Anatomist Larry Witmer never expected to find tissues engorged with blood--erectile tissues--in bird beaks. But new research on a pink flamingo reveals two pads of tissue under the tongue that can expand, provide mechanical support and stiffness to the bottom of the mouth, and probably help the birds eat with their heads upside down, as flamingos are wont to do. The finding might help biologists determine how filter feeding--seen in birds and mammals--first evolved.
Pink flamingos eat by sucking water through their mouths and straining out diatoms and plankton. Ducks and geese also filter feed, but flamingos do so with their heads upside down, using their tongues like pistons to drag water in and push it through their mouths' filtering apparatus. Little else is known about the birds' unique technique for eating or how it evolved.
Witmer, of Ohio University in Athens, and his colleagues originally planned to compare blood vessels in the skulls of flamingos to those of other birds. The Brevard Zoo in Melbourne, Florida, offered Witmer and his team a recently deceased pink flamingo. First, they infused barium-rich latex into the animal's arteries and veins. Using more barium in the veins than arteries allowed them to distinguish which were which in a CT scanner. The team was surprised to find two blood-filled sinuses under and on either side of the tongue, expandable structures that had never been observed in birds before.
Finding such erectile tissue--more commonly associated with sexual organs--in the mouth is unusual. "It had us scratching our heads for awhile," says Witmer. Based on the tissue's proximity to the feeding apparatus under the tongue, Witmer's team suggests the tissues help flamingoes filter food from water, a novel role for blood vessels in the head. Although they haven't measured these so-called paralingual sinuses in live flamingoes yet, the scientists believes their fattened state is several times larger than when deflated. In addition, CT scans revealed that the tissues wear down the jawbone and form a depression, the team reports in the October issue of The Anatomical Record. A cursory check of other flamingo skeletons revealed the depression on all of them, but not the skeletons of other birds.
"This is pioneering work," says ornithologist Richard Prum of Yale University. "Flamingos evolved a whole new way to feed, with a new orientation of the head," he says. "And nobody has come up with a role for the vascular system in foraging before." Evolutionary biologist Marcel van Tuinen of the University of North Carolina, Wilmington, says the depression in the jawbone can be used to trace the origins of filter feeding in evolution. In birds, this type of feeding is unique to flamingos, but it also resembles the feeding strategy of the baleen whale, he says.
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