The Flamingo’s Smile Read Free Page A

tongue fills a large channel in the lower beak. It moves rapidly back and forth, up to four times a second, drawing water through the filters on the backwards pull and expelling it on the forward drive. The tongue’s surface also sports numerous denticles that scrape the collected food from the filters (just as whales collect krill from their baleen plates).
    The extensive literature on feeding in flamingos has highlighted the unique filters—and often neglected another, intimately related, feature equally remarkable and long appreciated by the great naturalists. Flamingos feed with their heads upside down. They stand in shallow water and swing their heads down to the level of their feet, subtly adjusting the head’s position by lengthening or shortening the s-curve of the neck. This motion naturally turns the head upside down, and the bills therefore reverse their conventional roles in feeding. The anatomical upper bill of the flamingo lies beneath and serves, functionally, as a lower jaw. The anatomical lower bill stands uppermost, in the position assumed by upper bills in nearly all other birds.
    With this curious reversal, we finally reach the theme of this essay: Has this unusual behavior led to any changes of form and, if so, what and how? Darwin’s theory, as a statement about adaptation to immediate environments (not general progress or global direction), predicts that form should follow function to establish good fit for peculiar life styles. In short, we might suspect that the flamingo’s upper bill, working functionally as a lower jaw, would evolve to approximate, or even mimic, the usual form of a bird’s lower jaw (and vice versa for the anatomical lower, and functionally upper, beak). Has such a change occurred?

    The enigmatic smile of a swan—or is it?
    Nature harbors a large suite of oddities so special that we scarcely know what to predict. But, in this case, we encounter a precise reversal of anatomy and usual function—leading to a definite expectation: upside-down animals should reorient the form of their bodies to a new function when current behavior and conventional anatomy conflict.
    We may begin by sparing the usual pontification (but only for a while) and looking at a picture. If this picture excites a vague feeling of familiarity slightly awry, your perceptions are acute, but ride with me for a while.
    We seem to see a long-necked swan with a broad smile. But look carefully, for details betray this impossible beast. Its mouth opens above the eyes; its feathers run the wrong way; and where are its legs? I now show you the celebrated original in its proper orientation (and with the legs restored)—the flamingo from J.J. Audubon’s Birds of America , and a sure entry on anyone’s hit parade of most famous pictures in natural history.
    This dramatic perceptual switch from happy swan to haughty flamingo recalls any standard item in the psychological arsenal of optical illusion—particularly the young well-dressed lady looking away who becomes the old hag in profile. In fact, any accurately executed picture of a flamingo produces the same jolting effect when viewed upside down (I have checked all historically important portraits)—and for an obvious reason. The jaws have evolved to fit their reversed function. The flamingo’s upper jaw does look like a typical bird’s lower bill, and we therefore see the upside-down flamingo not as an absurdity, but as an only slightly odd swan-like bird.
    The morphological alterations extend far beyond the changes in external form that produce such a striking perceptual shift from upright flamingo to inverted “swan.” But note first the peculiar curve of the beak itself. The flamingo’s bill projects out from its face, but then makes a sharp angular turn, producing the pronounced hump that looks like a trough (and works like one) when inverted for feeding. Some Near Eastern peoples call flamingos “camels of the sea,” not because the inclined bill