Erectile function: bats mop up nectar with “hairy” tongues

Blood forces hairs to stand up as the bat’s tongue is extended.

As sweet as the reward is, it isn’t easy being a nectar-feeder. Bats, hummingbirds, and bees face the difficult task of sipping as much nectar as possible from a tiny floral tube, all while hovering delicately in the air. And since hovering is such an energy-demanding task, the more efficiently these animals can slurp up nectar, the better they do on each visit to a flower.

Hummingbirds have evolved a creative adaptation to deal with this challenge: the tips of their long tongues are bifurcated, or split. During feeding, nectar is trapped between the two tongue tips and carried into the bird’s mouth. Long-tongued bees have solved the problem in a slightly different way: they have a brush-like structure on the end of their tongue to help lap up nectar. Nectar-feeding bats have similar hairy projections on the end of their tongue, and researchers have long assumed that these were simply static structures that increased the surface area of the tongue to make nectar feeding more efficient.

However, a new study in PNAS suggests that a nectar-feeding bat’s tongue is far more efficient—and more complicated—than scientists previously assumed.

The tongue of Glossophaga soricina, a tiny nectar-feeding bat native to Central and South America, is covered with hundreds of tiny hair-like projections called papillae. Using high-speed video, a group of researchers found that rather than remaining still during feeding, these papillae actually become erect, swelling each time the bat laps nectar (you can watch the awesome slow-mo footage here and here). At the beginning of each “tongue cycle,” the papillae are lying flat against the tongue, but just before the tongue is completely extended, the tiny projections extend outwards. Nectar gets trapped between each row of papillae, enabling the bats to mop up the nectar more efficiently with each flick of the tongue.

The video also showed that even if the bat’s extended tongue doesn’t contact any nectar, the papillae still jump to attention. This suggests that instead of responding to the surface tension of the nectar (as hummingbirds’ tongues do), a different mechanism is driving the extension of the papillae.

Via examination and dissection, the scientists discovered that a specialized network of veins, arteries, and sinus crisscross the tongue; many have corrugated linings, suggesting the capability to become engorged with blood. By experimentally injecting saline solution into dead bats’ tongues, the researchers found that a hemodynamic pump drives the erection of the papillae. As the bat’s tongue emerges from its mouth, the muscle fibers embedded in the tongue contract, compressing the veins and arteries and displacing blood to its tip. Here, blood enters the sinuses and a network of tiny veins at the base of each hair-like projection, helping the papillae pop up as the tongue plunges into the nectar.

This all happens incredibly quickly: one tongue “lap,” from beginning to end, takes just 100 milliseconds. To put that speed in perspective, it takes us nearly four times that long to simply blink once.

But beyond being a spectacular example of evolution’s ingenuity, why should we care about the tongues of nectar-feeding bats? The researchers suggest that the structure of these critters’ tongues could inspire tiny robots (or tiny parts of a larger robot) to be used inside patients’ bodies during surgery. Medical procedures such as angioplasties and gastric endoscopies require work in cramped, awkward spaces inside the body; incorporating technology based on these bats’ tongues could help create robots that are small, extremely dexterous, and capable of changing their shape and size at a moment’s notice.

PNAS, 2013. DOI: 10.1073/pnas.1222726110 (About DOIs).