Do Locusts Understand Prime Numbers?

41. DO LOCUSTS UNDERSTAND PRIME NUMBERS?

Just to drive home the point about the independence of numbers and numerals, free-floating rationales and represented reasoning, consider the case of the seventeen-year locusts. In 1977, Stephen Jay Gould wrote an insightful and admiring chapter about the curious fact that cicadas (such as the “seventeen-year locust”) have reproductive cycles with lengths, in years, equal to prime numbers—thirteen years, or seventeen, but never fifteen or sixteen, for instance. “As evolutionists,” Gould (p. 99) says, “we seek answers to the question, why. Why, in particular, should such striking synchronicity evolve, and why should the period between episodes of sexual reproduction be so long?” The answer—which makes beautiful sense, in retrospect—is that by having a large prime number of years between appearances, the cicadas minimize the likelihood of being discovered and later tracked as a predictable feast by predators who themselves show up every two years, or three years, or five years. If the cicadas had a periodicity of, say, sixteen years, then they would be a rare treat for predators who showed up every year, but a more reliable source of food for predators who showed up every two or four years, and an even-money gamble for predators who got in phase with them on an eight-year schedule. If the length of their reproductive period is not a multiple of any lower number, however, they are not worth “trying” to track for any species that isn’t lucky enough to have exactly the same periodicity (or some multiple of it—the mythical Thirty-Four-Year Locust-Muncher would be in fat city).

It should be clear that the soundness of this explanation (which may not yet be established) does not depend on any hypothesis suggesting that locusts understand arithmetic, let alone prime numbers. Nor does it depend on the process of natural selection understanding prime numbers. The mindless, uncomprehending process of natural selection can exploit this important property of some numbers without having to understand it at all. For another example: neither bees nor Mother Nature need to understand the geometry that declares the hexagon the ideal shape for cells in a honeycomb. Many more examples of evolution’s mathematical competence-without-comprehension could be cited.