happens. When the tongue is retracted into the bill at the point of air/nectar interface the forces that curl the lamellae take charge again and the nectar becomes now trapped in the grooves of the tongue, which prevents any potential nectar leakage (figure 2 E and video 2).
Video 2. Hummingbird retracting its tongue from nectar. As the tongue is retracted see how at the point of air/liquid interface (a kind of a black line travelling along the tongue) lamellae curl back to their initial state.
Obviously, the nectar that is now trapped in the tongue needs to be offloaded inside the bill. While it is not quite clear as yet how that is achieved, one hypothesis has emerged from observation that when the bird initially extends its tongue to reach the nectar the opening of the bill through which the tongue emerges is narrower than the width of the opening when the tongue is retracted. This suggest that at the same time that the bird extends its tongue it also squeezes it to partially offload the nectar that was taken up previously (as the squashed lamellae in figure 2A and video 3). This hypothesis is also supported by the observed flattened tongue shape as it emerges from the bill and also the fact that hummingbirds have been observed to extend and retract their tongues even when they have finished feeding from a flower.
Video 3. squeezing of hummingbird’s tongue. The first capture is during extension and the second during retraction, note the difference in tongues diameter.
Notably, the mechanism described above only explains how nectar is loaded onto a tongue that is submerged in the nectar. However, as seen in video 4, the use of coloured nectar has shown that the liquid can be drawn upwards through the parts of tongue that never reached its source. For a long while the though was that the nectar was sucked up by simple capillary force action, however recent field experiments have shown that it is not the case. Now a new model has been proposed, which envisions the hummingbird’s tongue acting as an elastic micropump. This model brings us back to the observed squeezing of the tongue as it initially protrudes from the bill. As the tongue is squeezed an elastic energy is loaded into the walls of the tongue that form the grooves. Once the tongue reaches the nectar the incoming fluid restores the grooves into their relaxed states and the energy released from the collapsed groove walls drive the nectar up to the tip of the bill.
Video 4. Uptake of coloured nectar by the hummingbird.
All in all I’d say that hummingbird’s tongue is an extraordinary example of the ‘form fits function’ principle at its best.
Rico-Guevara, A., & Rubega, M. (2011). The hummingbird tongue is a fluid trap, not a capillary tube Proceedings of the National Academy of Sciences, 108 (23), 9356-9360 DOI: 10.1073/pnas.1016944108
Alejandro Rico-Guevara, Tai-Hsi Fan, Margaret A. Rubega “Hummingbird tongues are elastic micropumps.”
Proc. R. Soc. B 2015 282 20151014; DOI: 10.1098/rspb.2015.1014. Published 19 August 2015
Suarez, R. K. “Hummingbird flight: sustaining the highest mass-specific metabolic rates among vertebrates.” Experientia 48.6 (1992): 565-570.