Paper: Washington Post
Title: No Cicadas on Ice
Biologists Theorize That 13- and 17-Year Broods Evolved to Survive Climatic Changes
Date: May 3, 2004
Evolution has shown living things a thousand ways to save themselves.
The leopard gecko's tail pulls off, leaving the cat clutching nature's version of the tear-away jersey. The female pea crab
Pinnotheres, unsatisfied with her own shell, spends its life inside an oyster. The bacterium Thermotoga maritima grows in
water just below boiling temperature, an environmental niche into which most organisms won't dip a toe.
Few strategies, however, are as strange and unlikely as the one periodical
These large, ungainly insects in the genus Magicicada spend either 13 or 17 years underground, then emerge nearly
simultaneously in densities that can exceed 1 million per acre. Their few weeks of life in the open air are spent molting, calling
for a mate (in the case of the buzzing males), copulating and depositing eggs in nests made in gashed twigs (in the case of the
They do little to defend themselves. They fly poorly, don't fight and taste great. In the parlance of animal behavior,
"predator foolhardy" -- they are always available for lunch. Birds consume them in the greatest numbers, but many other
animals get in
the act. Squirrels, dogs, cats, turtles, fish and spiders all eat
, which for a few weeks are the protein
equivalent of manna from heaven.
Eventually, though, everyone gets full -- and there are still billions of
alive. This is the survival strategy known as
"predator satiation." It is a passive strategy that depends almost entirely
timing. If too few
emerge, or if they come
out over an extended period of time, they are likely to be wiped out by predators. If this occurs before they find a mate and
create a new generation to carry their genes forward, they will eventually disappear completely.
Most species of
have life cycles between two and eight years, with a fair amount of variability. If five years is the
dominant length, for example, many members of a population may come out in four years -- or not until the sixth.
Of course, this isn't apparent to the casual observer. That's because in most places a person hears
-- often more than
one species -- every summer. A fraction of the population reaches maturity and emerges every year, making the insects
annual, not periodical.