The Flight of the Iguana by David Quammen

in the passing of time adapted themselves especially to this way of life. A delicate and mutually satisfactory balance seems to have developed.
    An ecologist named Michael Goulding, after two years of fieldwork along the Rio Madeira, a major Amazon tributary in western Brazil, has produced the first broad study (The Fishes and the Forest: Explorations in Amazonian Natural History) of this interrelationship in the flooded forest. The featured players in Goulding’sstudy are a group of fish known as the characins, a highly diversified and successful clan that numbers up to a thousand species in Amazon waters. Included among the seed-eating characins is a species called the tambaqui, one of the largest fish found in the Amazon and possibly the single most important source of protein for the region’s human population. Also included, unexpectedly, are some of the piranhas.
    Tambaqui are seed predators of huge appetite. They migrate into the flooded forest and gather in crowds beneath their favorite food-furnishing species, the rubber tree Hevea spruceana. Like most tree species in the Amazon, H. spruceana grows not in clusters or groves but as widely separated individuals, surrounded by other tree species yet with a large distance between each spruceana. Consequently, a whole gang of tambaqui may simultaneously address themselves to a single spruceana tree. They could conceivably eat every seed that it drops. To crush the hard nut walls of those seeds, tambaqui have evolved large jaws and strong, broad, molar-like teeth. They crunch up the spruceana seeds (which are about as tough as Brazil nuts) and swallow the nut shells as well as the seed tissue—though only the seed tissue gives them any nutrition. A single gorged tambaqui, weighing thirty pounds, might carry two pounds of ground-up seeds in its stomach. That’s like you or I tucking away twelve pounds of peanuts, shells and all, in the course of an afternoon ball game.
    The seed-eating piranhas are a bit more fastidious, and it seems to be precisely those pointed, razor-edge teeth that make such fastidiousness possible.
    Even the most notorious flesh-eating piranhas—for instance, that large species commonly known as the black piranha—evidently move up into the flooded forest on a seasonal search for food. They aren’t necessarily there for the seeds. In the igapó these flesh-eaters continue to function as secondary consumers, preying upon other fish, insects, occasionally a bird or a mammalor any other animal that might inhabit the forest waters or be so unlucky as to fall in. Black piranha do seem to be omnivorous rather than strictly carnivorous, opportunistic enough to make seeds and fruit a small fraction of their diet when those foods are easily available. But the piranhas that specialize in seed-eating, though closely related, are distinct.
    Michael Goulding has identified at least two species of piranha —Serrasalmus serrulatus and Serrasalmus striolatus —that live mainly on a diet of seeds. Each of these species retains jaw and tooth structures almost identical to those seen in its infamous, flesh-eating cousins. Serrulatus and striolatus merely put those structures to different use. According to Goulding: “Piranhas shell the nuts they eat and ingest only the soft seed contents. . . . After the nut wall is broken, the endosperm contents are removed and the shell is discarded. By doing this the piranha does not fill its stomach and intestines with material that cannot be digested but that will take up space. The sharp teeth of piranhas allow them to masticate the soft seed contents into small bits that are usually of nearly equal size.” By slicing through the nut wall and rejecting it, eating only the seed tissue itself (and ignoring also the fleshy fruits in which some igapó trees wrap their seeds), the piranha maximize their nutritional benefit from each belly-load of food. It is a delicacy of