Some bats use their hairy tongues to slurp up nectar from flowers, and now we know why they’re so good at drinking. The spacing of their tongue hairs has evolved to be almost exactly optimal for sucking up as much nectar as possible.
To figure out how the hair-like protuberances on a bat’s tongue help it drink, at the Massachusetts Institute of Technology and her colleagues molded rubbery surfaces with differently-spaced nubs on them, and dipped the model tongues into a viscous oil.
As they pulled these artificial bat tongues out of the oil, they took pictures to determine how the liquid was flowing out of the forest of hairs and then weighed them to find out how much liquid was left behind.
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“Their hairs allow them to take out 10 times more liquid than they would be able to if they didn’t have any hairs on their tongue,” Nasto says.
Slowing the flow
It works much like a honey dipper – channels carved into wood trap the honey and force it to flow more slowly, allowing you to transport more honey from the jar onto your toast. Hairs on bat tongues work the same way.
When a bat drinks, the blood vessels in its tongue hairs get engorged with blood, making them stick straight out. The spaces between the spiky hairs act just like the channels in a honey dipper, stopping the nectar from slipping off before it gets into the bat’s mouth. The slower the flow, the more liquid the bat can scoop into its mouth with each dip.
Through their experiments and mathematical models, Nasto and her colleagues figured out how the hairs should be spaced for optimum nectar transportation.
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Not too close
“You want your hairs to be as closely spaced as possible in order to minimize the drainage of nectar from between the hairs. But if you have too many hairs, there isn’t much space between them for the nectar to occupy,” Nasto says.
On average, each hair on a nectar-drinking bat’s tongue is about 0.3 millimetres wide. The researchers found that the optimum distance between these hairs is 0.13 millimetres. Bats have an average tongue hair spacing of about 0.125 millimetres, so they are getting just about as much nectar from each dip as possible.
Physical Review Fluids
Read more: Tongues may have evolved from a mouthful of water
