The Pallas's long-tongued bat uses blood to change the shape of its mop-like tongue as it feeds in mid-air, researchers have discovered. High-speed video footage has revealed that an increased flow of blood to the tip of the bat's tongue causes scores of tiny hair-like projections to become swollen and erect, allowing the bat to maximise its nectar-gathering potential with each lap.
Like the hummingbird, the Pallas's long-tongued bat (Glossophaga soricina) from South and Central America expends a great deal of energy hovering over flowers while lapping up nectar. To sustain this activity, these small-bodied animals have to maintain the highest metabolic rates of all vertebrates, relying on the sugary substance they ingested mere minutes ago to almost exclusively fuel their hovering activity. A 2008 study in the Journal of Experimental Biology led by Kenneth Welch from the Department of Ecology, Evolution and Marine Biology at the University of California found that in Pallas's long-tongued bats, 78% of the energy they need for hovering flight is provided by the processing of recently ingested sugar, while hummingbirds can fuel around 95% of their hovering flight in the same way. Compare this to the relatively inefficient 25 to 30% of exercise metabolism that humans and other mammals can fuel using dietary sugars, because we rely so heavily on other types of fuel such as glycogen and triglyceride, and you can see how impressive these creatures are.
It's impressive, but still pretty stressful business. Both hummingbirds and Pallas's long-tongued bats are living in constant states of just-fed or starving, and they consume most of their body fat every day just to make it through. So they've evolved several morphological features to make them nectar-processing machines, including light, compact bodies (Pallas's long-tongued bats weigh on average 9 grams), specialised intestines that are super effective at assimilating sugar, flight muscles made up of fast-twitch muscle fibres that can sustain high-intensity hovering flight, and their tongues are so long, they've had to evolve special places to keep them. The hummingbird keeps its tongue in an elongated bill, while the Pallas's long-tongued bat has a snout-like rostrum to fold his inside. When elongated, their tongues will grow to about double their resting size.
The tip of the Pallas's long-tongued bat's tongue is covered in organised rows of slender filamentous papillae, or little hair-like projections that sit flat against the tongue when it's at rest, and fan out from the tongue when it's being used. Scientists have known about these papillae for a long time, but it was assumed that they existed to increase the surface area of the tongue and catch the odd piece of pollen or droplet of nectar in the space between them, like a simple, static brush.
Then in 2011, Alejandro Rico-Guevara and Margaret Rubega from the University of Connecticut published a study in Proceedings of the National Academy of Sciences (PNAS) using high-speed cameras to watch 30 hummingbirds lap at nectar inside clear tubes (video here). Hummingbird tongues are split into two at the tip, and the space between the fork is filled with tiny, interlocking plates called lamellae. No one had suspected that these structures could be deliberately moved in any way, but by watching the hummingbirds feed in slow motion, Rico-Guevara and Rubega saw that as the tongue extends towards the nectar, its two tips separate, causing the lamellae inside to unfurl. When the hummingbird's tongue is withdrawn, the lamellae roll back inwards, and the nectar is trapped between the tongue tips, ready to be safely delivered to the mouth. It was this discovery that led Cally Harper, Sharon Swartz and Elizabeth Brainerd from the Department of Ecology and Evolutionary Biology and School of Engineering at Brown University to try out the same technique on the nectar-eating Pallas's long-tongued bat.
Describing their findings in today's edition of PNAS, the team first saw that the hair-like papillae became erect during feeding, extending off the surface of the tongue with each lap just as the tongue approached maximum extension. The papillae were even manoeuvred to change their orientation so they could sit perpendicular to the tongue's long axis, thereby maximising its nectar-catching surface area. More footage revealed that this movement occurred even when the tongue had not made contact with the sticky nectar, which proved that surface tension release did not drive the changes in the shape of the bat's tongue. This was significant, because the hummingbird's tongue movements are reliant on this surface tension release, so the researchers reasoned that some other mechanism - something internal - was responsible for papilla erection in Pallas's long-tongued bats.
Based on what they observed in the vascular morphology of the Pallas's long-tongued bat's tongue, they suggested that rapid blood flow to the area was causing the papillae to become swollen and erect during nectar feeding. More video footage confirmed this. "A colour high-speed movie shows increased blood flow to the vascular sinuses and engorgement of the papillary veins during nectar feeding," they wrote, describing how the tongue was pale pink in the footage as it first extended from the mouth, which indicated the presence of relatively little blood in the vessels. Then, "As the tongue reaches maximum extension, the vascular sinuses and papillary veins engorge with blood and become bright red as the papillae become erect. Blood is temporarily trapped within these vessels and the papillae remain erect throughout tongue retraction."
During lapping, the length of the tongue tip was also increased by more than 50%, and the researchers think that as the tongue extends from the mouth, this causes the contraction of certain muscle fibres, which not only causes it to decrease in width or diameter and increase in length, but it also compresses the arteries and veins in the tongue to displace more blood down into the tongue tip.
While this kind of mechanism has not yet been found in any other mammals, the researchers suggest that it could also be in play in the tiny, nectarivorous Australian honey possum (Tarsipes rostratus), because its tongue is equipped with enlarged blood vessels and a significant artery sitting right in the tip.
Welch, K., Herrera M., L., & Suarez, R. (2008). Dietary sugar as a direct fuel for flight in the nectarivorous bat Glossophaga soricina Journal of Experimental Biology, 211 (3), 310-316 DOI: 10.1242/jeb.012252
Rico-Guevara A, & Rubega MA (2011). The hummingbird tongue is a fluid trap, not a capillary tube. Proceedings of the National Academy of Sciences of the United States of America, 108 (23), 9356-60 PMID: 21536916
Cally J. Harper, Sharon M. Swartz, and Elizabeth L. Brainerd (2013). Specialized bat tongue is a hemodynamic nectar mop
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