Tongues can do delightful and astonishing things. I am thinking of the way a frog fires its sticky tongue halfway across the universe to snag a passing insect (see below). Or how an alligator snapping turtle wriggles its tongue like a worm as a dinner invitation to fish. And now the Pallas’s long-tongued bat (Glossophaga sorcina) joins this elite club of astonishing animal tongue artists.
These bats, found from Argentina to northern Mexico, and sometimes into Arizona and New Mexico, have the fastest metabolism ever recorded in a mammal, says a 2007 study in the journal Nature. They burn half their body fat each day, and have to make up for it at night by consuming as much as 150 percent of their body weight in nectar from flowers. And of course, they have to do it on the wing. According to a study published today in the Proceedings of the National Academy of Science, the secret to its success is a remarkable ability to change the shape of its tongue into a hemodynamic—or blood-swollen—“nectar mop.”
When lead author Cally Harper, a doctoral candidate in biomechanics at Brown University in Rhode Island, began her study, specialists already knew that bats of this species have an unusual fringe of hair-like structures around the tip of the tongue. They assumed these were useful for collecting nectar—but passively, like raking icing off a cake using your fingernails. Biologists also knew these bats have enlarged blood vessels in their tongues. But they didn’t know what to make of them. Harper had a hunch that the two features might be connected, especially since hovering is a challenging way to get dinner. (Try it sometime.) She figured that the bats might be rapidly forcing blood into the tips of their tongues, swelling those hair-like structures to glom up nectar far more actively and efficiently, as if a human could lengthen its finger nails by inflating them on a moment’s notice to better rake that icing off the cake.
To test her hypothesis, Harper, who had never worked with bats before, learned how to use high-speed filming equipment. Then she locked herself in a dark room with her test animals. “It took a long time to get used to, because bats are nocturnal,” she says. “It’s quite an experience to walk into a roomful of bats where they know where you are but you can’t see them. They can echolocate, and they’re disturbed by your presence.”
Harper previously studied a fatty deposit that influences echolocation in dolphins. Her parents, she says, think her research on bat tongues and other “soft, flexible structures that do amazing things” is “very weird.” But she also credits her father, who works in construction, with getting her started. “My Dad and I used to build things and try to figure out how stuff worked,” she says, and it stuck.
She spent the next six or seven months, for 12 hours a day, getting the study to work. First she trained the bats to feed from a square feeder filled with sugar water. Then she had to catch the crucial moment on film. “One of the biggest challenges was that we couldn’t control when they would feed and how they would feed, so I just had to wait. And because they are nocturnal, they didn’t like to be in the light. And high-speed cameras require a lot of light.”
Eventually, she figured out where the tongue usually lands in the feeder. Then she used fiber optics to illuminate only that spot, while the rest of the room stayed dark. She also had to stay alert, because an entire “feeding bout”—that is, the arrival of the bat, the tongue flicking in and out two or three times, and then departure—happened literally in the blink of an eye, about 330 milliseconds.
The resulting video demonstrated that the bat’s tongue lengthens by 50 percent during lapping, and the hairs, or papillae, become erect and change their orientation so they’re perpendicular to the length of the tongue. The nectar gets trapped between the erect hairs and drawn into the mouth. Harper says the tongue changes shape even when it flicks out into the air, rather than into nectar. So it isn’t just a byproduct of surface tension. It’s a hydraulic process in which blood flow changes the shape of the tongue to turn it into a perfect tool for rapidly mopping up nectar.
Harper and her co-authors, Sharon M. Swartz and Elizabeth L. Brainerd, both Brown University biomechanists, believe the discovery could serve as a valuable model for developing improved surgical tools. Minimally invasive surgery already relies on robotic devices that move with an inchworm-like motion through narrow spaces. Adapting the bat’s hydraulic approach, says Harper, could allow the tip of a surgical tool to widen and hold open a space, creating a better working field for fiber optic video and other surgical tools.
Harper collects her doctorate this month, dreams of going on to work in engineering or robotics—and like many graduates these days, has not yet found a job.