Cicadas Primed for Defense

The periodical cicada is one of the world’s longest-living insects, but nobody knows why it times its death with bizarre precision: It either lives for 13 years or 17 years, on the dot. Now, Japanese researchers have developed a model that may explain the animals’ mysteriously accurate biological clocks.

The noisy winged critters spend more than 99 percent of their 13 or 17 years as juveniles, sucking on roots in underground lairs. In the summertime, they crawl out en masse — up to 40,000 can emerge from under a single tree within days. Their subterranean tenures are intriguing not only because 13 and 17 years are long periods over which to remain synchronized, but also because both numbers are prime — divisible only by themselves and the number 1.

A leading theory is that long, prime-numbered life cycles minimize the likelihood that the 13-year broods and 17-year broods will ever mate. If the animals lived smaller prime-numbered lives, like 5 and 7, they’d sync up every 35 years; if their lifespans were large, non-prime numbers, like 12 and 16 years, they might inadvertently mate every 48 years. But the large prime numbers 13 and 17 only match up every 221 years.

Though this theory is mathematically sound, no one could say why the animals would need to minimize hybridization, so a researcher developed a mathematical model to explore the rationale. He thought if 13-year and 17-year broods interbred, they might produce offspring with intermediate lifecycles — for example 15 years. This would result in their emergence two years before or after the vast majority of their fellow cicadas.

This is a problem, Cooley said, because periodical cicadas find strength in numbers. They’re easy to catch and don’t bite or sting, so they easily become snacks for hungry predators. But by buzzing around with hundreds of thousands of other cicadas, the probability of any one being eaten is close to zero.

The researcher's model shows that this negative consequence of hybridization could explain the prime life cycles. In his model, which starts with all possible life cycles, the only way to arrive at enduring 13- and 17- year life cycles is to include this density-dependent effect.

Read more in the original article here.