Primed to Survive

质数化生存

Cicadas, those tubby brown bugs that spend most of their lives underground, emerge from dormancy every 7, 13, or 17 years-all prime numbers. Physicists Eric Goles of the University of Chile and Mario Markus of the Max Planck Institute in Germany sensed an evolutionary logic behind this pattern. To prove it, the researchers designed a computer model of how cicadas interact with their predators. In the simulation, predator and prey follow randomly assigned life-cycle durations. If the cicadas appear when many predators are waiting, their population drops; if the cicadas arise when few predators are around, they flourish. Eventually the cycles settled into the most beneficial pattern, and sure enough, the periods were prime numbers. Because of their metabolic demands, predators cannot remain dormant for many years. It's better for the predator's life cycle to be a small, even number because then they most often have the same peak year as their prey,* says a physicist on the German team. That explains why there are no even-cycle cicadas. For the cicada, 12 years is bad because predators on 2-, 3-, 6-, and 12-year cycles would eat them,* he says. But 13 (as with 7 and 17) offers safety in numbers because it is divisible only by itself and 1.


　　蝉一生中的大部分时间都在地下度过，这些圆滚滚的褐色虫子结束休眠来到地面的间隔期是7年，13年或是17年--都是质数。智利大学的物理学家埃里克·科尔斯和德国马克斯·普朗克学院的马里奥·马库斯感到这种数字模型背后存在着某种进化逻辑。为了证明这一点，研究者们设计了一个关于蝉与其天敌之间互动关系的计算机模型。在这个模型中，天敌和猎物按照随机设定的生命周期活动。如果蝉出来的时候有许多天敌环伺在侧，其数量就会下降；而如果它们在天敌很少的时候出来就可以大量繁衍。最终，蝉的生命周期以一个最有利的模式固定下来，这个数字果然是质数。由于新陈代谢的需要，天敌们无法长时间地休眠。德国小组的一位物理学家说�"对蝉的天敌来说，生命周期最好是一个小的偶数，这样它们最有可能和猎物一同进入高峰年。"这也解释了为什么没有生命周期为偶数的蝉。他说："对蝉来说，12年无疑是不妙的周期，那样的话周期是2、3、6和12年的天敌都有机会捕食它们。"不过13（以及7和17）是安全的数字，因为它只能被它本身和1整除。

Remarks：我们把蝉叫做"知了"多少带点讽刺的意思，但是看起来它们还是知道一些东西的。