Now, a new study using high-speed video shows that the tongue's mopping ability is powered by blood, a phenomenon unique in nature, said study co-author Cally J. Harper of Brown University.
Two high-speed videos show how increased blood flow to a nectar-feeding bat's tongue helps it mop up food.
Elongated hairs at the tongue's tip that typically lie flat become engorged with blood and stand erect in rows when the tongue protrudes, allowing the animal to soak up the food.
"We've known of these elongated hairs, or papillae, at the tongue tip for a long time," Harper said.
"But it was really only when we looked at them with color, high-speed videos that we could see they actually changed their orientation and rose up perpendicular to the tongue during feeding, and that it was due to the flow of blood into the tongue."
For the study, Harper and colleagues identified enlarged vascular sinuses that run through the bat's tongue and are directly connected to the hairs.
When drawn to a flower by enticing smells or other triggers, the bat stretches out its tongue to feed, and the tongue tip can grow in length by more than 50 percent. That, in turn, contracts the tongue's width and squeezes those sinuses.
A color video of live bat tissue also showed that their capillaries go from pink—with little blood—to dark red when the hairs stiffened.
This led the team to suspect that blood flow through these sinuses might be what erects the hairs. To find out, the scientists took high-speed video of what happened when they injected fluid that mimicked blood flow into the tongues of dead bat specimens.
The experiment produced the same type of erection seen in the living animals, even though the muscles were long dead—suggesting blood is the culprit.
"We noticed the networks of blood vessels that run into the tip of the tongue are surrounded by muscle fibers, so that when the muscles contract they compress the blood vessels in the tongue and displace blood into the tip where it would erect the hairs," Harper explained.
Indeed, in some cases the bats barely protruded their tongues during tentative feeding and the hairs did not become erect. But they always did so just before the tongue reached maximum extension.
Harper suspects the end of the tongue is an adaptation that allows Pallas's long-tongued bat to survive on limited food sources.
Found in much of South America, Mexico, and Jamaica, the animal gets most of its sustenance from flowers—which isn't as easy as one might think.
"In order to feed, these bats hover in front of flowers, which takes a lot of energy," said Harper, whose study was published May 6 in the journal Proceedings of the National Academy of Sciences.
"They need to get as much nectar as possible, as quickly as possible. That's very important for them to be able to fuel their energy-intensive lifestyle."
Kenneth Welch, who studies vertebrate physiology at the University of Toronto Scarborough, said that the finding reminded him of another recent study showing how hummingbird tongues may function as fluid traps to capture nectar in the curled ends of the tongue.
"This is another example of the astounding ways evolution can arrive at unique solutions to similar challenges in each animal," said Welch, who is unaffiliated with the research.
"Nectar bats and hummingbirds have converged over evolutionary time in many remarkable ways that seem connected to their specialized diet of floral nectar."
Both are capable of hovering, he said, though they do it differently. And both fuel up on simple sugars, though they likely have different ways of controlling blood sugar levels.
"And now we see that both groups have tongues specialized to enhance the removal of nectar from flowers, though using tongues that are quite different in design and function," Welch said.
A Model for Miniature Robots?
A shape-changing tongue and blood-driven erectile hair system helps such bats survive.
Harper said the natural system could conceivably be mimicked as a model for miniature surgical robots that could function as improved endoscopes or other types of surgical devices.
"Because the tongue is flexible, and can simultaneously increase length and change its surface configuration," she explained, "similar designs might be used to open up blood vessels or regions of the small intestines during surgery."