New species of mammals aren’t found very often, but if one is, chances are it will be a bat. Bats are secretive, often nocturnal, and numerous. With more than 1200 species in the order Chiroptera, they represent 20% of all mammalian species (red in the pie chart below); the only bigger order is the rodents:
In 2005 a new species of bat was discovered in the cloud forests of Ecuador (perhaps reader Lou Jost has seen this one): Anoura fistulata, otherwise known as the tube-lipped nectar bat. (The short paper describing it, by Nathan Muchhala et al., is free for download here, and the reference is at the bottom of this post.) As National Geographic News reported last week, its unusual method of feeding—inserting an absurdly long tongue into flowers and lapping up the nectar—has just been filmed for the first time.
What is unusual about the bat is that its tongue is longer than its body!
The creature is only about two inches (five centimeters) long, but its tongue is nearly three and a half inches (nine centimeters) long—one and a half times longer than the bat’s body.
When not collecting nectar from the Centropogon nigricans flower, the bat’s tongue is retracted and stored in the animal’s rib cage.
That’s the longest tongue relative to its body size of any mammal, even longer than that of Gene Simmons! Just as a comparison (no griping, readers), if my own tongue were of similar relative size, it would be eight and a half feet long.
A. fistulata is distinguished from other species in the genus by having a lower lip that is elongated, extends further than the upper lip, and is rolled into a tube covered with papillae. And of course there’s that tongue. Both of these features are shown in the figure below, taken from the paper.
But of course what we want to see is how they use that tongue. This was filmed just recently by a National Geographic crew. As their article reports:
In the new high-def video—which aired Sunday as part of the National Geographic Channel’s Untamed Americas documentary series—the bat is shown feeding on the wing. (The Channel and National Geographic News are affiliated within the National Geographic Society.)
“These bats can hover,” said biologist Nathan Muchhala, who helped discover the species in an Andean cloud forest. “They’re like hummingbirds in that sense.”
In a close-up, the animal’s tongue slithers, snakelike, down the flower’s long neck. When the tongue reaches the pool of sweet nectar at the bottom, the tip transforms, becoming suddenly prickly as hairlike structures called papillae extend outward.
“Just before the bat retracts the tongue, the [papillae] stick straight out sideways,” said Muchhala, of the University of Nebraska-Lincoln. “That maximizes the surface area, allowing it to act like a mop and sop up as much nectar as possible.”
Here’s the video, which shows the bat hovering like a hummingbird (this, by the way, is an example of “convergent evolution,” since these bats and hummingbirds independently evolved nectar feeding, long tongues, and hovering):
The narration is very good here: I love the line “necessity is the mother of evoluiton.”
As National Geographic reports, getting that video was no easy matter:
To get the super-tongue footage, National Geographic filmmakers flew to Ecuador, where Muchhala and his team were waiting with a bat they’d already netted. Filming took place in a special tent, in which the bat could freely fly and feed. To make the tongue visible to the camera, a small hole was cut at the base of the flower. “They put the camera behind the hole and got that amazing close-up shot,” Muchhala said. At first, the bats were bothered by the humans and the bright lights in the tent and would not approach the flower to feed, but they eventually adjusted.”They actually get so used to it that after a while,” Muchhala said, “you come into the tent and they come up to you and will land on your hand looking for nectar.”
The long tongue was first discovered before they knew anything about the bat’s ecology, but, like Darwin’s orchid and its pollinating moth, a long tongue in a nectar-feeding bat implies a flower with a long corolla tube. Muchhala published another paper in 2006 (reference below) verifying this and giving a diagram showing how the bat’s tongue is retracted into its abdomen with special muscles:
Dietary studies of Anoura in four reserves on the eastern and western slopes of the Andes confirm this prediction. During 129 nights of mist-netting in 2003–05, I captured 46 A. geoffroyi, 38 A. caudifer, and 21 A. fistulata, and identified the pollen on their fur and in their faeces. Pollen from Centropogon nigricans, which has corollas 8–9 cm long, was carried only by A. fistulata (Fig. 1d), as might be expected, given that other Anoura could not reach this flower’s nectar. During 55 hours of nocturnal and diurnal videotaping of 12 flowers of C. nigricans, ten bats were the only visitors. This observation, combined with the finding that A. fistulata was the only bat visitor, supports the conclusion that A. fistulata is the only pollinator of this plant.
Specialization on one species of pollinator is exceedingly rare in angiosperms, and C. nigricans is the only example known in flowers pollinated by bats. After the initial evolution of a glossal tube, the extreme tongue length of A. fistulata probably coevolved with long flowers such as those of C. nigricans. In an example of convergent evolution, pangolins (scaly anteaters) also have a glossal tube; despite their different diets, ant-eating and nectar-feeding animals face similar evolutionary pressures for highly protrusible tongues, and pangolins and A. fistulata have independently converged on a similar solution.
How did this evolve? Well, of course we’re not sure, but, as Muchhala (2006) suggests, it could be a form of “coevolution,” in which the bat evolved its long tongue in concert with the flower evolving its long corolla tube. A flower that is already being pollinated by bats (and whose nectar is being nommed) might accrue a reproductive advantage by making the nectar a little bit more inaccessible—a little farther away from the opening. That forces the bat to press its head up against the flower, ensuring a better contact between bat and flower (the pollen is carried between flowers on the bat’s head). And if the flower tube gets longer, that then gives an advantage to bats who have a longer tongue, since those individuals will be better at getting the nectar, and hence will be better fed and leave more offspring. And so tongue and flower, spurred on by each other’s evolution, mutually elongate.
True “coevolution” occurs when two species act as selective forces for each other, so that they evolve in concert. The word is often misused to imply one species adapting to another, as in certain forms of mimicry, but true coevolution involves reciprocal evolutionary adjustments between a pair of species. Although it’s probably common in nature, we don’t have a lot of good examples.
Will this system evolve until the flower gets three-foot nectar tubes and bats have three-foot tongues? Probably not. There’s a limit to how far this type of coevolution can proceed, presumably imposed by the evolutionary costs of making longer corolla tubes or producing longer tongues. For the same reason, the very long tails of male African widowbirds don’t become ten feet long, for although females prefer longer tails than males actually have, at some point the added sexual benefits of having a longer, more attractive tail are outweighed by the loss in fitness such a tail confers (it could, for example, seriously impede the male’s ability to fly).
There are no new evolutionary principles demonstrated by this example, but it’s nevertheless thrilling, for it shows us another unexpected way that natural selection has worked, producing adaptations that seem a priori inconceivable.
Muchhala, N., P. Mena, and L. Albuja. 2005. A new species of Anoura (Chiroptera: Phyllostomidae) from the Ecuadorian Andes. J. Mammalogy 86:457-461.
Muchhala, N. 2006. Nectar bat stows huge tongue in its rib cage. Nature 444:701-702. [Isn’t that a terse and informative title? Hemingway could have written it.]