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Archive - May 29, 2009

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Reason for Long, Prime-Numbered Life Cycles in Cicadas

Scientists may have discovered the reason for the long, prime-numbered life cycles (13 and 17 years) in periodical cicadas. A mathematical model used to predict the life cycles of these insects yielded these prime-numbered periods only when the so-called Allee effect was considered. This effect is a general model used to predict evolution that links large numbers of organisms to high survival rates. These results suggest that natural selection during times of low population size has favored the evolution of non-overlapping broods; broods emerging at the same time ultimately drive both populations to extinction. With 13- and 17-year life cycles, two neighboring broods of cicadas co-emerge approximately once every 221 years (13 times 17), greatly limiting the accompanying population crash. As cicadas depend on their overwhelming numbers to ward off predators, a smaller cicada population would result in increased mortality. This work was published online on May 18 in PNAS. [PNAS abstract]