That old, airy injunction—Pursue your dream—takes on an almost literal flavor when you peer into the world of the lepidopterist. For butterflies can seem like creatures born no less of the mind than of the earth. They are vivid like dreams, and shifting and balky like dreams, and when like dreams they expire, they leave behind an all but weightless residue. In Sue Halpern’s bright new book, Four Wings and a Prayer, the species under scrutiny is Danaus plexippus, the monarch butterfly, which, although a sizable specimen by North American standards, typically weighs less than a gram. Its lipid reserves—the stored fat that fuels its transcontinental flights—are measured by the milligram. We’re in the realm of motes and minims—not far from the world attended by Peaseblossom and Cobweb in A Midsummer Night’s Dream.
Yet these pretty lightweights congregate so densely—in such staggering multitudes—as to bow the branches of the trees they roost upon. They winter in the Transverse Neovolcanic Mountains, some fifty to a hundred miles west of Mexico City, in oyamel pine groves located at an altitude of roughly ten thousand feet. (Smaller winter roosts, for monarchs west of the Rockies, are found in California.) Some of them will have flown nearly two thousand miles, from Minnesota, or perhaps from Quebec, in order to settle there. Among the 165,000 or so known species in the insect order Lepidoptera—which includes both butterflies and moths—there isn’t another species that attempts a journey of remotely comparable magnitude. In the common imagination, butterflies often play the role of pilgrims—wandering souls in life’s garden—but only the monarch initiates an annual pilgrimage marked by such miraculous navigational skills and odds-defying pertinacity.
The mysteries of the monarch’s migration lie at the core of Sue Halpern’s tightly focused book, whose greatest virtue is its clear delineation of the enormous, precarious network of contingencies that arise when so small a creature undertakes so vast a voyage; the monarch’s migration is an elongated chain with any number of vulnerable links. To begin with, there are a host of traditional dangers—drought, fire, predators, storms. In addition, monarchs face a number of imponderable man-made perils that raise the theoretical possibility of the creature’s eventual decimation or destruction: deforestation; transplanted predators, like the fire ant; land development, with its discontinuities of food and shelter; the possibility of poisoning through genetically engineered crops; wholesale climatic change brought about by greenhouse warming.
Halpern does a splendid job of giving the big picture of a creature that lives on a bigger scale than other insects. When the monarch is viewed in this way—with the maps and the statistics of its migration before us—we’re beguiled into a realm of pure mathematics. The migratory flights take on the ideal one-dimensional purity of Euclidean lines, and the butterflies themselves the ideal no-dimensional purity of Euclidean points (even more so when you realize …