The hummingbird tongue is a fluid trap, not a capillary tube

Abstract

Hummingbird tongues pick up a liquid, calorie-dense food that cannot be grasped, a physical challenge that has long inspired the study of nectar-transport mechanics. Existing biophysical models predict optimal hummingbird foraging on the basis of equations that assume that fluid rises through the tongue in the same way as through capillary tubes. We demonstrate that the hummingbird tongue does not function like a pair of tiny, static tubes drawing up floral nectar via capillary action. Instead, we show that the tongue tip is a dynamic liquid-trapping device that changes configuration and shape dramatically as it moves in and out of fluids. We also show that the tongue-fluid interactions are identical in both living and dead birds, demonstrating that this mechanism is a function of the tongue structure itself, and therefore highly efficient because no energy expenditure by the bird is required to drive the opening and closing of the trap. Our results rule out previous conclusions from capillarity-based models of nectar feeding and highlight the necessity of developing a new biophysical model for nectar intake in hummingbirds. Our findings have ramifications for the study of feeding mechanics in other nectarivorous birds, and for the understanding of the evolution of nectarivory in general. We propose a conceptual mechanical explanation for this unique fluid-trapping capacity, with far-reaching practical applications (e.g., biomimetics).biomechanics | fluid dynamics | nectar trapping | surface tension

The hummingbird's tongue: a self-assembling capillary syphon

“…The tongue must be flexible for this unloading process. In addition, when only small quantities of nectar are available in the target flowers, a flexible tongue may make it easier for the tongue lamellae to sweep the corolla tube [11,25]. Hummingbirds feed from plants with a variety of floral morphologies; jewelweed (Impatiens capensis) even has forward-pointing nectar spurs that require the tongue to bend at a 1808 angle [26].…”

“…Py et al [13] presented the first example of capillary origami, demonstrating that thin sheets with bending stiffness B can be folded up by the surface tension s of a water droplet placed on them, provided that the largest sheet dimension exceeds the elastocapillary length l E ¼ (B/s) 1/2 . Recently, Rico-Guevara & Rubega [11] demonstrated that a hummingbird tongue closes around nectar, thus representing a natural example of capillary origami [14]. Their high-speed videography indicates that when the tongue is withdrawn from the nectar, the formerly immersed portion of the tongue changes shape, so that the thin membrane curls inwards and traps liquid inside its grooves.…”

Hummingbird tongues are elastic micropumps

“…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 [14]. Expansive filling accounts for nectar uptake by the portions of a hummingbird's tongue that remain outside the nectar pool.…”

“…During protrusion, they use their bill tips to squeeze the nectar off the tongue grooves inside the bill [14]. Consequently, as the tongue emerges, it is compressed along the grooves' entire length [15].…”

Dogs lap using acceleration-driven open pumping

“…Archerfish adjust for index of refraction when shooting water jets through the interface to catch prey (1, 2), some marine copepods leap into the air to avoid predation (3-6), and lizards and frogs run or skip across the water surface to escape predators (7-9). However, almost all terrestrial animals interact regularly with the air-water interface when they drink or feed (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22). These animals use a wide array of mechanisms to breach the interface and transport fluid into their bodies, including viscous dipping, capillary suction, viscous suction, licking, and lapping (17).…”