Sunday, 11 February 2018

Hairy tongues help bats drink up

February 7, 2018 by Jennifer Chu, Massachusetts Institute of Technology

Animals have evolved all manner of adaptations to get the nutrients they need. For nectar-feeding bats, long snouts and tongues let them dip in and out of flowers while hovering in mid-air. To help the cause, their tongues are covered in tiny hairs that serve as miniature spoons to scoop and drag up the tasty sap.

Now engineers at MIT have found that, for bats and other hairy-tongued nectar feeders, the key to drinking efficiently lies in a delicate balance between the spacing of hairs on the tongue, the thickness of the fluid, and the "speed of retraction," or how fast an animal darts its tongue back to slurp up the nectar. When all three of these parameters are in balance, a good amount of nectar reaches the animal's mouth instead of dribbling away.

As it happens, the same goes for other hairy-tongued nectar feeders, such as honeybees and honey possums, which the researchers found also exhibit optimal "viscous entrainment," which refers to the amount of fluid that hairy surfaces can drag up from a bath.

"There are lots of different drinking techniques for animals, and what we think is normal when we drink is really a singular way of drinking," says Pierre-Thomas, a former instructor in MIT's Department of Mathematics. "We hope that our theory explains what are the main trending mechanisms of this hairy drinking method, and we believe we have rationalized this peculiar drinking technique."

Brun, who is now an assistant professor at Princeton University, carried out this current work at MIT with Alice Nasto, a graduate student in MIT's Deparment of Mechanical Engineering, and Anette "Peko" Hosoi, professor of mechanical engineering and associate dean of engineering at MIT. The researchers have published their results, which are based on a combination of mathematical modeling and lab experiments, today in the journal Physical Review Fluids.

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