Why do cicadas have prime-number (13- or 17-year) life-cycles?
Glenn Webb, a mathematician at Vanderbilt University in Nashville, Tennessee, has demonstrated mathematically that prime-numbered lifecycles could help cicadas avoid damaging “resonances” with the two- and three-year population fluctuations of their predators. These would result in lots of predators being around in years when there were lots of prey. Dr Webb's model shows that, over a 200-year period, average predator populations during hypothetical outbreaks of 14- and 15-year cicadas would be up to 2% higher than during outbreaks of 13- and 17-year cicadas. That may not sound like much, but it is enough to drive natural selection towards a prime-numbered life-cycle.Only one predator—or, rather, parasite—is known to have overcome this anti-resonance strategy, by developing its own 17-year clock. Massospora cicadina, a fungus, lives in cicada larvae and passes between adults when they mate. But according to Gene Kritsky, an entomologist at the College of Mount St Joseph, in Cincinnati, Ohio, natural selection is working against this resonance too. In 2000, he recorded thousands of Brood X members emerging four years early—in other words, shifting to a 13-year cycle that Massospora is not equipped to match. Lo and behold, when Dr Kritsky examined several dozen members of the “accelerated” Brood X that emerged in 2000, he found only one infected female among them, and she had but one fungal spore. By contrast, he found from 50 to 300 spores in each cicada female from another brood that emerged on time that year in North Carolina. Brood X, it seems, is splitting up, and a new 13-year cicada population is evolving.
The article's also got a really rocking photo of a cicada. I do miss hearing them over the summers...
(I found the article thanks to the fine folks at Crooked Timber.)
Posted by aloysius at May 09, 2004 11:58 AM | TrackBack |