Leonardo’s Mountain of Clams and the Diet of Worms

by Stephen Jay Gould

  Charles Darwin, as a young man in South America on the Beagle, took up the subject and, as usual, both asked the key question in the most fruitful way, and got the answer basically right. He did a crude experiment on the Andean condor (Vultur gryphus) and concluded that this species does not smell. Darwin wrote:

  Remembering the experiments of Mr. Audubon . . . I tried the following experiment: the condors were tied, each by a rope, in a long row at the bottom of a wall; and having folded up a piece of meat in white paper, I walked backwards and forwards, carrying it in my hand at the distance of about three yards from them, but no notice whatever was taken. I then threw it on the ground, within one yard of an old male bird; he looked at it for a moment with attention, but then regarded it no more. With a stick I pushed it closer and closer, until at last he touched it with his beak; the paper was then instantly torn off with a fury, and at the same moment, every bird in the long row began struggling and flapping its wings. Under the same circumstances, it would have been quite impossible to have deceived a dog.

  But Darwin recognized that other species might smell, and he mentioned that evidence for the turkey vulture favored olfaction as an important sense:

  The evidence in favor of and against the acute smelling powers of carrion-vultures is singularly balanced. Professor Owen has demonstrated that the olfactory nerves of the turkey-buzzard (Cathartes aura) are highly developed; and on the evening when Mr. Owen’s paper was read at the Zoological Society, it was mentioned by a gentleman that he had seen the carrion-hawks in the West Indies on two occasions collect on the roof of a house, when a corpse had become offensive from not having been buried: in this case, the intelligence could hardly have been acquired by sight.

  (I might add that Darwin also developed quite a fondness for Andean condors, despite their dubious lifestyle in human terms. He ended his discussion of this species by writing: “When the condors are wheeling in a flock round and round any spot, their flight is beautiful . . . It is truly wonderful . . . to see so great a bird, hour after hour, without any apparent exertion, wheeling and gliding over mountain and river.”)

  The issue of olfaction in turkey vultures was not conclusively solved until 1964, when Kenneth E. Stager presented overwhelming evidence, based on years of clever and careful experimentation, that Cathartes aura does indeed rely upon a keen sense of smell to find carrion. Turkey vultures will often make an initial identification by sight (though first clues can also be olfactory). They then circle the carcass far above and in a wide arc until they catch a sniff downwind. The famous circle (our conventional icon for vultures) then shortens considerably in radius as the birds home in by odor before descending for the feast.

  Ironically, Stager discovered that previous researchers often misread the evidence for olfaction because they assumed “the more putrid the better”—and therefore tested birds with truly rotten meat. In fact, turkey vultures prefer food only slightly rotten and will reject highly putrid flesh if any alternative exists (or unless severe hunger demands compromise with usual standards). Stager writes: “Cathartes shows a preference for food that is relatively fresh rather than putrid. If food is in short supply in a given area, the turkey vulture will feed on carrion that is well advanced in putrefaction. Tests . . . indicated that captive Cathartes showed a decided preference for recently dead, newly hatched chicks, rather than for putrefied carrion.”

  I am grateful for all this good information, but I would so prefer my unobtainable minute in a turkey vulture’s brain, particularly at the first sight or whiff of a good dead meal on the plains below.

  Such conjectures inevitably bring up the contentious theme of animal “consciousness.” J confess that I find this subject, as usually debated, both tedious and utterly fruitless as a dispute about subjective words pursued by people who mistakenly think that they are arguing about important and resolvable things. If I ask “does a dog have consciousness,” the endless and passionate arguments that ensue usually reduce to different definitions of this confusing word, rather than focusing upon interesting and empirically resolvable questions about what dogs can and don’t do. (I also admit, of course—for this essay takes the point as its major theme—that many questions raised by this fascinating topic do treat the genuine (things rather than words] but unknowable issue of a dog’s internal state of mind.) Whether or not a dog “thinks” or “has consciousness” depends upon a chosen definition. Some people won’t grant “consciousness” to any creature that can’t abstract a general concept—truth or religion, for example—from particulars and then apply the apparatus of formal logic to make inferences even further away from starting points. Others confer “consciousness” upon creatures that recognize kin and remember places of previous danger or pleasure. By the first criterion, dogs don’t; by the second, they do. But dogs remain dogs, feeling what they feel without regard to our chosen labels.

  In the context of Costa Rica, and international efforts to preserve biodiversity, this issue assumes centrality because the classical argument for why a supposedly decent and moral creature like Homo sapiens can mistreat and even extirpate other species rests upon an extreme position in a continuum. The Cartesian tradition, formulated explicitly in the seventeenth century, but developed in “folk” and other versions throughout human history no doubt, holds that other animals are little more than unfeeling machines, with only humans enjoying “consciousness,” however defined. Under extreme versions of this theory, even the overt pain and suffering of other mammals (so palpable to us in the most visceral way because the vocal and facial expressions of such close evolutionary relatives match our own reactions to the same stimuli) only record an automatic response with no internal representation in feeling—because other animals have no consciousness. Thus, taking the argument further, we might worry about extinction for other reasons, but not for any aggregate pain or distress in the requisite killing.

  I don’t think that many people today hold such a strong version of the Cartesian position, but the tradition of viewing “lower” animals as “less feeling” certainly persists as a Band-Aid of justification for our rapacity—just as our racist ancestors argued that “insensitive” Indians couldn’t feel conceptual or philosophical pain in the loss of environment or lifestyle (so long as reservations provided bodily needs of food and shelter), and that “primitive” Africans wouldn’t lament a forcibly lost land and family so long as slavery provided corporeal security.

  I don’t want to press the counterargument to extremes. Any definition of consciousness must involve gradations. I am willing to believe that my unobtainable sixty seconds within a sponge or a flatworm might not reveal any mental acuity that I would care to call consciousness. But I am also confident—without wrapping myself in unresolvable arguments about definitions—that vultures and sloths, as close evolutionary relatives with the same basic set of organs, lie on our side of any meaningful (and necessarily fuzzy) border—and that we are therefore not mistaken when we look them in the eye and see a glimmer of emotional and conceptual affinity. I feel sure that I could make something of those sixty seconds if I could ever get in. Vultures must have an aesthetic, and sloths must have a sense of pace.

  Modern sloths include but a small remnant of their former diversity—two small-bodied, tree-dwelling genera. As recently as ten to fifteen thousand years ago, giant ground sloths as big as elephants still inhabited the Americas. (Their large and well-preserved skeletons are often mistaken for dinosaurs by people who don’t read museum labels.) The other groups of modern edentates were also decimated in the recent past; the giant glyptodonts, better armored than turtles, are fossil armadillos, for example.

  South America had been an island continent, far bigger and far more diverse than Australia, for tens of millions of years before the Isthmus of Panama rose just a couple of million years ago. The resulting flood of North American mammals across the new land bridge corresponds in time with the decimation of the native South American fauna (though the causal link
s remain much disputed). In fact, most large mammals generally considered distinctively South American—jaguars, llamas, and tapirs, for example—are all recent migrants from North America. A few South American forms also managed to move north—including the armadillo of our Southern states, and the so-called (and misnamed) Virginia opossum. But most distinctive South American lineages simply died out—including the borhyaenid marsupial carnivores, the giant and rapacious phorusrhacid ground birds, the horselike (but unrelated) litopterns, and the camel-like (but similarly unrelated and phyletically unique) Macrauchenia—see chapter 7. I so wish that this wondrously diverse and evolutionarily disparate fauna had survived—and I do blame the entirely natural rise of the Isthmus of Panama for triggering this particular biological tragedy.

  Thus, older sloths saw what nature can achieve in her unplanned fortuity. Living species are now experiencing what humans can do with more pressing and conscious power. If I could have those sixty seconds within Bradypus, would I not hear a lament for lost and giant brethren; would I not receive a plea for humans to pause, reassess—and, above all, slow down?

  21

  REVERSING ESTABLISHED ORDERS

  WE ALL KNOW HOW THE WORLD WORKS. A FISHERMAN ASKS HIS BOSS IN Shakespeare’s Pericles: “Master, I marvel how the fishes live in the sea,” and receives the evident response, “Why, as men do a-land; the great ones eat up the little ones.” Consequently, when humorists invent topsy-turvy worlds, they reverse such established orders and then emphasize the rightness of their absurdity. Alice’s Wonderland works on the principle of “sentence first—verdict afterwards.” In Gilbert and Sullivan’s town of Titipu, the tailor Ko-ko, condemned to death by decapitation, is elevated instead to the rank of Lord High Executioner because—it is so obvious, after all—a man “cannot cut off another’s head until he’s cut his own off.” Pish-tush explains all this in a spirited song with a rousing chorus: “And I am right, and you are right, and all is right too-loora-lay.”

  Social and literary critics of the so-called postmodernist movement have emphasized, in a cogent and important argument often buried in the impenetrable jargon of their discourse, that conventional support for established orders usually relies upon claims for the naturalness of “dualisms” and “hierarchies.” In creating dualisms, we divide a subject into two contrasting categories; in imposing hierarchy upon these dualisms, we judge one category as superior, the other as inferior. We all know the dualistic hierarchies of our social and political lives—from righteous versus infidel of centuries past to the millionaire CEOs who deserve tax cuts versus single mothers who should lose their food stamps in our astoundingly mean-spirited present. The postmodernists correctly argue that such dualisms and hierarchies represent our own constructions for political utility (often nefarious), rather than nature’s factual and inevitable dictate. We may choose to parse the world in many other ways with radically different implications.

  Our categorizations of nature also tend to favor dualistic hierarchies based upon domination. We often divide the world ecologically into predators and prey, or anatomically into complicated and dominant “higher” animals versus simpler and subservient “lower” forms. I do not deny the utility of such parsings in making predictions that usually work—big fish do generally eat little fish, and not vice versa. But the postmodernist critique should lead us to healthy skepticism, as we scrutinize the complex and socially embedded reasons behind the original formulations of our favored categories. Dualism with dominance may primarily record a human imposition upon nature, rather than a lecture directed to us by the birds and bees.

  Natural historians tend to avoid tendentious preaching in this philosophical mode (though I often fall victim to such temptations in these essays). Our favored style of doubting is empirical: if I wish to question your proposed generality, I will search for a counterexample in flesh and blood. Such counterexamples exist in abundance, for they form a staple in a standard genre of writing in natural history—the “wonderment of oddity” or “strange ways of the beaver” tradition. (Sorry to be so disparaging—my own ignoble dualism, I suppose. The stories are terrific. I just often yearn for more intellectual generality and less florid writing.)

  Much of our fascination with “strange cases” lies in their abrogation of accepted dualisms based on dominance—the “reversing established orders” of my title. As an obvious example, and paragon of this literature, carnivorous plants have always elicited primal intrigue—and the bigger and more taxonomically “advanced” the prey, the more we feel the weirdness. We yawn when a Venus’s-flytrap ensnares a mosquito, but shiver with substantial discomfort when a large pitcher plant devours a bird or rodent.

  I keep a file marked “Reversals” to house such cases. I have long been on the lookout for optimal examples, where all three of the most prominent dualisms based on dominance suffer reversal: predator and prey, high and low, large and small—in other words, where a creature from a category usually ranked as small in body, primitive in design, and subject to predation eats another animal from a category generally viewed as bigger, anatomically superior, and rapacious. I now have four intriguing examples, more than enough for an essay. Since we postmodernists abjure hierarchical ranking, I will simply present my stories in the nonjudgmental chronological order of their publication (though postmodernism in this sense—and truly I am not a devotee of this movement—may be a cop-out and an excuse for not devising a better logical structure for this essay!)

  1. FROGS AND FLIES. Frogs eat flies. If flies eat frogs, then we might as well be headed for bedlam or the apocalypse. My colleague Tom Eisner of Cornell University is revered throughout our profession as the past master of natural oddities with important and practical general messages. One day in August 1982, at a small pond in Arizona, Eisner and several colleagues noted thousands of spadefoot toads congregating on the muddy shore as they emerged to adulthood in near synchrony from their tadpole stage. Eisner and colleagues described their discovery in a technical publication (see the 1983 article by R. Jackman and others, listed in the bibliography):

  Spaced only centimeters apart in places, they were all of minimal adult size (body length, 1.5 to 2 cm [less than an inch]). Conspicuous among them were toads that were dead or dying, apparently having been seized by a predator in the mud and drawn partly into the substrate, until only their head, or head and trunk, projected above ground. We counted dozens of such semisubmerged toads.

  They then dug deeper and, to their great surprise, found the predator: “a large grublike insect larva, subsequently identified as that of the horsefly Tabanus punctifer.” In other words, flies can eat toads! (Although astonishment may be lessened in noting that the tiny toads are much smaller than enormous fly larvae.) Unusually large insects and maximally small vertebrates have also been featured in the few other recorded cases of such reversals—frogs, small birds, even a mouse, consumed by praying mantids, for example.

  The fly larvae force themselves into the mud, rear end first, until their front end, bearing the mouthparts, lies flush with the surface. The larvae then catch toads by hooking their pointed mandibles into the hind legs or belly, and then dragging the toad partway into the mud. The larvae—please remember that many tales in natural history are not pleasant by human standards—then suck the toad dry (and dead) by ingesting blood and body fluids only.

  I loved the wry last sentence of the paper by Eisner and his colleagues—unusual in style for a technical article, but odd stories have always permitted some literary license:

  The case we report is a reversal of the usual toad-eats-fly paradigm, although . . . the paradigm may also prevail in its conventional form. Adult Scaphiopus [the spadefoot toad] might well on occasions have predatory access to the very Tabanus flies that as larvae preyed upon their conspecifics.

  J. Greenberg, reporting for Science News (November 5, 1983), began his commentary with the emotional impact of such reversals:

  This is the Okeechobee Fla. Little League team thr
ashing the New York Yankees; this is Wally Cox beating out Burt Reynolds for the girl; this is Grenada invading the United States. “This is unlike anything I’ve ever seen,” says Thomas Eisner.

  2. LOBSTERS AND SNAILS. Decapod crustaceans (lobsters, crabs, shrimp) eat snails, as all naturalists know. In fact, the classic case of an extended evolutionary “arms race,” elegantly documented over many years by my colleague Geerat Vermeij, involves increased strength of crab claws correlated with ever more efficient protective devices (spines, ribs, thicker and wavier shells) in snails over geological time. Land crabs are the overwhelmingly predominant predator of my own favorite subject for research, the Caribbean land snail Cerion. If snails eat decapods, we might as well retire.

  Amos Barkai and Christopher McQuaid studied rock lobsters and whelks (snails of middling size) in waters around two islands, Marcus and Malgas, located just four miles apart in the Saldanha Bay area of South Africa. On Malgas, as all God-fearing folk would only rightly suspect, rock lobsters eat mollusks, mostly mussels and several species of whelks. Barkai and McQuaid write in their 1988 account: “The rock lobsters usually attacked the whelks by chipping away the shell margin with their mouthparts.”

  The local lobstermen report that, twenty years ago, rock lobsters were equally common on both islands. But lobsters then disappeared from Marcus Island, for unclear reasons, perhaps linked to a period of low oxygen in surrounding waters during the 1970s. In the absence of lobsters as the usual top predator, extensive mussel beds have become established, and the population density of whelks has soared. Barkai and McQuaid asked themselves: “Why do rock lobsters not recolonize Marcus Island despite the high availability of food?”