Bugs That Count
Primes and Other Conundrums
One factor not addressed by the models I have described so far is
feedback from prey to predator species. In an emergence year, birds
that feast on cicadas should be able to raise more young than usual;
the resulting increase in predator numbers will make life even
harder on straggler cicadas the following year, thus sharpening the
population peak. Many authors have drawn attention to this linkage,
and have offered it as the key to understanding why the
Magicicada species chose prime numbers for their
life–cycle periods. Because a prime has no divisors other than
itself and 1, the cicadas avoid falling into resonance with
predators whose abundance fluctuates on some shorter cycle. For
example, a hypothetical 12–year cicada would be susceptible to
predators with cycle lengths of 2, 3, 4 or 6 years.
G. F. Webb of Vanderbilt University has constructed computer
simulations in which such interactions of prey and predator cycles
favor the 13–year and 17–year cicada periods. Eric Goles
of the University of Chile and Oliver Schulz and Mario Markus of the
Max–Planck Institut für molekulare Physiologie in
Dortmund, Germany, have also published on this subject, referring to
cicadas as "a biological generator of prime numbers."
Whether the cyclic predator species exist remains an open question.
And some quite different explanations have also been put forward.
For example, Randel Tom Cox of Arkansas State University and C. E.
Carlton of Louisiana State University argue that the heart of the
matter is not predation but hybridization. Interbreeding between
broods that differ in period could disrupt synchronization for both
groups. Thus 13–year and 17–year broods are favored
because they emerge together only once every 221 years.
Much else about the lives of cicadas remains mysterious. In the
1960s Monte Lloyd of the University of Chicago and Henry S. Dybas of
the Field Museum of Natural History in Chicago offered a curious
meta–theory of cicada evolution. Any theory that seems too
plausible, they argued, is automatically suspect. "If there
were a broad, easy evolutionary highway towards periodicity, then
why would not more species have taken it?"
If one day we run out of cicada mysteries to solve, there are still
harder problems waiting. Synchronized, periodic breeding is known in
plants as well as animals. Daniel H. Janzen of the University of
Michigan has written about the bamboo Phyllostachys
bambusoides, which apparently maintains synchronized flowering
even when seeds are planted continents apart. And the length of the
plant's period makes the cicadas seem as ephemeral as mayflies. The
bamboo knows how to count not just to 17 but to 120.
© Brian Hayes