T1I2

8/10

WEEK 1 DAY2 CD1-05112530354045参考lack the means to protect themselves, and easily fall prey to predators, from birds to snakes to large and small mammals. However, by emerging at the same time in huge numbers, often they can overwhelm their predators. Even though many of them are consumed by voracious predators, others survive to create a new generation. Some researchers also point out that their density makes finding mates easier.Another explanation has to do with atmospheric cooling during the Pleistocene epoch, which began about 2.6 million years ago. During that period, glaciers repeat-edly advanced into and retreated from North America. For cicadas to survive their life stages above ground, a certain period of warm temperature is needed. Cicadas that emerged in cold summers were easily destroyed, but ones that developed more slowly and emerged less often were less likely to experience the killing summers, and thus more likely to survive. So were their offspring, who inherited the slow- development genes of their parents. Or as some suggest, the cooler conditions may have slowed down their development, extending their lifespans to 13 or 17 years.Then, what explains cicadas’ lifespans being prime numbers, which can be divided only by themselves and one? It has been hypothesized that their life cycles of 13 or 17 years minimize the risk that their adult life will synchronize with the shorter life cycles of predators. For instance, if a predator species comes and goes over a five-year life cycle, the emergence of a 17-year cicada species would synchro-nize with the predator’s cycle only every 85 years. Also, prime numbers help avoid interbreeding between the 13-year and 17-year species, or between other, shorter- lived cicadas, which may produce hybrids with non-prime-cycle lifespans in low density.As with many questions involving evolution, nothing can be said for certain. Nevertheless, further findings, along with logical thinking, will help us formulate the best possible hypotheses.