LSAT Practice Test 6

Passage

Every thirteen or seventeen years, depending on the brood, the forests of eastern North America fill with periodical cicadas. The nymphs, having spent the intervening years underground feeding on fluid drawn from tree roots, emerge within a few weeks of one another in densities that can exceed a million per acre, molt, mate, lay eggs, and die. Emergence itself appears to be cued by soil temperature in spring, but such a cue cannot explain why it comes only in the thirteenth or seventeenth year. Two features of this life cycle have demanded explanation: the synchrony of emergence and the length of the cycle—in particular, the fact that both lengths are prime numbers.

Synchrony is the less mysterious of the two. An insect that emerges in overwhelming numbers satiates its predators: birds, squirrels, and even fish gorge on cicadas, but they cannot eat more than a small fraction, and the survivors breed. A cicada that emerged a year early would face predators almost alone, and studies of such stragglers have found them consumed at far higher rates than members of a main emergence. Selection would thus punish any departure from the common schedule.

The prime-number lengths are another matter. One long-standing hypothesis holds that prime cycles minimize coincidence with predators whose populations rise and fall on shorter multiyear cycles. A predator with a three-year cycle would coincide with a twelve-year cicada at every emergence but with a thirteen-year cicada only once every thirty-nine years. The entomologist Hollis Marrin has pointed out a difficulty: no predator of periodical cicadas has been shown to have a multiyear population cycle tied to them, and a hypothesis that requires such predators cannot be confirmed by the cicadas alone. Marrin favors a second hypothesis, which concerns not predators but the cicadas themselves. If broods with different cycle lengths emerged in the same year and interbred, their offspring might inherit mismatched timing and emerge in small, unprotected numbers. Prime cycles make such joint emergences rare: thirteen- and seventeen-year broods coincide only once every 221 years.

Marrin's hypothesis has the merit of drawing on a mechanism, predator satiation, whose operation is well documented. Yet it carries its own burden. It presumes that, at some earlier stage, broods of many cycle lengths coexisted, so that the nonprime cycles could be winnowed out—an episode for which direct evidence would be difficult to obtain. Recent genetic work suggests that cycle length can shift between thirteen and seventeen years through a change in the timing of a single developmental stage, which shows at least that the trait is less fixed than once assumed. Whether it also shows that other lengths were once common remains, for now, an open question.