Natural Acts: A Sidelong View of Science and Nature - David Quammen (1996)
ELOQUENT PRACTICES, NATURAL ACTS
LIKE WOODY ALLEN, the English poet John Donne was obsessed in his younger days by love and death. Throughout Donne’s early work those two motifs recur again and again, linked so closely together that they come to seem almost logically inseparable, two sides of a macabre equation, evoking each other almost interchangeably. Love is a manner of dying, Donne suggests; and vice versa. For instance: “I cannot say I lov’d, for who can say / Hee was kill’d yesterday?” Another poem contains the tender sentiment “Since thou and I sigh one another’s breath, / Who e’r sighes most, is cruelest, and hastes the other’s death.” In still another, a man speaking from his own grave declares himself “love’s martyr,” dead of an excess of passion. During the sixteenth century in England, this oxymoronic linkage of the two concepts, love and death, was enough to get an ardent young man eventually categorized as one of the “metaphysical” poets. Nowadays it would make him a theoretical population ecologist.
The notion with which John Donne was flirting, in all that love-and-death poetry, is studied today as a curious phenomenon of evolutionary ecology and denoted by the label semelparity. That fancy word is, of course, just another bit of the formal jargon that scientists take cruel joy in inventing. The same thing is more casually known, with a lewd nod from one science to another, as Big Bang reproduction.
Semelparity: An animal or a plant waits a very long time to procreate only once, does so with suicidal strenuousness, and then promptly dies. The act of sexual reproduction proves to be ecstatically fatal, fatally ecstatic. And the rest of us are left merely to say, Wow.
As a strategy for perpetuation of a species, semelparity is not well understood. But the list of known semelparous creatures is intriguingly diverse. Bamboos do it. A group of hardy desert plants called the agaves do it. Pacific salmon do it. The question is why. What can these three kinds of organisms, apparently so dissimilar, have in common? Why should all three, living in drastically different environments with drastically different life histories, be similarly committed to dying for one taste of love?
The answer, according to cautious speculation by some ecologists, may be as simple as a few symbols. Natural selection, these researchers say, tends to maximize, for each age I, the sum Bi+ pi(vi+1/v0). They may be right, but if you’re like me, any such clot of algebraic erudition immediately causes alarm bells to ring in your head, sprinkler systems to begin dousing your overheated brain, and your eyes to slide straight off the page like a cheap ballpoint skidding across oilpaper. But wait. The idea wrapped in that ugly cryptogram happens to be rather interesting, and just possibly it can also be said in English.
First, a few concrete facts. Five different species of salmon migrate regularly from the Pacific Ocean into the rivers of western North America. They are headed upstream to spawn, and for each individual fish the journey is a return trip back to that same freshwater tributary where it began its life. Some of these salmon (the Chinook of the Yukon River, for instance) will travel as much as 2,000 miles, climbing through rapids, making ten-foot leaps to clear the cascades, dodging predators, fighting constantly upriver at an unflagging pace of perhaps 50 miles per day. The effort and determination involved are prodigious, but it is a one-way trip. Soon after having spawned, every male and every female of these five species is dead. Decomposing corpses pile up in the eddies, turning clear mountain water funky with rotting flesh. Among those lucky fish that have completed the trip, won a mate, found genetic fulfillment in the gentle current above a gravel spawning nest, there are no survivors.
Certain types of bamboo make a long journey to breed also, but the distance they cover is measured in time. The common Chinese species Phyllostachys bambusoides, for instance, has a regular life cycle of 120 years. Historical sources back into the tenth century record its episodes of massive synchronous flowering. Each time around, a vast number of P. bambusoides individuals begin life together as new sprouts; for 120 years they grow taller and sturdier, putting out leaves and branches, storing away energy, adding clones of themselves to the population by a nonsexual budding process, maturing together into a dense grove; then, after the appointed twelve decades, they suddenly and simultaneously produce an awesome profusion of flowers. The blossoms fertilize one another by wind. Seeds fall like heavy hail, coating the ground, ankle-deep to a man. And the 120-year-old progenitors all immediately die.
Members of the plant genus Agave, meanwhile, are content through long years of growth and celibacy to resemble giant artichokes peeking out between desert rocks. They are succulents, related to lilies but preferring a hard life on dry hillsides, such as those of the Sonoran Desert. They unfold their leaves in a spiral rosette, each leaf tipped with a sharp spine for protection against browsers, and they range by species from the size of a small porcupine to the size of a 300-pound octopus. Like the bamboo, they can set clones of themselves by budding, but their primary mode of reproduction is sexual. Years go by uneventfully, an agave grows large and vigorous until, with startling abandon, one season it produces a towering flower stalk. This great inflorescence will be notable not for the beauty or fragrance of its blossoms but for its sheer height. From a big agave, in just three or four months the stalk might shoot up thirty feet, sturdy and straight as a young lodgepole pine. It’s a flower that you’d cut with a chainsaw. Pollination is done by insects and bats; seeds ripen and drop. And down below, shriveled and brown, the agave dies away, as though speared through the heart by its own flowery stalk.
This is semelparity, at work in the moist tropics of Asia, on the sere slopes of southern Arizona, at the headwaters of the Yukon River. A curious person naturally wants to know why, in each case, a single act of sex should prove deadly. But even more interesting, it seems to me, is the first derivative of that question: Is there any one answer that can explain why sex is terminal in three such disparate cases?
A man named William M. Schaffer says yes, and offers this:
Hold the sprinklers, hold the alarm. What he is talking about is simply a delicate balance between love and death.
Dr. Schaffer is a respected theoretical ecologist at the University of Arizona. In a half-dozen papers published over the past decade, alone and with various coauthors, he has proposed a theory of how evolution tends to produce optimal life-history strategies for different animals and plants. This theory entails enough mathematical bebop to make the average human swoon. Descriptive numbers for a particular species can be inserted into Schaffer’s equations, a lever is then pulled, a crank is turned, and the theory will posit how the creature should act if it wants to survive the long-term Darwinian struggle. That’s the kind of thing theoretical ecologists do. Semelparity is quite useful in such theorizing, because it involves the love-and-death balance taken to one logical extreme.
Dr. Schaffer calls that balance “the trade-off function.” The particular trade-off at issue is between present and future—that is, the immediate prospect of producing offspring versus the chance of surviving to produce other broods later. An underlying premise here is that an animal or a plant has, at each stage in its life, only a limited total amount of available energy that it can spend on the business of living. That limited energy must be shared out among three fundamental categories of effort—routine metabolism, growth, and reproduction—and an optimal life history is one that balances the shares most efficiently to produce evolutionary success. Evolutionary success, of course, is measured on one simple scale: How many offspring does the creature leave behind? All this is quite basic.
Schaffer’s trade-off equation merely codifies that crucial balance between present and future, between short-term and long-term concerns, between effort devoted to immediate self-preservation and effort devoted to parenthood. His Bi represents the reproductive potential of a given creature right now. The factor pi stands for the probability (or improbability) that the given creature, having bred once, will survive to breed again later. The parenthesis (vi+1/v0) will be the remaining reproductive potential that an experienced parent can expect still to possess at that hypothetical later time. Balance all these considerations against one another, with just the proper commitment of energy (at each age) to metabolism, to growth, to reproduction, and the result is high evolutionary success. This is the burden of
Now let’s try it in English. If a female Chinook salmon, having swum 2,000 miles up the Yukon River, having climbed rapids and dodged otters and leapt cascades, having beaten her fins to tatters digging a gravel nest and fought off other salmon to keep the spot inviolate—if this poor haggard creature has virtually no chance of surviving to accomplish the same entire feat again, then she will be required by the forces of natural selection to sacrifice herself totally, in one great suicidal effort of unstinting motherhood from which she cannot possibly recover. She will lay about five thousand eggs. And then she will croak.
Likewise for those Sonoran agaves, for those Chinese bamboos. The terms of the trade-off are the same, the results are the same, only the numbers and the details are different. Bamboos seem to sacrifice themselves for the sake of predator satiation—that is, producing so many seeds that after all the rats and jungle fowl of China have eaten their fill, a few seeds will still be left to germinate. The agaves compete with each other to produce taller and yet taller flowers, apparently because their pollinators deign to visit only the tallest. The evolutionary consequence in each case, as with salmon, is semelparity.
But Dr. Schaffer’s neat mathematical model is not without gaps, not without weaknesses. What about the Atlantic salmon, for instance, which faces an almost identical set of circumstances to those the Pacific species do yet which doesn’t resort to semelparity? It can be argued that Schaffer’s equations constitute an unduly abstract version of reality. To some observers, such airy theorizing has little more connection to the untidy actualities of ecological fieldwork than it does to, say, metaphysical poetry.
At Christmas of the year 1600, John Donne secretly married a young girl named Anne More, a gentle but dignified sixteen-year-old whose hotheaded father was a powerful nobleman, serving as Queen Elizabeth’s Lieutenant of the Tower. Donne himself was twenty-seven and employed as private secretary to the girl’s uncle. It was a reckless move, this marriage, putting passion before prudence, and Donne knew that. He suffered consequences: dismissed from his job, briefly imprisoned, denied the dowry that Anne otherwise would have brought, and left to struggle for years on the margins of poverty with his adored wife and their many children. Around that time he wrote:
Love with excesse of heat, more yong than old,
Death kills with too much cold…
Once I lov’d and dy’d; and am now become
Mine Epitaph and Tombe.
Here dead men speake their last, and so do I;
Love-slaine, loe, here I lye.
It’s highly unlikely that John Donne ever set eyes upon a Pacific salmon, or a Sonoran agave, or even a transplanted grove of Phyllostachys bambusoides in some London botanical garden. But we can safely assume that he would have understood.