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<em>Science Advances</em>: Deep-Dipping, Grooved Tongue Helps Bats Feast on Nectar

Researchers have captured on video what appears to be a new tongue mechanism some bats use to feast on nectar. The research appears in the 25 September issue of the journal Science Advances.

Nectar is a primary food source for many animals but a few, including hummingbirds, honey eaters and sun birds and bats, possess mouthparts specifically designed to slurp up the sweet liquid found in flowers. Specialized nectar-feeding bats typically siphon nectar from flowers using extremely long, protruding tongues sprinkled with hair-like papillae. But some bats sport another type of grooved tongue — one that scientists haven't studied in depth.

Marco Tschapka at the University of Ulm and colleagues show that grooved-tongued bats in the group of New World leaf-nosed bats known as the Lonchophyllinae display a unique feeding behavior. Using high-speed cameras focused on bats trained to obtain nectar from artificial "test tube" flowers containing honey water in the lab, the researchers' video captured bats hovering in short flights (rarely lasting a second) over the feeders.

Unlike bats with hairy papillae, which moved their tongues in short lapping movements resembling a cat, the grooved-tongued bats lowered their tongues into the test tubes and did not move them during the entire visit. The bats' grooved tongues appear to never separate from the liquid nectar, acting a bit like a conveyor belt to transport the food up the tongue and straight into the animals' mouths.

The researchers aren't exactly sure how the bats are able to do this, but suspect it occurs by some combination of tongue deformation and the ability of fluids like nectar to flow without external force in certain narrow spaces, a phenomenon known as capillary action.

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The groove-tongued Lonchophylla robusta bat visits a bromeliad flower. | M. Tschapka/ University of Ulm

Both methods for obtaining nectar were effective, but the researchers think that the grooved-tongue bats may have an advantage in acquiring nectar from flowers of certain shapes that hold the sweet liquid differently.

For example, some flowers have diffusely distributed nectar, while others offer one small pool of nectar. Different nectar extraction mechanisms might be useful depending on the flower. The pumping mechanism of Lonchophyllinae bats might work more efficiently in flowers that allow a more complete submersion of the tongue in the nectar pool, the authors suggest.

Another important factor might be nectar viscosity. Nectar from bat-visited flowers is generally dilute, but the sugar concentration in nectar varies between flowers. Nectar with a low sugar concentration that is less viscous and more free-flowing might be more easily harvested by the pumping mechanism compared to nectar of high sugar concentration and viscosity.