Congratulations to the Rico-Guevara lab members, David Cuban, Amanda Hewes, and Alejo Rico-Guevara, on being featured in Science on their work on hummingbird tongues!
Excerpt from the Science article:
TONGUE EVOLUTION helped reptiles and amphibians capture animal prey, but in birds, some of the most outlandish tongue adaptations reflect a taste for plants. Most avian tongues are a stiff sliver of keratin (think fingernails) or bone, with little muscle or other living tissue. They “are just a conveyor belt to move food from front to back,” Schwenk says. But there are exceptions—most notably in hummingbirds and other birds that feed on nectar. “The tongue is probably the most vital component for nectar feeding in birds,” says David Cuban, a graduate student at the University of Washington (UW) who works with behavioral ecophysicist Alejandro Rico-Guevara.
Nectar is packed with energy and easy to find. But each flower offers just a drop or so, often sequestered in a long, narrow blossom. Many nectar-eating hummingbirds, sunbirds, and other unrelated groups of birds cope with these constraints by being small—usually less than 20 grams—and having long slender bills and highly specialized tongues.
Researchers used to assume these birds relied on capillary action—the tendency of a liquid to flow up a narrow tube—to take in nectar. And some of them do, including the pied honeyeater (Certhionyx variegatus), Rico-Guevara’s student Amanda Hewes and her collaborators have found. In this species the tongue has a paintbrush-like tip for picking up nectar, which is then drawn inward along grooves that run the length of the tongue.
But for hummingbirds, which flick their tongues 15 times per second as they drain each flower and quickly move on, capillary action just isn’t fast enough, Rico-Guevara says. His team captured high-speed videos as Anna’s hummingbirds (Calypte anna), white-necked jacobins (Florisuga mellivora), sparkling violetears (Colibri coruscans), festive coquettes (Lophornis chalybeus), and other hummingbirds visited transparent artificial flowers loaded with artificial nectar. The movies revealed that the hummingbird tongue works like a tiny nectar pump.
Two grooves run from the tip about halfway back, lined with fringes that trap liquid. As the tip of the birds’ flexible bill closes, it wrings nectar from fringes near the front of the tongue, pushing the liquid inward; then the bill opens at the base to help move nectar the rest of the way into the mouth, Rico-Guevara’s team reported on 3 April in the Journal of Experimental Biology.
He and his collaborators have recently turned their attention to some of the oddest nectar-feeding birds: parrots. At 30 centimeters tall and 100 grams, the rainbow lorikeet towers over most nectarivorous birds and is utterly incapable of hovering in midair like a hummingbird. It has the typical short, stout, hooked parrot beak and a muscular tongue much like our own—all traits that make slurping nectar from long, thin blossoms impossible. But Rico-Guevara and Cuban have identified adaptations that enable these parrots to get the sweet stuff.
To start, the birds target flatter, more open blooms. And instead of hovering, they land on a nearby branch and contort their bodies around the flower. Then they open their beak and stick out their tongue, which undergoes an amazing transformation as it extends into a flower. The hard, scratchy tongue tip opens into a circular array of fine protrusions, Rico-Guevara recently discovered. “It looks like an anemone, almost,” he says. These protrusions work like the bristles of a paintbrush to sop up nectar.
In one experiment, Rico-Guevara laced the test nectar solution with a barium compound, a diluted version of what doctors give patients to look for obstructions in the digestive tract, then took x-ray movies of lorikeet feeding. Once the tongue tip is saturated with a large drop of nectar, he found, the bird presses it against the top of the mouth, squeezing out the liquid. Then it closes its bill, nudging the nectar back toward the throat, and repeats the process until all the nectar goes down.
It’s not the only way parrots consume nectar. Last year, Cuban filmed feeding in the more diminutive hanging parrots—so named because they sleep upside down. Instead of a bushy tongue tip like the lorikeet’s, these parrots have a grooved tongue tip, and Cuban’s videos reveal that they vibrate their tongues very quickly to pump tiny amounts of nectar back toward the esophagus and down the throat.
By describing in detail how these birds feed and calculating the energy they expend in the process, Cuban, Hewes, and Rico-Guevara hope to learn how their feeding strategies may have shaped their evolution—and that of the plants they feed on. Since evolving 22 million years ago, for example, hummingbirds have influenced how much nectar their partner plants produce and how deep their flowers are, and this in turn has influenced the length of the hummingbirds’ beaks, their eagerness to monopolize flowers by chasing off competitors, and other traits. It’s a coevolutionary dance of birds and flowers—mediated by their tongues.
Read the full article in Science.